Illustration of Comet 67P/C-G brought down to Earth in the city of Los Angeles, Calif. Compare to the same image (below) as viewed in space. Credit: ESA and anosmicovni
All the pictures we’ve seen of Rosetta’s target comet 67P/C-G show it reflecting brightly against the background of outer space. And well they should. Space is black as night. But if we were to see the comet against a more familiar earthly backdrop, we’d be shocked by its appearance. Instead of icy white, Rosetta’s would appear the color of a fresh asphalt parking lot. Most comets, including Rosetta’s, are no brighter than the charcoal briquettes you use to grill hamburgers.
Astronomers rank an object’s reflectivity by its albedo (al-BEE-do). A body that reflects 100% of the light is said to have an albedo of 1.0. Venus’ albedo is .75 and reflects 75% of the light it receives from the sun, while the darker Earth’s average is 30%. Trees and the darker-toned continents reflect much less light compared to Venus’ pervasive cloud cover. In contrast, the coal-dark moon reflects only 12% of the sunlight falling on it and fresh asphalt just 4% – smack in the middle of the 2-6% rangeof most known comets.
Photo of Comet 67P/C-G taken by Rosetta on August 6, 2014. Against the blackness of space, it appears whitish-grey. Credit: ESA
The brightest object in the solar system is Saturn’s icy moon Enceladus with a reflectivity of 99%. So why are comets so dark? It’s funny because before we sent the Giotto spacecraft to snap close-up pictures of Halley’s Comet in 1986, astronomers thought comets, being made of reflective ice, were naturally white. Not Halley and not every comet seen up close since then.
Comets are as dark as charcoal but appear light only because the sun illuminates them against the blackness of outer space. I shone a flashlight on a charcoal briquette (left) to simulate comet lighting. The same charcoal, when viewed in normal light, appears black. Credit; Bob King
Astronomers hypothesize that a comet grows a dark ‘skin’ both from accumulated dust and irradiation of its pristine ices by cosmic rays. Cosmic rays loosen oxygen atoms from water ice, freeing them to combine with simple carbon molecules present on comets to form larger, more complex and darker compounds resembling tars and crude oil.
Comet colored parking lots have been the rage for years. Both comets and fresh asphalt reflect about the same amount of light. Credit: Bob King
Over time, the comet can become insulated by dust and complex organic materials. Combined with a loss of ice to vaporization at each repeated swing past the sun, they stop outgassing and become inert or defunct comets similar to asteroids. And that might not be the end of the story. Occasionally, a dead comet or an object originally discovered as an asteroid can unexpectedly fire back up after years of inactivity and become a comet again temporarily. Astronomers call these peculiar critters ‘damocloids’.
One wonders what you’d see if you could slice open a 67P/Churyumov-Gerasimenko. Would it resemble an Oreo cookie with a dark exterior and creamy white inside? One of NASA’s instruments aboard Rosetta named Alice began mapping the comet last month. In its first far ultraviolet spectra of the surface, we learned just this week that 67P is “darker than charcoal black”. Alice also detected hydrogen and oxygen in the comet’s coma, or atmosphere.
Oreo cookies – a model of a comet nucleus? Credit: Evan-Amos
Rosetta scientists also discovered the comet’s surface so far shows no large water-ice patches. The team expected to see ice patches on the comet’s surface because it’s too far away for the sun’s warmth to turn its water into vapor.
“We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows,” said Alan Stern, Alice principal investigator at the Southwest Research Institute in Boulder, Colorado.
The Sun. Credit: NASA & European Space Agency (ESA)
Approximately every 11 years the Sun becomes violently active, putting on a show of magnetic activity for aurora watchers and sungazers alike. But the timing of the solar cycle is far from precise, making it hard to determine the exact underlying physics.
Typically astronomers use sunspots to map the course of the solar cycle, but now an international team of astronomers have discovered a new marker: brightpoints, small bright spots in the solar atmosphere that allow us to observe the constant turmoil of material inside the Sun.
The new markers provide a new method in understanding how the Sun’s magnetic field evolves over time, suggesting a deeper and longer cycle.
A well-behaved Sun flips its north and south magnetic poles every 11 years. The cycle begins when the field is weak and dipolar. But the Sun’s rotation is faster at its equator than at its poles, and this difference stretches and tangles the magnetic field lines, ultimately producing sunspots, prominences, and sometimes flares.
“Sunspots have been the perennial marker for understanding the mechanisms that rule the sun’s interior,” said lead author Scott McIntosh, from the National Center for Atmospheric Research, in a news release. “But the processes that make sunspots are not well understood, and far less, those that govern their migration and what drives their movement.”
So McIntosh and colleagues developed a new tracking devise: spots of extreme ultraviolet and X-ray light, known as brightpoints in the Sun’s atmosphere, or corona.
“Now we can see there are bright points in the solar atmosphere, which act like buoys anchored to what’s going on much deeper down,” said McIntosh. “They help us develop a different picture of the interior of the sun.”
McIntosh and colleagues dug through the wealth of data available from the Solar and Heliospheric Observatory and the Solar Dynamics Observatory. They noticed that multiple bands of these markers also move steadily toward the equator over time. But they do so on a different timescale than sunspots.
At solar minimum there might be two bands in the northern hemisphere (one positive and one negative) and two bands in the southern hemisphere (one negative and one positive). Due to their close proximity, bands of opposite charge easily cancel one another, causing the Sun’s magnetic system to be calmer, producing fewer sunspots and eruptions.
But once the two low-latitude bands reach the equator, their polarities cancel each other out and the bands abruptly disappear — a process that takes 19 years on average.
The Sun is now left with just two large bands that have migrated to about 30 degrees latitude. Without the nearby band, the polarities don’t cancel. At this point the Sun’s calm face begins to become violently active as sunspots start to grow rapidly.
Solar maximum only lasts so long, however, because the process of generating a new band of opposite polarity has already begun at high latitudes.
In this scenario, it is the magnetic band’s cycle that truly defines the solar cycle. “Thus, the 11-year solar cycle can be viewed as the overlap between two much longer cycles,” said coauthor Robert Leamon, from Montana State University in Bozeman.
The true test, however, will come with the next solar cycle. McIntosh and colleagues predict that the Sun will enter a solar minimum somewhere in the last half of 2017, and the first sunspots of the next cycle will appear near the end of 2019.
The findings have been published in the Sept. 1 issue of the Astrophysical Journal and are available online.
Sprite with Airglow and Gravity Waves over South Dakota. Credit and copyright: Randy Halverson.
Look! Fast! Sprite lightning occurs only at high altitudes above thunderstorms, only last for a thousandth of a second and emit light in the red portion of the visible spectrum, so they are really difficult to see. But one of our favorite astrophotographers and timelapse artists, Randy Halverson captured sprites during a recent thunderstorm in South Dakota. But wait, there’s more!
In his timelapse video, above, you’ll also see some faint aurora as well as green airglow being rippled by gravity waves.
See some imagery from the storm, below:
More sprites with airglow and gravity waves over South Dakota on August 20, 2014. Credit and copyright: Randy Halverson.
See more images and information about Randy’s fun night of observing these phenomena on his website, dakotalapse.
NASA's Mars rover Curiosity took this self-portrait, composed of more than 50 images using its robotic arm-mounted MAHLI camera, on Feb. 3, 2013. The image shows Curiosity at the John Klein drill site. A drill hole is visible at bottom left. Credit: NASA / JPL / MSSS / Marco Di Lorenzo / Ken Kremer- kenkremer.com
NASA’s planetary senior review panel harshly criticized the scientific return of the Curiosity rover in a report released yesterday (Sept. 3), saying the mission lacks focus and the team is taking actions that show they think the $2.5-billion mission is “too big to fail.”
While the review did recommend the mission receive more funding — along with the other six NASA extended planetary missions being scrutinized — members recommended making several changes to the mission. One of them would be reducing the distance that Curiosity drives in favor of doing more detailed investigations when it stops.
The role of the senior review, which is held every two years, is to help NASA decide what money should be allocated to its extended missions. This is important, because the agency (as with many other departments) has limited funds and tries to seek a balance between spending money on new missions and keeping older ones going strong.
Engineering acumen means that many missions are now operating well past their expiry dates, such as the Cassini orbiter at Saturn and the Opportunity rover on Mars. In examining the seven missions being reviewed, the panel did recommend keeping funding for all, but said that 4/7 are facing significant problems.
Opportunity rover’s 1st mountain climbing goal is dead ahead in this up close view of Solander Point at Endeavour Crater. Opportunity has ascended the mountain looking for clues indicative of a Martian habitable environment. This navcam panoramic mosaic was assembled from raw images taken on Sol 3385 (Aug 2, 2013). Credit: NASA/JPL/Cornell/Marco Di Lorenzo/Ken Kremer (kenkremer.com)
In the case of Curiosity, the panel called out principal investigator John Grotzinger for not showing up in person on two occasions, preferring instead to interact by phone. The review also said there is a “lack of science” in its extended mission proposal with regard to “scientific questions to be answered, testable hypotheses, and proposed measurements and assessment of uncertainties and limitations.”
Other concerns were the small number of samples over the prime and extended missions (13, a “poor science return”), and a lack of clarity on how the ChemCam and Mastcam instruments will play into the extended mission. Additionally, the panel expressed concern that NASA would cut short its observations of clays (which could help answer questions of habitability) in favor of heading to Mount Sharp, the mission’s ultimate science destination.
“In summary, the Curiosity … proposal lacked scientific focus and detail,” the panel concluded, adding in its general recommendations for the reviews that principal investigators must be present to avoid confusion while answering questions. The other missions facing concern from the panel included the Lunar Reconnaissance Orbiter, Mars Express and Mars Odyssey.
Lunar Reconnaissance Orbiter. Image Credit: NASA
LRO: Its extended mission (the second) is supposed to look at how the moon’s surface, subsurface and exosphere changes through processes such as meteorites and interaction with space. The panel was concerned with a “lack of detail” in the proposal and in answers to follow-up questions. The panel also recommended turning off certain instruments “at the end of their useful science mission”.
Mars Express: The extended mission is focusing on the ionosphere and atmosphere as well as the planet’s surface and subsurface. Concerns were raised about matters such as why funding is needed to calibrate its high-resolution stereo camera after 11 years — especially given the instrument has been rarely cited in published journal reports lately — and how people involved in the extended mission would meet the goals. The panel also saw a “lack of communication” in the team.
Mars Odyssey: If approved, the spacecraft will move to the day/night line of Mars to look at the planet’s radiation, gamma rays, distribution of water/carbon dioxide/dust in the atmosphere, and the planet’s surface. The panel, however, said there are no “convincing arguments” as to how the new science relates to the Decadal Survey objectives for planetary science. Odyssey, which is in its 11th year, may also be nearing the end of its productive lifespan given fewer publications using its data in recent years, the panel said.
The panel also weighed in on the success of the Cassini and Opportunity missions:
Artist’s conception of the Mars Odyssey spacecraft. Credit: NASA/JPL
Cassini received the highest rating — “Excellent” — due to its scientific merit, the only mission this time around to do so. The panel was particularly excited about seasonal changes that will be seen on Titan in the coming years, as well as measurements of Saturn’s rings and magnetosphere and its icier moons (such as Enceladus). The spacecraft is noted to be in good condition and the new mission will be a success because of “the unique aspect of the new observations.”
Opportunity, which is more than 10 years into its Mars exploration, is still “in sufficiently good condition” to do science, although the panel raised concerns about software and communication problems. The panel, however, said more time with the rover would allow it to look for evidence of past water on Mars that would not be visible from orbit — even though it’s unclear if phyllosilicates around its current location (Endeavour crater) are from the Noachian period, the earliest period in Mars’ history.
The panel is just one step along the road to figuring out how NASA chooses to spend its money in the coming years. Funding availability depends on how much money Congress allocates to the agency.
An illustration of a Titanic lake by Ron Miller. All rights reserved. Used with permission.
Titan — that moon of Saturn that has what some scientists consider precursors to elements for life — is a neat place to study because it also has a liquid cycle. But how the hydrocarbons move from the moon’s hundreds of lakes and seas into the atmosphere and the crust is still being examined.
A new study suggests that rainfall on Titan changes when it interacts with underground icy clathrates, which are watery structures that can include methane or ethane. This can make it easier for reservoirs to be created.
“We knew that a significant fraction of the lakes on Titan’s surface might possibly be connected with hidden bodies of liquid beneath Titan’s crust, but we just didn’t know how they would interact,” stated lead author Olivier Mousis, a Cassini research associate at the University of Franche-Comté in France. “Now, we have a better idea of what these hidden lakes or oceans could be like.”
Artist’s conception of a possible structure for underground liquid reservoirs on Saturn moon’s Titan. From top, the layers include a porous icy crust, alkanofer in porous icy crust, expanding clathrate layer in porous icy crust and a non-porous icy crust. Credit: ESA/ATG medialab
This information is based on models of how the reservoirs would move through the crust of the icy moon. Clathrates would form at the bottom of reservoirs (which are filled with methane) and gradually split its molecules into solid and liquid components. Over time, this would transform the methane into propane or ethane.
“Importantly, the chemical transformations taking place underground would affect Titan’s surface,” the Jet Propulsion Laboratory stated.
“Lakes and rivers fed by springs from propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain a significant fraction of methane. This means researchers could examine the composition of Titan’s surface lakes to learn something about what is happening deep underground.”
More about the research is available in the print version of the Sept. 1 edition of Icarus. Of note, the Cassini spacecraft is going to do another flyby of Titan in 17 days — its 105th, according to the spacecraft website.
An artist's conception of a circumbinary planet. Credit: NASA/JPL-Caltech/T. Pyle
There are few environments more hostile than a planet circling two stars. Powerful tidal forces from the stars can easily destroy the rocky building blocks of planets or grind a newly formed planet to dust. But astronomers have spotted a handful of these hostile worlds.
A new study is even suggesting that these extreme systems exist in abundance, with roughly half of all exoplanets orbiting binary stars.
NASA’s crippled Kepler space telescope is arguably the world’s most successful planet hunter, despite the sudden end to its main mission last May. For nearly four years, Kepler continuously monitored 150,000 stars searching for tiny dips in their light when planets crossed in front of them.
As of today, astronomers have confirmed nearly 1,500 exoplanets using Kepler data alone. But Kepler’s database is immense. And according to the exoplanet archive there are over 7,000 “Kepler Objects of Interest,” dubbed KOIs, that might also be exoplanets.
There are a seeming endless number of questions waiting to be answered. But one stands out: how many exoplanets circle two stars? Binary stars have long been known to be commonplace — about half of the stars in the Milky Way are thought to exist in binary systems.
A team of astronomers, led by Elliott Horch from Southern Connecticut State University, has shown that stars with exoplanets are just as likely to have a binary companion. In other words, 40 to 50 percent of the host stars are actually binary stars.
“It’s interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago,” said Horch in a news release.
The research team made use of the latest technology, speckle imaging, to take a second look at KOI stars and search for any companion stars. In using this technique, astronomers obtain rapid images of a small portion of the sky surrounding the star. They then combine the images using a complex set of algorithms, which yields a final picture with a resolution better than the Hubble Space Telescope.
Speckle imaging allows astronomers to detect companion stars that are up to 125 times fainter than the target, but only a small distance away (36,000 times smaller than the full Moon). For the majority of Kepler stars, this equates to finding a companion within 100 times the distance from the Sun to the Earth.
The team was surprised to find that roughly half of their targets had companion stars.
“An interesting consequence of this finding is that in the half of the exoplanet host stars that are binary we can not, in general, say which star in the system the planet actually orbits,” said coauthor Steve B. Howell from the NASA Ames Research Center.
The new findings, soon to be published in the Astrophysical Journal, further advance our need to understand these exotic systems and the harrowing environments they face.
This graphic depicts the passage of asteroid 2014 RC past Earth on September 7, 2014. At time of closest approach, the space rock will be about one-tenth the distance from Earth to the moon. Times indicated on the graphic are Universal Time. Subtract 4 hours for Eastern Daylight Time. Credit: NASA/JPL-Caltech
Guess who’s dropping by for a quick visit this weekend? On Sunday, a 60-foot-wide (20-meters) asteroid named 2014 RC will skim just 25,000 miles (40,000 km) from Earth. That’s within spitting distance of all those geosynchronous communication and weather satellites orbiting at 22,300 miles.
Size-wise, this one’s similar to the Chelyabinsk meteorite that exploded over Russia’s Ural Mountains region in February 2013. But it’s a lot less scary. 2014 RC will cleanly miss Earth this time around, and although it’s expected back in the future, no threatening passes have been identified. Whew!
2014 RC will pass along the outer edge of the geosynchronous satellite belt, home to many weather and communications satellites. The chance of a hit is close to infinitesimal. Click for more information and detailed finder charts. Credit: SatFlare
NEOs or Near Earth Asteroids are defined as space rocks that come within about 28 million miles of Earth’s orbit. Nearly once a month astronomers discover an Earth-crossing asteroid that passes within the moon’s orbit. In spite of hype and hoopla, none has threatened the planet. As of February 2014, we know of 10,619 near-Earth asteroids. It’s estimated that 93% of all NEOs larger than 1 km have been discovered but 99% of the estimated 1 million NEOs 100 feet (30-meters) still remain at large.
No surprise then that new ones pop up routinely in sky surveys. Take this past Sunday night for example, when the Catalina Sky Survey nabbed 2014 RA, a 20-foot (6-meter) space rock that whistled past Earth that evening at 33,500 miles (54,000 km). It’s now long gone.
Artist view of an asteroid (with companion) passing near Earth. Credit: P. Carril / ESA
2014 RC was picked up on or about September 1-2 by both the Catalina Sky Survey and Pan-STARRS 1 survey telescope atop Mt. Haleakala in Maui. The details are still being worked out as to which group will take final discovery credit. Based on current calculations, 2014 RC will pass closest to Earth around 2:15 p.m. EDT (18:15 UT) on Sunday, September 7th. When nearest, the asteroid is expected to brighten to magnitude +11.5 – too dim for naked eye observing but visible with a good map in 6-inch and larger telescopes.
Seeing it will take careful planning. Unlike a star or planet, this space rock will be faint and barreling across the sky at a high rate of speed. Discovered at magnitude +19, 2014 RC will brighten to magnitude +14 during the early morning hours of September 7th. Even experienced amateurs with beefy telescopes will find it a challenging object in southern Aquarius both because of low altitude and the unwelcome presence of a nearly full moon.
64-frame movie showing Toutatis tumbling through space only 4.3 million miles from Earth on Dec. 12-13. Credit: NASA/Goldstone radar
Closest approach happens in daylight for North and South America , but southern hemisphere observers might spot it with a 6-inch scope as a magnitude +11.5 “star” zipping across the constellations Pictor and Puppis. 2014 RC fades rapidly after its swing by Earth and will quickly become impossible to see in all amateur telescopes, though time exposure photography will keep the interloper in view for a few additional hours.
2014 RC accelerates across the sky between 4 a.m. to 4 p.m EDT September 7 in this path created by Gianluca Masi using SkyX Pro software and the latest positions from JPL.
Most of us won’t have the opportunity to run outside and see the asteroid, but Gianluca Masi and his Virtual Telescope Project site will cover it live starting at 6 p.m. EDT (22:00 UT). Lance Benner, who researches radar imaging of near-Earth and main-belt asteroids, hopes to image 2014 RC with 230-foot (70-m) radar dish at the Goldstone complexon September 5-7 and possibly the big 1,000-foot (305-m) radar dish at Arecibo. Both provide images based on radar echoes that show asteroids up close with shapes, craters, ridges and all.
After a ten year journey, Rosetta and Philae will attempt the first soft landing upon a comet's surface. (Credits: ESA, Composite, T.Reyes)
ESA has announced that on September 15, the team from the Rosetta mission will reveal the landing site for the Philae lander. After traveling on a 10-year, 6.4 billion kilometer journey, Rosetta has been gently captured by comet 67P/Churyumov-Gerasimenko, an oddly-shaped and mysterious two-lobed comet. Yet, how will the small Philea attempt the landing? Very carefully, because a second chance is not possible. Philae cannot pull up and try again.
In contrast to NASA’s Deep Impact mission which directed a high speed impactor onto the surface of comet Tempel 1, ESA’s Philae lander is designed to execute the first soft landing. The landing must be as gentle as any landing that a respectable bird might accomplish. Philae’s nominal landing speed is about 1.0 meter/sec, that is, 2.2 mph. But like the Deep Impact impactor, Philae is flying solo. Software onboard will function alone without assistance from ground control.
The circumstances surrounding this momentous event – the first landing on a comet – has quite an amazing history and geography. Philae is truly a European Union mission with the design distributed across Europe, spanning from Hungary to Finland to Spain, Ireland to Italy and including UK and Germany.
As is common, the project development spanned several years. A sample return mission was considered the next step after ESA’s Giotto mission that studied Halley’s Comet, but Rosetta evolved out of the cancelled NASA mission Comet Rendezvous Asteroid Flyby (CRAF). ESA could not afford a sample return mission on its own, so Rosetta used the CRAF design but without sample return. Instead it would rendezvous and orbit a comet and include a lander.
Rosetta’s mission began on March 2, 2004 from the Guiana Space Centre in French Guiana and it now flies quietly alongside a comet 400 million kilometers from Earth. 67p is falling towards the Sun and perihelion will be on August 13, 2015.
Escape from Devil’s Island, 14 Km off the coast from the Guiana Space Centre is no longer through the jungles of South America. For Rosetta, it was straight up, then eastward and then finally into a Solar orbit to catch 67P/Churyumov-Gerasimenko. (Photo Credit: ESA)Illustration of Philae on a cometary surface. The actual surface of 67P/Churyumov-Gerasimenko is actually as dark as a barbecue briquette. (Credit: ESA)An illustration of the elements of the Philae landing dynamics. (Credit: “Simulation of the Landing of Rosetta Philae on Comet 67P”, M. Hilchenbach, et al., Max Planck Institute)
The technology of Philae is 1990s technology. However, the landing mechanisms may not be much different if designed today. Consider that the 7 minutes of terror – Entry, Descent and Landing of the Mars Rovers (MER) was also accomplished with 1990s computer hardware and you can express some relief and assurance that such technology is up to the task of landing on a comet.
Illustration and Photo of Philae Pendulum Tests. The force of impact on the wall simulates the force due to descent velocity, comet’s gravity and cold thrusters upon touchdown. (Credit: Simulation of the Landing of Rosetta Philae on Comet 67P, M. Hilchenbach, et al., Max Planck Institute)
How will the team make their choice of landing spots? The performance specifications of the lander and the mechanisms it can employ to attach to the surface sets definite constraints on the choice of landing location.
The landing mechanisms are: landing legs with ice screws, propulsion system and harpoons. The legs were designed with the intent of landing softly. The harpoons are designed to secure Philae to the surface. The gravity of the comet is so weak, Philae could bounce off the surface or roll over. The purpose of the harpoons — to be fired at the moment of contact — is to prevent bouncing off the surface or tipping over. The direction and strength of gravity at the landing site will not be absolutely known so there is the risk of roll over after landing, albeit very slowly. Tipping over is mitigated by screws under the footpads to penetrate the surface immediately after landing.
The Philae Lander anchoring harpoon with the integrated MUPUS-accelerometer and temperature sensor. (Credit: “Philae Lander Fact Sheet”, ESA)
Philae also has a flywheel for stabilization during descent and landing and a dampening system between the landing legs’ carriage and the probe’s body. The dampener is meant to make the landing inelastic — meaning no bouncing. However, there is a set limit to how much the probe’s body can tilt (or twist) upon surface contact. Any tilt will impose a rotating force on the probe which will need to be countered by the propulsion pushing down and the harpoons. Philae does not carry a stick of bubble gum or any duct tape, which have been known by Earthlings to come in handy in a pinch.
Clean Room photo of Philae with Principal Investigator Dr. Helmut Rosenbauer, Director at the Max-Planck-Institute for Aeronomy. Philae’s mass is 100 kg including 21 kg of instrument payload It’s dimensions are 1 × 1 × 0.8 meters (3.3 × 3.3 × 2.6 ft) Photo Credit: Max Planck Institute, Filser)
The Philae design was actually developed with a different comet in mind, 46P/Wirtanen, which is smaller (~.5 to 1 mile) than 67P. So the speed at landing on the surface was nominally 0.5 m/sec, however, now with the larger 67P/Churyumov–Gerasimenko, the landing speed could be 2 or 3 times greater. In December 2002, there was an Ariane 5 launch failure, one month before launch of Rosetta and Philae to comet Wirtanen. Because of the necessary failure investigation, the launch was scrubbed and the only launch window to undertake the trajectory to Wirtanen was lost. The present comet 67P was then chosen. Mission engineers were aware of the mass difference and consequently had to modify Philae’s landing gear to withstand the greater forces upon landing on 67P/Churyumov–Gerasimenko.
Philae illustration showing the landing feet ice screws and the two harpoons (blue), below the center pedestal (dampener) (Credit: ESA)
Knowing the comet’s gravity, rotation axis and period are critical. Rosetta mission planners are working feverishly to determine the direction of gravity at the possible landing sites.
Philae has a simple cold gas propulsion system and its purpose is not to slow down the descent, as we often imagine for landers, but rather to push the lander onto the surface. Rosetta will accurately push off Philae at the right time, speed and direction to reach the landing spot.
So imagine if you will that it is the mid-1990s and you are designing a lander. It must accomplish the landing on its own, without help from Earth — except for what is built into the mechanisms and software. Philae’s software operates on a simple computer chip in the Command, Data and Management System (CDMS) jointly developed and tested in Hungary – Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences and Germany – the Max Planck Institute. The Hungarian Institute also constructed the Power Subsystem (PSS) which is critical to Philae’s success. The PSS must produce and store power while enduring extremes in temperature and periods of no sunlight.
The computer processing power is about the same as that of a 1990s hand calculator, however, the chips used were radiation hardened to survive space conditions. Philae’s systems will be watching and making navigation corrections throughout the descent. Nothing fancy, this is a simple and straightforward execution with a modest control system on board. Nevertheless, it has everything necessary to accomplish the soft landing on a comet.
When studying the design, I first imagined that Philae would make a long descent and the comet would make a full rotation. But rather, Rosetta will be navigated to somewhere between 2 to 10 km above the comet surface then release Philae. Because of the comet’s odd shape, the probes could be 4 km above the surface at one time and then just 2 km at another, due to the rotation of the comet. The odd rotating shape means that the gravity field effecting the descent will be constantly changing. One might compare the effects of 67P’s gravity on Philae as similar to the motion of a well thrown knuckleball (e.g., Wakefield, Wilhelm). Catchers resort to using a larger catchers mitt and likewise, the landing zone (or ellipse) is 1 square kilometer, sizable considering 67P’s dimensions are 3.5 × 4 km (2.2 × 2.5 miles).
Five candidate landing sites on 67P as viewed from three perspectives. Down selection from 10 to 5 was announced August 25. The final selection is to be announced by September 14th for the landing scheduled on November 11th. (Photo Credit: ESA)
There is a also a modest tug of war going on between the mission planners and the researchers. For any mission that lands on a surface, for example, landers on Mars, there is the need to weigh safety against the return on investment. For the latter, the return is scientific return: measurements and observations of the most incredibly fascinating places you can imagine. For Philae, it gets one chance to land and one location to study, in contrast to the Mars Rovers which have traversed diverse terrain away from its landing site.
If anyone recalls the lander simulations that one could play on a computer or even a hand calculator, the simulations for Philae are a bit more challenging. Mission planners must have a good estimate of the comet’s gravity field, as strange as it is. They must know the rotation axis and rate of the comet accurately, and also know the relative position of Rosetta and Philae at the beginning of the descent.
The steps for the landing are: 1) release Philae towards the comet, 2) Descent: the comet is rotating and its gravity is weirdly pulling on the little probe during descent. Sounds like fun and one can be certain that mission planners are loving it. The descent that is undertaken is likely to be about 2 hours long. With a rotation period of 12.7 hours, the comet will rotate about 20%. But wait, there’s more. Rosetta is moving too and its orbital motion will be carried by Philae. This motion will offset the comet’s rotation to some degree.
Artist’s illustration of Philae upon touchdown. The lander is capable of landing on up to 30% slopes. (Credits: ESA)
3) Touchdown is when the CDMS will earn its badge of honor. Upon touchdown, the control system will fire the cold thrusters to push Philae snugly onto the surface. At the same time, the two harpoons will be fired to, hopefully, pierce and latch onto the cometary surface. To further prevent bounce or tipping, the dampener will absorb energy of the touchdown. Philae is likely to have some transverse velocity on touchdown and this will translate into a torque and a tipping action which the Harpoons and cold thrusters will reckon with.
So one can imagine that all the variables and possibilities have been considered by the mission planners. But not so fast. This is Humanity’s first visit to the surface of a comet. The name Rosetta and Philae were chosen because comets are like a Rosetta Stone that is revealing the secrets of our origins – the early formation of the planets. Carl Sagan explained that we are all made of star stuff but more recently, about 4.3 billion years ago, it was comet stuff that may have delivered the building blocks of life and possibly even the water that fills our oceans. We do not know for certain but studying, landing upon, touching and analyzing 67P/Churyumov–Gerasimenko will increase our understanding of the link between comets and the Earth.
The Journey of the Rosetta probe to the comet 67P/Churyumov-Gerasimenko. Image, linked to the ESA animation of the 10 year journey. (Credit: ESA)
A slice of the Laniakea Supercluster -- a local basin of attraction. This structure contains many galaxies and clusters, including our own Milky Way Galaxy. Credit: SDvision interactive visualization software by DP at CEA/Saclay, France.
Our cosmic address extends well beyond Earth, past the Milky Way and toward the farthest reaches of the universe. But now astronomers are adding another line: the Laniakea Supercluster, which takes its name from the Hawaiin term “lani” meaning heaven and “akea” meaning spacious or immeasurable.
And the name is true to its meaning. The supercluster extends more than 500 million light-years and contains the mass of 100 quadrillion Suns in 100,000 large galaxies. This research is the first to trace our local supercluster on such a large scale.
“We have finally established the contours that define the supercluster of galaxies we can call home,” said lead researcher R. Brent Tully, from the University of Hawaii’s Institute for Astrophysics, in a news release. “This is not unlike finding out for the first time that your hometown is actually part of much larger country that borders other nations.”
Superclusters — aggregates of clusters of galaxies — rank among the largest structures in the universe. Although these structures are interconnected in a web of filaments, their exact outlines and boundaries are hard to define.
Large three-dimensional maps (think Sloan Digital Sky Survey) calculate a galaxy’s location based on its galactic redshift, the shifts in its spectrum due to its apparent motion as space itself expands. But Tully and colleagues used peculiar redshifts, the shifts in a galaxy’s spectrum due to the local gravitational landscape, instead.
In other words, the team is mapping the galaxies by examining their impact on the motions of other galaxies. A galaxy caught in the midst of multiple galaxies will find itself in a massive tug-of-war, where the balance of the surrounding gravitational forces will dictate its motion.
Typically this method is only viable for the local universe where the peculiar velocities are high enough compared with the expansion velocities, which increase with distance (a galaxy recedes faster the farther away it is). But Tully and colleagues used a new algorithm, which revealed the large-scale patterns created by galaxies’ motions.
Not only did this allow them to map our home supercluster, but to clarify the role of the Great Attractor, a dense region in the vicinity of Centaurus, Norma, and Hydra clusters that influences the motion of our Local Group and other groups of galaxies. They revealed that the Great Attractor is a large gravitational valley that draws all galaxies inward.
The team also discovered other structures, including a region named Shapley, toward which Laniakea is moving.
The findings have been published in the Sept. 4 issue of Nature.
An artist's conception of the European Space Agency's ExoMars rover, scheduled to launch in 2018. Credit: ESA
Picking a landing site on Mars is a complex process. There’s the need to balance scientific return with the capabilities of whatever vehicle you’re sending out there. And given each mission costs millions (sometimes billions) of dollars — and you only get one shot at landing — you can bet mission planners are extra-cautious about choosing the right location.
A recent paper in Eos details just how difficult it is to choose where to put down a rover, with reference to the upcoming European ExoMars mission that will launch in 2018.
In March, scientists came together to select the first candidate landing sites and came up with four finalist locations. The goal of ExoMars is to look for evidence of life (whether past or present) and one of its defining features is a 2-meter (6.6-foot) drill that will be able to bore below the surface, something that the NASA Curiosity rover does not possess.
“Among the highest-priority sites are those with subaqueous sediments or hydrothermal deposits,” reads the paper, which was written by Bradley Thomson and Farouk El-Baz (both of Boston University). Of note, El-Baz was heavily involved in landing site selection for the Apollo missions.
Curiosity snaps selfie at Kimberley waypoint with towering Mount Sharp backdrop on April 27, 2014 (Sol 613). Inset shows MAHLI camera image of rovers mini-drill test operation on April 29, 2014 (Sol 615) into “Windjama” rock target at Mount Remarkable butte. MAHLI color photo mosaic assembled from raw images snapped on Sol 613, April 27, 2014. Credit: NASA/JPL/MSSS/Marco Di Lorenzo/Ken Kremer – kenkremer.com
“For example,” the paper continues, “some of the clearest morphological indicators of past aqueous activity are channel deposits indicative of past fluvial activity or the terminal fan, or delta deposits present within basins.”
But no landing site selection is perfect. The scientists note that Curiosity, for all of its successes, seems unlikely to achieve its primary science objectives in its two-year mission because the commissioning phase took a while, and the rover moves relatively slowly.
Outcrops in Yellowknife Bay are being exposed by wind driven erosion. These rocks record superimposed ancient lake and stream deposits that offered past environmental conditions favorable for microbial life. This image mosaic from the Mast Camera instrument on NASA’s Curiosity Mars rover shows a series of sedimentary deposits in the Glenelg area of Gale Crater, from a perspective in Yellowknife Bay looking toward west-northwest. The “Cumberland” rock that the rover drilled for a sample of the Sheepbed mudstone deposit (at lower left in this scene) has been exposed at the surface for only about 80 million years. Credit: NASA/JPL-Caltech/MSSS
What could change the area of the landing could be using different types of entry, descent and landing technologies, the authors add. If the parachute opened depending on how far the spacecraft was from the ground — instead of how fast it was going — this could make the landing ellipse smaller.
This could place the rover “closer to targets of interest that are too rough for a direct landing and reducing necessary traverse distances,” the paper says.
You can read the paper in its entirety at this link, which also goes over the history of selecting landing sites for the Apollo missions as well as the Mars Exploration Rovers (Spirit and Opportunity).
