Gerard Kuiper, founder of the Lunar and Planetary Laboratory. Credit: lpl.arizona.edu
In the outer reaches of the Solar System, beyond the orbit of Neptune, lies a region permeated by celestial objects and minor planets. This region is known as the “Kuiper Belt“, and is named in honor of the 20th century astronomer who speculated about the existence of such a disc decades before it was observed. This disc, he reasoned, was the source of the Solar Systems many comets, and the reason there were no large planets beyond Neptune.
Gerard Kuiper is also regarded by many as being the “father of planetary science”. During the 1960s and 70s, he played a crucial role in the development of infrared airborne astronomy, a technology which led to many pivotal discoveries that would have been impossible using ground-based observatories. At the same time, he helped catalog asteroids, surveyed the Moon, Mars and the outer Solar System, and discovered new moons.
Europa is probably the best place in the Solar System to go searching for life. But before they’re launched, any spacecraft we send will need to be squeaky clean so don’t contaminate the place with our filthy Earth bacteria. Continue reading “Will We Contaminate Europa?”
Robotic sampling arm and capture mechanism to collect a multi-ton boulder from an asteroid are under development at NASA Goddard and other agency centers for NASA’s unmanned Asteroid Redirect Vehicle and eventual docking in lunar orbit with Orion crew vehicle by the mid 2020s. Credit: Ken Kremer/kenkremer.com
NASA GODDARD SPACE FLIGHT CENTER, MD – Rigorous testing has begun on the advanced robotic arm and boulder extraction mechanisms that are key components of the unmanned probe at the heart of NASA’s Asteroid Redirect Robotic Mission (ARRM) now under development to pluck a multi-ton boulder off a near-Earth asteroid so that astronauts visiting later in an Orion crew capsule can harvest a large quantity of samples for high powered scientific analysis back on Earth. Universe Today inspected the robotic arm hardware utilizing “leveraged robotic technology” during an up close visit and exclusive interview with the engineering development team at NASA Goddard.
“The teams are making great progress on the capture mechanism that has been delivered to the robotics team at Goddard from Langley,” NASA Associate Administrator Robert Lightfoot told Universe Today.
“NASA is developing these common technologies for a suite of missions like satellite servicing and refueling in low Earth orbit as well as autonomously capturing an asteroid about 100 million miles away,” said Ben Reed, NASA Satellite Servicing Capabilities Office (SSCO) Deputy Project Manager, during an exclusive interview and hardware tour with Universe Today at NASA Goddard in Greenbelt, Maryland, regarding concepts and goals for the overall Asteroid Redirect Mission (ARM) initiative.
NASA is leveraging technology originally developed for satellite servicing such as with the Robotic Refueling Mission (RRM) currently on board the International Space Station(ISS) and repurposing them for the asteroid retrieval mission.
“Those are our two near term mission objectives that we are developing these technologies for,” Reed explained.
The unmanned Asteroid Redirect Robotic Mission (ARRM) to grab a boulder is the essential first step towards carrying out the follow on sample retrieval with the manned Orion Asteroid Redirect Mission (ARM) by the mid-2020s.
ARRM will use a pair of highly capable robotic arms to autonomously grapple a multi-ton (> 20 ton) boulder off the surface of a large near-Earth asteroid and transport it to a stable, astronaut accessible orbit around the Moon in cislunar space.
“Things are moving well. The teams have made really tremendous progress on the robotic arm and capture mechanism,” Bill Gerstenmaier, NASA Associate Administrator for Human Exploration and Operations, told Universe Today.
Then an Orion crew capsule can fly to it and the astronauts will collect a large quantity of rock samples and gather additional scientific measurements.
“We are working on a system to rendezvous, capture and service different [target] clients using the same technologies. That is what we are working on in a nut shell,” Reed said.
This engineering design unit of the robotic servicing arm is under development to autonomously extract a boulder off an asteroid for NASA’s asteroid retrieval mission and is being tested at NASA Goddard. It has seven degrees of freedom and mimics a human arm. Credit: Ken Kremer/kenkremer.com
“Right now the plan is to launch ARRM by about December 2020,” Reed told me. But a huge amount of preparatory work across the US is required to turn NASA’s plan into reality.
Key mission enabling technologies are being tested right now with a new full scale engineering model of the ‘Robotic Servicing Arm’ and a full scale mockup of the boulder snatching ARRM Capture Module at NASA Goddard, in a new facility known as “The Cauldron.”
Capture Module comprising two robotic servicing arms and three boulder grappling contact and restraint system legs for NASA’s Asteroid Redirect Robotic Mission (ARRM). Credit: NASAThe ARRM capture module is comprised of two shorter robotic arms (separated by 180 degrees) and three lengthy contact and restraint system capture legs (separated by 120 degrees) attached to a cradle with associated avionics, computers and electronics and the rest of the spacecraft and solar electric power arrays.
“The robotic arm we have here now is an engineering development unit. The 2.2 meter-long arms can be used for assembling large telescopes, repairing a failed satellite, removing orbital debris and capturing an asteroid,” said Reed.
“There are two little arms and three big capture legs.”
“So, we are leveraging one technology development program into multiple NASA objectives.”
“We are working on common technologies that can service a legacy orbiting satellite, not designed to be serviced, and use those same technologies with some tweaking that we can go out with 100 million miles and capture an asteroid and bring it back to the vicinity of the Moon.”
“Currently the [capture module] system can handle a boulder that’s up to about 3 x 4 x 5 meters in diameter.”
Artists concept of NASA’s Asteroid Redirect Robotic Mission capturing an asteroid boulder before redirecting it to a astronaut-accessible orbit around Earth’s moon. Credits: NASA
The Cauldron is a brand new Goddard facility for testing technologies and operations for multiple exploration and science missions, including satellite servicing and ARRM that just opened in June 2015 for the centers Satellite Servicing Capabilities Office.
Overall project lead for ARRM is the Jet Propulsion Laboratory (JPL) with numerous contributions from other NASA centers and industrial partners.
“This is an immersive development lab where we bring systems together and can do lifetime testing to simulate what’s in space. This is our robotic equivalent to the astronauts NBL, or neutral buoyancy lab,” Reed elaborated.
“So with this same robotic arm that can cut wires and thermal blankets and refuel an Earth sensing satellite, we can now have that same arm go out on a different mission and be able to travel out and pick up a multi-ton boulder and bring it back for astronauts to harvest samples from.”
“So that’s quite a technical feat!”
The Robotic Servicing Arm is a multi-jointed powerhouse designed to function like a “human arm” as much as possible. It builds on extensive prior research and development investment efforts conducted for NASA’s current Red Planetrovers and a flight-qualified robotic arm developed for the Defense Advanced Research Projects Agency (DARPA).
“The arm is capable of seven-degrees-of-freedom to mimic the full functionally of a human arm. It has heritage from the arm on Mars right now on Curiosity as well as ground based programs from DARPA,” Reed told me.
“It has three degrees of freedom at our shoulder, two at our elbow and two more at the wrist. So I can hold the hand still and move the elbow.”
The arm will also be equipped with a variety of interchangeable “hands” that are basically tools to carry out different tasks with the asteroid such as grappling, drilling, sample gathering, imaging and spectrometric analysis, etc.
View of the robotic arm above and gripper tool below that initially grabs the asteroid boulder before the capture legs wrap around as planned for NASA’s upcoming unmanned ARRM Asteroid Redirect Robotic Mission that will later dock with an Orion crew vehicle. Credit: Ken Kremer/kenkremer.com
The ARRM spacecraft will carefully study, characterize and photograph the asteroid in great detail for about a month before attempting the boulder capture.
Why does the arm need all this human-like capability?
“When we arrive at an asteroid that’s 100 million miles away, we are not going to know the fine local geometry until we arrive,” Reed explained to Universe Today.
“Therefore we need a flexible enough arm that can accommodate local geometries at the multi-foot scale. And then a gripper tool that can handle those geometry facets at a much smaller scale.”
“Therefore we chose seven-degrees-of-freedom to mimic humans very much by design. We also need seven-degrees-of-freedom to conduct collision avoidance maneuvers. You can’t do that with a six-degree-of-freedom arm. It has to be seven to be a general purpose arm.”
How will the ARRM capture module work to snatch the boulder off the asteroid?
“So the idea is you come to the mother asteroid and touch down and make contact on the surface. Then you hold that position and the two arms reach out and grab the boulder.”
“Once its grabbed the boulder, then the legs straighten and pull the boulder off the surface.”
“Then the arms nestle the asteroid onto a cradle. And the legs then change from a contact system to become a restraint system. So the legs wrap around the boulder to restrain it for the 100 million mile journey back home.
“After that the little arms can let go – because the legs have wrapped around and are holding the asteroid.”
“So now the arm can also let go of the gripper system and pick up a different tool to do other things. For example they can collect a sample with another tool. And maybe assist an astronaut after the crew arrives.”
“During the 100 million mile journey back to lunar orbit they can be also be preparing the surface and cutting into it for later sample collection by the astronauts.”
Be sure to watch this video animation:
Since the actual asteroid encounter will occur very far away, the boulder grappling will have to be done fully autonomously since there will be no possibility for real time communications.
“The return time for communications is like about 30 minutes. So ‘human in the loop’ control is out of the question.
“Once we get into hover position over the landing site we hit the GO button. Then it will be very much like at Mars and the seven minutes of terror. It will take awhile to find out if it worked.”
Therefore the team at Goddard has already spent years of effort and practice sessions just to get ready for working with the early engineering version of the arm to maximize the probability of a successful capture.
“In this facility we put systems together to try and practice and rehearse and simulate as much of the mission as is realistically possible.”
“It took a lot of effort to get to this point, in the neighborhood of four years to get the simulation to behave correctly in real time with contact dynamics and the robotic systems. So the arm has to touch the boulder with force torque sensors and feed that into a computer to measure that and move the actuators to respond accordingly.”
“So the capture of the boulder is autonomous. The rest is teleoperated from the ground, but not the capture itself.”
How realistic are the rehearsals?
“We are practicing here by reaching out with the arm to grasp the client target using autonomous capture [procedures]. In space the client [target] is floating and maybe tumbling. So when we reach out with the arm to practice autonomous capture we make the client tumble and move – with the inertial properties of the target we are practicing on.”
“Now for known objects like satellites we know the mass precisely. And we can program all that inertial property data in very accurately to give us much more realistic simulations.”
“We learned from all our astronaut servicing experiences in orbit is that the more we know for the simulations, the easier and better the results are for the astronauts during an actual mission because you simulated all the properties.”
“But with this robotic mission to an asteroid there is no backup like astronauts. So we want to practice here at Goddard and simulate the space environment.”
ARRM will launch by the end of 2020 on either an SLS, Delta IV Heavy or a Falcon Heavy. NASA has not yet chosen the launch vehicle.
Several candidate asteroids have already been discovered and NASA has an extensive ongoing program to find more.
Orion crew capsule docks to NASA’s asteroid redirect vehicle grappling captured asteroid boulder orbiting the Moon. Credit: NASA
Again, this robotic technology was selected for development for ARRM because it has a lot in common with other objectives like fixing communications satellites, refueling satellites and building large telescopes in the future.
NASA is also developing other critical enabling technologies for the entire ARM project like solar electric propulsion that will be the subject of another article.
Therefore NASA is leveraging one technology development program into multiple spaceflight objectives that will greatly assist its plans to send ‘Humans to Mars’ in the 2030s with the Orion crew module launched by the monster Space Launch System (SLS) rocket.
The maiden uncrewed launch of the Orion/SLS stack is slated for November 2018.
Stay tuned here for Ken’s continuing Earth and Planetary science and human spaceflight news.
At NASA Goddard robotics lab Ben Reed/NASA Satellite Servicing Capabilities Office (SSCO) Deputy Project Manager and Ken Kremer/Universe Today discuss the robotic servicing arm and asteroid boulder capture mechanism being tested for NASA’s upcoming unmanned ARRM Asteroid Redirect Robotic Mission that will dock with an Orion crew vehicle in lunar orbit by the mid 2020s for sample return collection. Credit: Ken Kremer/kenkremer.com
The high gain antenna and solar arrays were installed on the OSIRIS-REx spacecraft prior to it moving to environmental testing. Credits: Lockheed Martin Corporation
OSIRIS-Rex, the first American spacecraft ever aimed at snatching pristine samples from the surface of an asteroid and returning them to Earth for exquisite analysis by researchers world-wide with the most advanced science instruments has successfully completed its assembly phase and moved into the “test drive” phase – just ten months before blastoff, following installation of all its science instruments at Lockheed Martin Space Systems facilities, near Denver, Colorado.
The launch window for OSIRIS-REx opens next fall on September 3, 2016 on a seven-year journey to asteroid Bennu and back. Bennu is a carbon-rich asteroid. OSIRIS-Rex will eventually return the largest sample from space since the American and Soviet Union’s moon landing missions of the 1970s.
The science payload installation was recently completed with attachment of the vehicles three camera instrument suite of cameras and spectrometers known as OCAMS (OSIRIS-REx Camera Suite), which was was designed and built by the University of Arizona’s Lunar and Planetary Laboratory.
OCAMS trio of instruments, PolyCam, MapCam and SamCam, will survey and globally map the surface of Bennu up close at a distance ranging from approximately 5 km to 0.7 km.
“PolyCam, MapCam and SamCam will be our mission’s eyes at Bennu,” said Dante Lauretta, principal investigator for OSIRIS-REx at the University of Arizona, Tucson, in a statement.
“OCAMS will provide the imagery we need to complete our mission while the spacecraft is at the asteroid.”
“All in all it was flawless installation, with the three cameras and the control electronics making it on the spacecraft well in advance of when we originally planned these activities. In general, the OSIRIS-REx ATLO (assembly, test and launch operations) flow has gone smoothly,” said Lauretta in a blog update.
The University of Arizona’s camera suite, OCAMS, sits on a test bench that mimics its arrangement on the OSIRIS-REx spacecraft. The three cameras that compose the instrument – MapCam (left), PolyCam and SamCam – are the eyes of NASA’s OSIRIS-REx mission. They will map the asteroid Bennu, help choose a sample site, and ensure that the sample is correctly stowed on the spacecraft. Credits: University of Arizona/Symeon Platts
For the next five months, NASA’s OSIRIS-REx which stands for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer will undergo a rigorous regime of critical environmental testing to ensure the probe will survive the unforgiving extremes of vacuum, vibration and extreme temperatures it will experience during launch and throughout the life of its planned eight year mission.
The asteroid sampling spacecraft is tracking on budget and ahead of schedule.
“OSIRIS-REx is entering environmental testing on schedule, on budget and with schedule reserves,” said Mike Donnelly, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement.
“This allows us to have flexibility if any concerns arise during final launch preparations.”
Bennu is a near-Earth asteroid and was selected for the sample return mission because it “could hold clues to the origin of the solar system and host organic molecules that may have seeded life on Earth,” says NASA.
The spacecraft is equipped with a suite of five science instruments to remotely study the 492 meter wide asteroid.
The instruments were all installed as planned on the spacecraft deck over the past few months so they can all be subjected to the environmental testing together with the spacecraft bus.
“This milestone marks the end of the design and assembly stage,” said Lauretta, in a statement.
“We now move on to test the entire flight system over the range of environmental conditions that will be experienced on the journey to Bennu and back. This phase is critical to mission success, and I am confident that we have built the right system for the job.”
The tests will “simulate the harsh environment of space, including acoustical, separation and deployment shock, vibration, and electromagnetic interference. The simulation concludes with a test in which the spacecraft and its instruments are placed in a vacuum chamber and cycled through the extreme hot and cold temperatures it will face during its journey to Bennu,” say NASA officials.
Video caption: Engineers at Lockheed Martin move the OSIRIS-REx spacecraft onto a rotation fixture. This fixture supports the full weight of the spacecraft and acts as a hinge, orienting the spacecraft at a 90 degree angle, which allows engineers to access the top of the spacecraft much more easily. Credits: Lockheed Martin Corporation
The testing is done to uncover any issues lurking prior next September’s planned liftoff.
“This is an exciting time for the program as we now have a completed spacecraft and the team gets to test drive it, in a sense, before we actually fly it to asteroid Bennu,” said Rich Kuhns, OSIRIS-REx program manager at Lockheed Martin Space Systems.
“The environmental test phase is an important time in the mission as it will reveal any issues with the spacecraft and instruments, while here on Earth, before we send it into deep space.”
After the testing is complete by next May, the spacecraft will ship from Lockheed Martin’s Denver facility to NASA’s Kennedy Space Center, where it will undergo final prelaunch preparations and transport to the launch pad at Cape Canaveral.
Artist concept of OSIRIS-REx, the first U.S. mission to return samples from an asteroid to Earth. Credit: NASA/Goddard
OSIRIS-REx is scheduled for launch in September 2016 from Cape Canaveral Air Force Station in Florida aboard a United Launch AllianceAtlas V 411 rocket, which includes a 4-meter diameter payload fairing and one solid rocket motor. Only three Atlas V’s have been launched in this configuration.
“This is an exciting time,” says Lauretta.
The spacecraft will reach Bennu in 2018. OSIRIS-REx will gather rocks and soil and bring at least a 60-gram (2.1-ounce) sample back to Earth in 2023 for study by researchers here with all the most sophisticated science instruments available.
Bennu is an unchanged remnant from the collapse of the solar nebula and birth of our solar system some 4.5 billion years ago, little altered over time.
OSIRIS-REx is the third mission in NASA’s New Frontiers Program, following New Horizons to Pluto and Juno to Jupiter, which also launched on Atlas V rockets.
NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is responsible for overall mission management.
OSIRIS-REx complements NASA’s Asteroid Initiative – including the Asteroid Redirect Mission (ARM) which is a robotic spacecraft mission aimed at capturing a surface boulder from a different near-Earth asteroid and moving it into a stable lunar orbit for eventual up close sample collection by astronauts launched in NASA’s new Orion spacecraft. Orion will launch atop NASA’s new SLS heavy lift booster concurrently under development.
OSIRIS-REx logo
Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.
Artist's impression of a Near-Earth Asteroid passing by Earth. Credit: ESA
In February of 2014, NASA put out the call for submissions for the thirteenth mission of their Discovery Program. In keeping with the program’s goal of mounting low-cost, highly focused missions to explore the Solar System, the latest program is focused on missions that look beyond Mars to new research goals. On September 30th, 2015, five semifinalists were announced, which included proposals for sending probes back to Venus, to sending orbiters to study asteroids and Near-Earth Objects.
Among the proposed NEO missions is the Near Earth Object Camera, or NEOCam. Consisting of a space-based infrared telescope designed to survey the Solar System for potentially hazardous asteroids, the NEOCam would be responsible for discovering and characterizing ten times more near-Earth objects than all NEOs that have discovered to date.
If deployed, NEOCam will begin discovering approximately one million asteroids in the Main Belt and thousands of comets in the course of its 4 year mission. However, the primary scientific goal of NEOCam is to discover and characterize over two-thirds of the asteroids that are larger that 140 meters, since it is possible some of these might pose a threat to Earth someday.
Artist’s concept of the NEOCam spacecraft, a proposed mission for NASA’s Discovery program that would search for potentially hazardous near-Earth asteroids. Credit: NASA/JPL-Caltech
The technical term is Potentially Hazardous Objects (PHO), and it applies to near-Earth asteroids/comets that have an orbit that will allow them to make close approaches to Earth. And measuring more than 140 meters in diameter, they are of sufficient size that they could cause significant regional damage if they struck Earth.
In fact, a study conducted in 2010 through the Imperial College of London and Purdue University found that an asteroid measuring 50-meters across with a density of 2.6 grams per cubic centimeter and a speed of 12.7 kps could generate 2.9 Megatons of airburst energy once it passed through our atmosphere. To put that in perspective, that’s the equivalent of about nine W87 thermonuclear warheads!
By comparison, the meteor that appeared over the small Russian community of Chelyabinsk in 2013 measured only 20 meters across. Nevertheless, the explosive airbust caused by it entering our atmosphere generated only 500 kilotons of energy, creating a zone of destruction tens of kilometers wide and injuring 1,491 people. One can imagine without much effort how much worse it would have been had the explosion been six times as big!
What’s more, as of August 1st, 2015, NASA has listed a total of 1,605 potentially hazardous asteroids and 85 near-Earth comets. Among these, there are 154 PHAs believed to be larger than one kilometer in diameter. This represents a tenfold increase in discoveries since the end of the 1990s, which is due to several astronomical surveys being performed (as well as improvements in detection methods) over the past two and a half decades.
The NEOCam sensor (right) is the lynchpin for the proposed Near Earth Object Camera, or NEOCam, space mission (left). Credit: NASA/JPL-Caltech
As a result, monitoring and characterizing which of these objects is likely to pose a threat to Earth in the future has been a scientific priority in recent years. It is also why the U.S. Congress passed the “George E. Brown, Jr. Near-Earth Object Survey Act” in 2005. Also known as the “NASA Authorization Act of 2005”, this Act of Congress mandated that NASA identify 90% of all NEOs that could pose a threat to Earth.
If deployed, NEOCam will monitor NEOs from the Earth–Sun L1 Lagrange point, allowing it to look close to the Sun and see objects inside Earth’s orbit. To this, NEOCam will rely on a single scientific instrument: a 50 cm diameter telescope that operates at two heat-sensing infrared wavelengths, to detect the even the dark asteroids that are hardest to find.
By using two heat-sensitive infrared imaging channels, NEOCam can also make accurate measurements of NEO and gain valuable information about their sizes, composition, shapes, rotational states, and orbits. As Dr. Amy Mainzer, the Principal Investigator of the NEOCam mission, explained:
“Everyone wants to know about asteroids hitting the Earth; NEOCam is designed to tackle this issue. We expect that NEOCam will discover about ten times more asteroids than are currently known, plus millions of asteroids in the main belt between Mars and Jupiter. By conducting a comprehensive asteroid survey, NEOCam will address three needs: planetary defense, understanding the origins and evolution of our solar system, and finding new destinations for future exploration.”
Dr. Mainzer is no stranger to infrared imaging for the sake of space exploration. In addition to being the Principal Investigator on this mission and a member of the Jet Propulsion Laboratory, she is also the Deputy Project Scientist for the Wide-field Infrared Survey Explorer (WISE) and the Principal Investigator for the NEOWISE project to study minor planets.
She has also appeared many times on the History Channel series The Universe, the documentary featurette “Stellar Cartography: On Earth”, and serves as the science consultant and host for the live-action PBS Kids series Ready Jet Go!, which will be debuting in the winter of 2016. Under her direction, the NEOCam mission will also study the origin and ultimate fate of our solar system’s asteroids, and finding the most suitable NEO targets for future exploration by robots and humans.
Proposals for NEOCam have been submitted a total of three times to the NASA Discovery Program – in 2006, 2010, and 2015, respectively. In 2010, NEOCam was selected to receive technology development funding to design and test new detectors optimized for asteroid and comet detection and discovery. However, the mission was ultimately overruled in favor of the Mars InSight Lander, which is scheduled for launch in 2016.
As one of the semifinalists for Discovery Mission 13, the NEOCam mission has received $3 million for year-long studies to lay out detailed mission plans and reduce risks. In September of 2016, one or two finalist will be selected to receive the program’s budget of $450 million (minus the cost of a launch vehicle and mission operations), and will launch in 2020 at the earliest.
In related news, NASA has confirmed that the asteroid known as 86666 (2000 FL10) will be passing Earth tomorrow. No need to worry, though. At its closest approach, the asteroid will still be at a distance of 892,577 km (554,000 mi) from Earth. Still, every passing rock underlines the need for knowing more about NEOs and where they might be headed one day!
Like me, you’re probably a little ego-geocentric about the importance of Earth. It’s where you were born, it’s where you keep all your stuff. It’s even where you’re going to die – I know, I know, not you Elon Musk, you’re going to “retire” on Mars, right after you nuke the snot out of it.
For the rest of us, Earth is the place. But in reality, when it comes to planets, this is somebody else’s racket. This is Jupiter’s Solar System, and we all sleep on its couch.
Jupiter accounts for 75% of the mass of the planets of the Solar System, nearly 318 times more massive than Earth, and isn’t just the name of everyone’s favorite secret princess. It’s the 1.9 × 10^27 kilogram gorilla in the room. Whatever Jupiter wants, Jupiter gets. Jupiter hungry? JUPITER HUNGRY.
What Jupiter apparently wants is to throw our stuff around the Solar System. Thanks to its immense gravity, Jupiter yanks material around in the asteroid belt, preventing the poor space rocks from ever forming up into anything larger than Ceres.
Jupiter gobbles up asteroids, comets, and spacecraft, and hurtles others on wayward trajectories. Who knows how much mayhem and destruction Jupiter has gotten into over the course of its 4.5 billion years in the Solar System.
Some scientists think we owe our existence to Jupiter’s protective gravity. It greedily vacuums up dangerous asteroids and comets in the Solar System.
Other scientists totally disagree and think that Jupiter is a bully, perturbing perfectly safe comets and asteroids into dangerous trajectories and flushing earth’s head in the toilet during recess.
Which is it? Is Jupiter our friend and protector, or evil enemy. We’ve already figured out how to dismantle you Jupiter, don’t make us put our plans into action.
Some of the most dangerous objects in the Solar System are long-period comets. These balls of rock and ice come from the deepest depths of the Oort cloud. Some event nudges these death missiles into trajectories that bring them into the inner Solar System, to shoot past the Sun and maybe, just maybe, smash into a planet and kill 99.99999% of the life on it.
The Solar System. Credit: NASA
There’s a pretty good chance some of the biggest extinctions in the history of the Earth were caused by impacts by long period comets.
As these comets make their way through the Solar System, they interact with Jupiter’s massive gravity, and get pushed this way and that. As we saw with Comet Shoemaker-Levy, some just get consumed entirely, like a tasty ice-rock sandwich.
The theory goes that Jupiter pushes these dangerous comets out of their murder orbits so they don’t smash into Earth and kill us all.
But a competing theory says that Jupiter actually diverts comets that would have completely missed our planet into deadly, Earth-killing trajectories.
Will the Sailor Scouts provide us any clues? Who can say?
At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)
Here’s friend of the show, Dr. Kevin Grazier, a planetary scientist and scientific advisor for many of your favorite sci-fi TV shows and movies.
… [ see video for Interview with Dr. Grazier about Jupiter]
So which is it? Is Jupiter our friend or enemy? We’ll need to run more simulations and figure this out with more accuracy. And until then, it’s probably best if we just tremble in fear and worship Jupiter as a dark and capricious god until the evidence proves otherwise. It’s what Pascal would wager.
What are some other theories you’ve heard about and you’d like us to dig in further? Make some suggestions in the comments below.
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All asteroids and comets visited by spacecraft as of November 2010 Credits: Montage by Emily Lakdawalla. Ida, Dactyl, Braille, Annefrank, Gaspra, Borrelly: NASA / JPL / Ted Stryk. Steins: ESA / OSIRIS team. Eros: NASA / JHUAPL. Itokawa: ISAS / JAXA / Emily Lakdawalla. Mathilde: NASA / JHUAPL / Ted Stryk. Lutetia: ESA / OSIRIS team / Emily Lakdawalla. Halley: Russian Academy of Sciences / Ted Stryk. Tempel 1, Hartley 2: NASA / JPL / UMD. Wild 2: NASA / JPL.
What are asteroids made of? Asteroids are made mostly of rock — with some composed of clay and silicate — and different metals, mostly nickel and iron. But other materials have been found in asteroids, as well.
Overview
Asteroids are solid, rocky and irregular bodies that are the rocky remnants of the protoplanetary disk of dust and gas that formed around our young Sun over 4.5 billion years ago. Much of the disk coalesced to form the planets, but some of the debris remained. During the chaotic, fiery days of the early Solar System, debris was constantly crashing together and so small grains became small rocks, which crashed into other rocks to form bigger ones.
Some of debris was shattered remnants of planetesimals – bodies within the young Sun’s solar nebula that never grew large enough to become planets — and large collisions pulverized these planetesimals while other debris never came together due to the massive gravitational pull from Jupiter. This is the how the asteroids originated.
The various elements that are found in asteroids. Credit: Planetary Resources.
Composition
An asteroid’s composition is mainly determined by how close it is to the Sun. The asteroids that are nearest the Sun are mostly made of carbon, with smaller amounts of nitrogen, hydrogen and oxygen, while the ones further away are made up of silicate rock. Silicates are very common on Earth and in the Solar System. They are made up of oxygen and silicon, the number one and number two most abundant elements in the Earth’s crust. The metallic asteroids are composed of up to 80% iron and 20% a mixture of nickel, iridium, palladium, platinum, gold, magnesium and other precious metals such as osmium, ruthenium and rhodium. There are a few that are made up of half silicate and half metallic.
The platinum group metals are some of the most rare and useful elements on Earth. According to Planetary Resources, a company that hopes to mine asteroids in space, those metals exist in such high concentrations on asteroids that a single 500-meter platinum-rich asteroid can contain more platinum group metals than have ever been mined on Earth throughout human history.
Other minerals have been found on asteroids that have been visited by our spacecraft. For example, the Hayabusa spacecraft landed on Itokawa, a spud-shaped, near-Earth asteroid, and found it consists mainly of the minerals olivine and pyroxene, a mineral composition similar to a class of stony meteorites that have pelted Earth in the past.
In addition to the metals, the elements to create water are present in asteroids and there are indications that asteroids contain water or ice in their interiors, and there’s even evidence that water may have flowed on the surface of at least one asteroid. Observations of Vesta from the Dawn mission show gullies that may have been carved by water. The theory is that when a smaller asteroid or comet slams into a bigger asteroid, the small asteroid or comet could release a layer of ice in the bigger asteroid. The force of the impact briefly turned the ice into water, which flowed across the surface, creating the gullies.
Metals that are abundant in asteroids. Credit: Planetary Resources.
But asteroids may have changed over time. It is also thought that chemical reactions over the millennia or more recent impacts they may have endured also effects the composition of asteroids. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. Only one such asteroid, Vesta, is known to have this type of surface.
Types of Asteroids
Generally, there are three main types of asteroids:
Dark C (carbonaceous) asteroids, which make up most asteroids and are in the outer belt. They’re believed to be close to the Sun’s composition, with little hydrogen or helium or other “volatile” elements.
Bright S (silicaceous) asteroids and are in the inner belt, closer to Mars. They tend to be metallic iron with some silicates of iron and magnesium.
Bright M (metallic) asteroids. They sit in the middle of the asteroid belt and are mostly made up of metallic iron.
There are also D type, known as the Trojan asteroids of Jupiter and are dark and carbonaceous in nature, and V type that are distant asteroids between the orbits of Jupiter and Uranus, and they may have originated in the Kuiper Belt. While these have not been studied extensively, it has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors.
Comparisons
Asteroids are different from comets, which are mostly rock and ice. Comets usually have tails, which are made from ice and debris sublimating as the comet gets close to the Sun. Asteroids typically don’t have tails, even those near the Sun. But recently, astronomers have seen some asteroids that have sprouted tails, such as asteroid P/2010 A2. Scientists have theorized this can happen when the asteroid has been hit or pummeled by other asteroids and dust or gas is ejected from their surfaces, creating a sporadic tail effect. These so-called “active asteroids” are a newly recognized phenomenon, and as of this writing, only 13 known active asteroids have been found in the main asteroid belt, and so they are very rare.
How Many Asteroids?
There are millions of asteroids in our Solar System. Scientists estimate the asteroid belt has between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Most of the undiscovered asteroids are likely the smaller ones (less than 100 km across) which are more difficult to detect. Some astronomers estimate there could be 150 million asteroids in the entire Solar System.
As of September 06, 2015, 13,024 Near-Earth objects have been discovered. About 875 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger. Also, 1,609 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs), but none at this time are expected to impact Earth. Check the NASA NEO website for updates.
All asteroids are covered in space dust called regolith. This dust is usually a rocky rubble more than dust. It is the result of the constant collisions the asteroids undergo in space.
Artist's depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra
4.6 billion years ago, our Solar System formed from a collection of gas and dust surrounding our nascent Sun. While much of the gas and dust in this protoplanetary disk coalesced to form the planets, some of the debris was left over.
Some of debris was shattered remnants of planetesimals – bodies within the young Sun’s solar nebula that never grew large enough to become planets, and scientists theorize that large collisions in the early, chaotic Solar System pulverized these planetesimals into smaller pieces. Other debris never came together due to the massive gravitational pull from Jupiter.
These rocky remnants are now the asteroids that travel about our Solar System. Since these “leftovers” contain clues about the early days of our Solar System, scientists are eager to study them.
Definition of an Asteroid
Asteroids are rocky, metallic bodies that orbit the Sun. They are made from different kinds of rock and metals, with the metals being mostly nickel and iron. They are sometimes called “minor planets” but they are much, much smaller than the planets or moons. They don’t have atmospheres, but about 150 asteroids are known to have small “moons” orbiting them, and some even have two moons. There are also binary (double) asteroids, where two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.
At least one asteroid has rings. This surprise discovery was made in 2013 when scientist watched Asteroid Chariklo pass in front of a star. The asteroid made the background star “blink” several times, which led to the discovery that two rings are surrounding the asteroid.
The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars. Credit: Wikipedia Commons
Location
The majority of known asteroids are in the asteroid belt, a large donut-shaped ring located between the orbits of Mars and Jupiter, and orbit approximately 2 to 4 AU (186 million to 370 million miles/300 million to 600 million kilometers) from the Sun. (*Note: 1 AU, or Astronomical Unit, equals the distance from the Earth to the Sun.)
Sometimes, the orbits of some asteroids get perturbed or altered from gravitational interactions with planets or other asteroids and they end up coming closer to the Sun, and therefore closer to Earth. These asteroids are known as Near Earth Asteroids, and are classified as NEAs if their orbits bring them within 1.3 AU (121 million miles/195 million kilometers) of the Earth.
Asteroids that actually cross Earth’s orbital path are known as Earth-crossers and, an asteroid is called a Potentially Hazardous Asteroid (PHA) if it will come less than .05 AU from Earth.
In addition to the asteroid belt, however, there have been recent discussions among astronomers about the potential existence of large number asteroids in the far reaches of our Solar System in the Kuiper Belt and Oort Cloud.
Number of Asteroids
There are millions of asteroids in our Solar System. Some scientists estimate the asteroid belt has between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Most of the undiscovered asteroids are likely the smaller ones (less than 100 km across) which are more difficult to detect. Other astronomers estimate there are over 150 million asteroids in the entire Solar System. New asteroids are being discovered all the time.
On average, three new NEAs are found every day. As of September 06, 2015, 13,024 Near-Earth objects have been discovered. About 875 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger. Also, 1,609 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs), but none at this time are expected to impact Earth. Check the NASA NEO website for updates.
Contrary to popular imagery that might be seen in science fiction movies and imagery, the asteroid belt is mostly empty. According to NASA, the average distance between objects in the asteroid belt is greater than 1-3 million km. The asteroids are spread over such a large volume that you likely would not run into an asteroid if you sent a spacecraft through the asteroid belt. Even though there may be millions of asteroids in the asteroid belt, most are small. Astronomers say if you put all of them together, the combination would be smaller than our moon.
Asteroids are not easy to spot because they often are made from dark material, and are difficult to find against the darkness of outer space. There are several dedicated surveys using both Earth-based telescopes and spacecraft searching the skies for asteroids. They include:
Catalina Sky Survey
Pan-STARRS
LINEAR
Spacewatch
NEOWISE
You can find more information about NASA’s NEO Search Program here.
Ceres compared to asteroids visited to date, including Vesta, Dawn’s mapping target in 2011. Image by NASA/ESA/JAXA. Compiled by Paul Schenck.
Shape and Size
Most asteroids are irregularly shaped, though some are nearly spherical, and they are often pitted or cratered from impacts with other asteroids. As they revolve around the Sun in elliptical orbits, the asteroids also rotate, and have some quite erratic movements, and literally tumble through space.
The size of what classifies as an asteroid is not extremely well defined, as an asteroid can range from a pebbles, to a few meters wide – like a boulder — to objects that are hundreds of kilometers in diameter. The largest asteroid is asteroid Ceres at about 952 km (592 miles) in diameter, and Ceres is so large that it is also categorized as a dwarf planet. Over 200 asteroids are known to be larger than 100 km (60 miles), with sixteen asteroids known to have diameters of 240 kilometers (150 miles) or greater.
Asteroids we’ve seen up close show various shapes. Credit: NASA
The following animation is based on a 2008 a study of the size distribution of asteroid families using data from the Sloan Digital Sky Survey and was created by Alex Parker.
Composition
Most asteroids are made of rock — with some composed of clay and silicate — and different metals, mostly nickel and iron. Other precious metals have been found on some asteroids, including platinum and gold. A wide variety of minerals have also been found on various asteroids including olivine and pyroxene, which are also found on meteorites that have landed on Earth.
Most asteroids contain vast amounts of carbon, which means they closely follow the elemental composition of the Sun. There are indications that asteroids also contain water or ice in their interiors, and observations by the Dawn mission shows indications that water may have flowed across the surface of Vesta.
You can find more details about what asteroids are made of at our article here.
Asteroids are different from comets, which are mostly rock and ice. Comets usually have tails, which are made from ice and debris sublimating as the comet gets close to the Sun. Asteroids typically don’t have tails, even those near the Sun. But recently, astronomers have seen some asteroids that have sprouted tails, such as asteroid P/2010 A2. This seems to happen when the asteroid has been hit or pummeled by other asteroids and dust or gas is ejected from their surfaces, creating a sporadic tail effect. These so-called “active asteroids” are a newly recognized phenomenon, and as of this writing, only 13 known active asteroids have been found in the main asteroid belt, and so they are very rare.
Asteroid classifications
Asteroids have a few different classifications based on their location and make-up.
Location classifications are:
Main Belt Asteroids: (which includes the majority of known asteroids which orbit within the asteroid belt between Mars and Jupiter)
Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan’s tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.
Near-Earth Asteroids: These objects have orbits that pass close by that of Earth.
Then, there are subgroups of Near-Earth asteroids, and are categorized by their orbits.
Atiras are NEAs whose orbits are contained entirely with the orbit of the Earth, having a distance of less than 1 AU. They are named after asteroid 163693 Atira.
Atens are Earth-crossing NEAs with semi-major axes smaller than Earth’s, with a distance of less than 1 AU. They are named after asteroid 2062 Aten.
Apollos are Earth-crossing NEAs with semi-major axes larger than Earth’s, with a distance of less than 1 AU. They are named after asteroid 1862 Apollo.
Amors are Earth-approaching NEAs with orbits outside of Earth’s but inside of Mars’ orbit. They are named after asteroid 1221 Amor.
Classification by the composition tell us what the asteroid is made of, and this is related to how far from the Sun an asteroid formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. Only one such asteroid, Vesta, survives to this day. There are three basic types of asteroids:
C-type (chondrite) asteroids are most common, making up about 75 percent of known asteroids. They are very dark in appearance and probably consist of clay and silicate rocks. They are among the most ancient objects in the solar system. Their composition is thought to be similar to the Sun, but depleted in hydrogen, helium, and other volatiles. C-type asteroids mainly are in the asteroid belt’s outer regions.
S-types (stony) are made up of silicate materials and nickel-iron, and accounts for about 17 percent of known asteroids. They are brighter than C-type and they dominate the inner asteroid belt.
M-types (metallic) are made from nickel and iron and accounts for about 8 percent of known asteroids. They are brighter than C-type and they can be found in the asteroid belt’s middle region.
Asteroid Impacts with Earth
How likely is it that our planet could be hit by a large asteroid or comet? We do know that Earth and the Moon have been struck many times in the past by asteroids whose orbits bring them into the inner Solar System. You can see pictures some of Earth’s largest and most spectacular impact craters here.
Studies of Earth’s history indicate that about once every 5,000 years or so (on average) an object the size of a football field hits Earth and causes significant damage. Once every few million years on average an object large enough to cause regional or global disaster impacts Earth.
Satellite views of the Chicxulub impact site. Image credit: NASA/JPL
There is strong scientific evidence that asteroid impacts played a major role in the mass extinctions documented in Earth’s fossil records. It is widely accepted that an impact 65 million years ago of an asteroid or comet at least 6 miles (10 kilometers) in diameter in the Yucatan peninsula, known as the Chicxulub crater is associated with the extinction of the dinosaurs.
We know of only a handful of recent large asteroid impacts. One is the forest-flattening 1908 Tunguska explosion over Siberia (which may have been the result of a comet) and another is the February 2013 meteor that exploded over Chelyabinsk, breaking windows and injuring many, mostly from broken glass.
But a recent study by the B612 Foundation found that there were 26 explosive airburst events similar to the Chelyabinsk event recorded from 2000 to 2013. The explosions asteroids ranged from one to 600 kilotons in energy output.
NASA says that about once a year, an automobile-sized asteroid hits Earth’s atmosphere, creates an impressive fireball, and burns up before reaching the surface.
NEOs still pose a danger to Earth today, but NASA, ESA and other space agencies have search programs that have discovered hundreds of thousands of main-belt asteroids, comets. None at this time pose any threat to Earth. You can find out more on this topic at NASA’s Near Earth Object Program website.
All asteroids and comets visited by spacecraft as of November 2010 Credits: Montage by Emily Lakdawalla. Ida, Dactyl, Braille, Annefrank, Gaspra, Borrelly: NASA / JPL / Ted Stryk. Steins: ESA / OSIRIS team. Eros: NASA / JHUAPL. Itokawa: ISAS / JAXA / Emily Lakdawalla. Mathilde: NASA / JHUAPL / Ted Stryk. Lutetia: ESA / OSIRIS team / Emily Lakdawalla. Halley: Russian Academy of Sciences / Ted Stryk. Tempel 1, Hartley 2: NASA / JPL / UMD. Wild 2: NASA / JPL.
History
We’ve gained knowledge of asteroids from three main sources: Earth-based remote sensing, data from spacecraft and laboratory analysis of meteorites.
Here are some important dates in the history of our knowledge and study of asteroids, including spacecraft missions that flew by or landed on asteroids:
1801: Giuseppe Piazzi discovers the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. 1898: Gustav Witt discovers Eros, one of the largest near-Earth asteroids. 1991-1994: The Galileo spacecraft takes the first close-up images of an asteroid (Gaspra) and discovers the first moon (later named Dactyl) orbiting an asteroid (Ida). 1997-2000: The NEAR Shoemaker spacecraft flies by Mathilde and orbits and lands on Eros. 1998: NASA establishes the Near Earth Object Program Office to detect, track and characterize potentially hazardous asteroids and comets that could approach Earth. 2006: Japan’s Hayabusa becomes the first spacecraft to land on, collect samples and take off from an asteroid. 2006: Ceres attains a new classification — dwarf planet — but retains its distinction as the largest known asteroid. 2007: The Dawn spacecraft is launched on its journey to the asteroid belt to study Vesta and Ceres. 2008: The European spacecraft Rosetta, on its way to study a comet in 2014, flies by and photographs asteroid Steins, a type of asteroid composed of silicates and basalts. 2010: Japan’s Hayabusa returns its asteroid sample to Earth. 2010: Rosetta flies by asteroid Lutetia, revealing a primitive survivor from the violent birth of our solar system. 2011-2015: Dawn studies Vesta, becoming the first spacecraft to orbit a main-belt asteroid. It now is studying the dwarf planet Ceres, located in the main asteroid belt.
Below is a list of links to articles about asteroids in general, asteroid related events in history, and some specific asteroids. Many hours of research are waiting for you. Enjoy!
In the 18th century, observations made of all the known planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) led astronomers to discern a pattern in their orbits. Eventually, this led to the Titius–Bode Law, which predicted the amount of space between the planets. In accordance with this law, there appeared to be a discernible gap between the orbits of Mars and Jupiter, and investigation into it led to a major discovery.
In addition to several larger objects being observed, astronomers began to notice countless smaller bodies also orbiting between Mars and Jupiter. This led to the creation of the term “asteroid”, as well as “Asteroid Belt” once it became clear just how many there were. Since that time, the term has entered common usage and become a mainstay of our astronomical models.
Discovery:
In 1800, hoping to resolve the issue created by the Titius-Bode Law, astronomer Baron Franz Xaver von Zach recruited 24 of his fellow astronomers into a club known as the “United Astronomical Society” (sometimes referred to the as “Stellar Police”). At the time, its ranks included famed astronomer William Herschel, who had discovered Uranus and its moons in the 1780s.
Ironically, the first astronomer to make a discovery in this regions was Giuseppe Piazzi – the chair of astronomy at the University of Palermo – who had been asked to join the Society but had not yet received the invitation. On January 1st, 1801, Piazzi observed a tiny object in an orbit with the exact radius predicted by the Titius-Bode law.
Ceres (left, Dawn image) compared to Tethys (right, Cassini image) at comparative scale sizes. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and NASA/JPL-Caltech/SSI. Comparison by J. Major.
Initially, he believed it to be a comet, but ongoing observations showed that it had no coma. This led Piazzi to consider that the object he had found – which he named “Ceres” after the Roman goddess of the harvest and patron of Sicily – could, in fact, be a planet. Fifteen months later, Heinrich Olbers ( a member of the Society) discovered a second object in the same region, which was later named 2 Pallas.
In appearance, these objects seemed indistinguishable from stars. Even under the highest telescope magnifications, they did not resolve into discs. However, their rapid movement was indicative of a shared orbit. Hence, William Herschel suggested that they be placed into a separate category called “asteroids” – Greek for “star-like”.
By 1807, further investigation revealed two new objects in the region, 3 Juno and 4 Vesta; and by 1845, 5 Astraea was found. Shortly thereafter, new objects were found at an accelerating rate, and by the early 1850s, the term “asteroids” gradually came into common use. So too did the term “Asteroid Belt”, though it is unclear who coined that particular term. However, the term “Main Belt” is often used to distinguish it from the Kuiper Belt.
One hundred asteroids had been located by mid-1868, and in 1891 the introduction of astrophotography by Max Wolf accelerated the rate of discovery even further. A total of 1,000 asteroids were found by 1921, 10,000 by 1981, and 100,000 by 2000. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.
The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars. Credit: Wikipedia Commons
Structure:
Despite common perceptions, the Asteroid Belt is mostly empty space, with the asteroids spread over a large volume of space. Nevertheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more. Over 200 asteroids are known to be larger than 100 km in diameter, and a survey in the infrared wavelengths has shown that the asteroid belt has 0.7–1.7 million asteroids with a diameter of 1 km (0.6 mi) or more.
Located between Mars and Jupiter, the belt ranges from 2.2 to 3.2 astronomical units (AU) from the Sun and is 1 AU thick. Its total mass is estimated to be 2.8×1021 to 3.2×1021 kilograms – which is equivalent to about 4% of the Moon’s mass. The four largest objects – Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea – account for half of the belt’s total mass, with almost one-third accounted for by Ceres alone.
The main (or core) population of the asteroid belt is sometimes divided into three zones, which are based on what is known as Kirkwood Gaps. Named after Daniel Kirkwood, who announced in 1866 the discovery of gaps in the distance of asteroids, these describe the dimensions of an asteroid’s orbit based on its semi-major axis.
Within this scheme, there are three zones. Zone I lies between the 4:1 resonance and 3:1 resonance Kirkwood gaps, which are 2.06 and 2.5 AU from the Sun respectively. Zone II continues from the end of Zone I out to the 5:2 resonance gap, which is 2.82 AU from the Sun. Zone III extends from the outer edge of Zone II to the 2:1 resonance gap at 3.28 AU.
The asteroid belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 AU), and the outer belt formed by those asteroids closer to Jupiter’s orbit.
The asteroids that have a radius of 2.06 AU from the Sun can be considered the inner boundary of the asteroid belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System.
The temperature of the Asteroid Belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73 °C) at 2.2 AU down to 165 K (-108 °C) at 3.2 AU. However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.
Composition:
Much like the terrestrial planets, most asteroids are composed of silicate rock while a small portion contains metals such as iron and nickel. The remaining asteroids are made up of a mix of these, along with carbon-rich materials. Some of the more distant asteroids tend to contain more ices and volatiles, which includes water ice.
Vesta seen from the Earth-orbit based Hubble Space Telescope in 2007 (left) and up close with the Dawn spacecraft in 2011. Hubble Credit: NASA, ESA, and L. McFadden (University of Maryland). Dawn Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Photo Combination: Elizabeth Howell
The Main Belt consists primarily of three categories of asteroids: C-type, or carbonaceous asteroids; S-type, or silicate asteroids; and M-type, or metallic asteroids. Carbonaceous asteroids are carbon-rich, dominate the belt’s outer regions, and comprise over 75% of the visible asteroids. Their surface composition is similar to that of carbonaceous chondrite meteorites while their spectra is similar to what the early Solar System’s is believed to be.
S-type (silicate-rich) asteroids are more common toward the inner region of the belt, within 2.5 AU of the Sun. These are typically composed of silicates and some metals, but not a significant amount of carbonaceous compounds. This indicates that their materials have been modified significantly over time, most likely through melting and reformation.
M-type (metal-rich) asteroids form about 10% of the total population and are composed of iron-nickel and some silicate compounds. Some are believed to have originated from the metallic cores of differentiated asteroids, which were then fragmented from collisions. Within the asteroid belt, the distribution of these types of asteroids peaks at a semi-major axis of about 2.7 AU from the Sun.
There’s also the mysterious and relatively rare V-type (or basaltic) asteroids. This group takes their name from the fact that until 2001, most basaltic bodies in the Asteroid Belt were believed to have originated from the asteroid Vesta. However, the discovery of basaltic asteroids with different chemical compositions suggests a different origin. Current theories of asteroid formation predict that the V-type asteroids should be more plentiful, but 99% of those that have been predicted are currently missing.
Families and Groups:
Approximately one-third of the asteroids in the asteroid belt are members of an asteroid family. These are based on similarities in orbital elements – such as semi-major axis, eccentricity, orbital inclinations, and similar spectral features, all of which indicate a common origin. Most likely, this would have involved collisions between larger objects (with a mean radius of ~10 km) that then broke up into smaller bodies.
This artist’s conception shows how families of asteroids are created. Credit: NASA/JPL-Caltech
Some of the most prominent families in the asteroid belt are the Flora, Eunomia, Koronis, Eos, and Themis families. The Flora family, one of the largest with more than 800 known members, may have formed from a collision less than a billion years ago. Located within the inner region of the Belt, this family is made up of S-type asteroids and accounts for roughly 4-5% of all Belt objects.
The Eunomia family is another large grouping of S-type asteroids, which takes its name from the Greek goddess Eunomia (goddess of law and good order). It is the most prominent family in the intermediate asteroid belt and accounts for 5% of all asteroids.
The Koronis family consists of 300 known asteroids which are thought to have been formed at least two billion years ago by a collision. The largest known, 208 Lacrimosa, is about 41 km (25 mi) in diameter, while an additional 20 more have been found that are larger than 25 km in diameter.
The Eos (or Eoan) family is a prominent family of asteroids that orbit the Sun at a distance of 2.96 – 3.03 AUs, and are believed to have formed from a collision 1-2 billion years ago. It consists of 4,400 known members that resemble the S-type asteroid category. However, the examination of Eos and other family members in the infrared show some differences with the S-type, thus why they have their own category (K-type asteroids).
Asteroids we’ve seen up close show cratered surfaces similar to yet different from much of the cratering on comets. Credit: NASA
The Themis asteroid family is found in the outer portion of the asteroid belt, at a mean distance of 3.13 AU from the Sun. This core group includes the asteroid 24 Themis (for which it is named) and is one of the more populous asteroid families. It is made up of C-type asteroids with a composition believed to be similar to that of carbonaceous chondrites and consists of a well-defined core of larger asteroids and a surrounding region of smaller ones.
The largest asteroid to be a true member of a family is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise, the HED meteorites may also have originated from Vesta as a result of this collision.
Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to a few hundred micrometers. This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Three prominent bands of dust have been found within the asteroid belt – which have similar orbital inclinations as the Eos, Koronis, and Themis asteroid families – and so are possibly associated with those groupings.
Origin:
Originally, the Asteroid Belt was thought to be the remnants of a much larger planet that occupied the region between the orbits of Mars and Jupiter. This theory was originally suggested by Heinrich Olbders to William Herschel as a possible explanation for the existence of Ceres and Pallas. However, this hypothesis has since fallen out of favor for a number of reasons.
Artist’s impression of the early Solar System, where collisions between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech
First, there is the amount of energy it would have required to destroy a planet, which would have been staggering. Second, there is the fact that the entire mass of the Belt is only 4% that of the Moon. Third, the significant chemical differences between the asteroids do not point towards them having been once part of a single planet.
Today, the scientific consensus is that, rather than fragmenting from a progenitor planet, the asteroids are remnants from the early Solar System that never formed a planet at all. During the first few million years of the Solar System’s history, when gravitational accretion led to the formation of the planets, clumps of matter in an accretion disc coalesced to form planetesimals. These, in turn, came together to form planets.
However, within the region of the Asteroid Belt, planetesimals were too strongly perturbed by Jupiter’s gravity to form a planet. These objects would continue to orbit the Sun as before, occasionally colliding and producing smaller fragments and dust.
During the early history of the Solar System, the asteroids also melted to some degree, allowing elements within them to be partially or completely differentiated by mass. However, this period would have been necessarily brief due to their relatively small size, and likely ended about 4.5 billion years ago, in the first tens of millions of years of the Solar System’s formation.
Though they are dated to the early history of the Solar System, the asteroids (as they are today) are not samples of its primordial self. They have undergone considerable evolution since their formation, including internal heating, surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites. Hence, the Asteroid Belt today is believed to contain only a small fraction of the mass of the primordial belt.
Computer simulations suggest that the original asteroid belt may have contained as much mass as Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt a million years after its formation, leaving behind less than 0.1% of the original mass. Since then, the size distribution of the asteroid belt is believed to have remained relatively stable.
When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a “snow line” below the freezing point of water. Essentially, planetesimals formed beyond this radius were able to accumulate ice, some of which may have provided a water source of Earth’s oceans (even more so than comets).
Exploration:
The asteroid belt is so thinly populated that several unmanned spacecraft have been able to move through it; either as part of a long-range mission to the outer Solar System, or (in recent years) as a mission to study larger Asteroid Belt objects. In fact, due to the low density of materials within the Belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.
Artist’s concept of the Dawn spacecraft arriving at Vesta. Image credit: NASA/JPL-Caltech
The first spacecraft to make a journey through the asteroid belt was the Pioneer 10 spacecraft, which entered the region on July 16th, 1972. As part of a mission to Jupiter, the craft successfully navigated through the Belt and conducted a flyby of Jupiter (which culminated in December of 1973) before becoming the first spacecraft to achieve escape velocity from the Solar System.
For the most part, these missions were part of missions to the outer Solar System, where opportunities to photograph and study asteroids were brief. Only the Dawn, NEAR and JAXA’s Hayabusamissions have studied asteroids for a protracted period in orbit and at the surface. Dawn explored Vesta from July 2011 to September 2012 and is currently orbiting Ceres (and sending back many interesting pictures of its surface features).
And someday, if all goes well, humanity might even be in a position to begin mining the asteroid belt for resources – such as precious metals, minerals, and volatiles. These resources could be mined from an asteroid and then used in space of in-situ utilization (i.e. turning them into construction materials and rocket propellant), or brought back to Earth.
It is even possible that humanity might one day colonize larger asteroids and establish outposts throughout the Belt. In the meantime, there’s still plenty of exploring left to do, and quite possibly millions of more objects out there to study.
