comet Archives

September 22, 2014December 23, 2015 by Tim Reyes

Rosetta’s Philae Lander: A Swiss Army Knife of Scientific Instruments

Rosetta's Philae lander is Like a modern-day Swiss Army Knife, now prepared for a November 11th dispatch to a comet's surface.

When traveling to far off lands, one packs carefully. What you carry must be comprehensive but not so much that it is a burden. And once you arrive, you must be prepared to do something extraordinary to make the long journey worthwhile.

The previous Universe Today article “How do you land on a Comet?” described Philae’s landing technique on comet 67P/Churyumov-Gerasimenko. But what will the lander do once it arrives and gets settled in its new surroundings? As Henry David Thoreau said, “It is not worthwhile to go around the world to count the cats in Zanzibar.” So it is with the Rosetta lander Philae. With the stage set – a landing site chosen and landing date of November 11th, the Philae lander is equipped with a carefully thought-out set of scientific instruments. Comprehensive and compact, Philae is a like a Swiss Army knife of tools to undertake the first on-site (in-situ) examination of a comet.

Now, consider the scientific instruments on Philae which were selected about 15 years ago. Just like any good traveler, budgets had to be set which functioned as constraints on the instrument selection that could be packed and carried along on the journey. There was a maximum weight, maximum volume, and power. The final mass of Philae is 100 kg (220 lbs). Its volume is 1 × 1 × 0.8 meters (3.3 × 3.3 × 2.6 ft) about the size of a four burner oven-range. However, Philae must function on a small amount of stored energy upon arrival: 1000 Watt-Hours (equivalent of a 100 watt bulb running for 10 hours). Once that power is drained, it will produce a maximum of 8 watts of electricity from Solar panels to be stored in a 130 Watt-Hour battery.

Side view schematics of the inner structure of the lander compartment showing the location of COSAC and PTOLEMY systems, the CONSERT antennas, the SESAME dust sensor and various ÇIVA cameras (Credits: "Capabilities of Philae, the Rosetta Lander, J. Biele, S. Ulamec, September 2007) — Side view schematics of the inner structure of the lander compartment showing the location of COSAC, PTOLEMY, the CONSERT antennas, the SESAME dust sensor and ÇIVA cameras. Philae is about the size of a dishwasher or four burner oven. (Credits: “Capabilities of Philae, the Rosetta Lander, J. Biele, S. Ulamec, September 2007)

Without any assurance that they would land fortuitously and produce more power, the Philae designers provided a high capacity battery that is charged, one time only, by the primary spacecraft solar arrays (64 sq meters) before the descent to the comet. With an initial science command sequence on-board Philae and the battery power stored from Rosetta, Philae will not waste any time to begin analysis — not unlike a forensic analysis — to do a “dissection” of a comet. Thereafter, they utilize the smaller battery which will take at least 16 hours to recharge but will permit Philae to study 67P/Churyumov-Gerasimenko for potentially months.

There are 10 science instrument packages on the Philae lander. The instruments use absorbed, scattered, and emitted light, electrical conductivity, magnetism, heat, and even acoustics to assay the properties of the comet. Those properties include the surface structure (the morphology and chemical makeup of surface material), interior structure of P67, and the magnetic field and plasmas (ionized gases) above the surface. Additionally, Philae has an arm for one instrument and the Philae main body can be rotated 360 degrees around its Z-axis. The post which supports Philae and includes a impact dampener.

CIVA and ROLIS imaging systems. CIVA represents three cameras which share some hardware with ROLIS. CIVA-P (Panoramic) is seven identical cameras, distributed around the Philae body but with two functioning in tandem for stereo imaging. Each has a 60 degree field of view and uses as 1024×1024 CCD detector. As most people can recall, digital cameras have advanced quickly in the last 15 years. Philae’s imagers were designed in the late 1990s, near state-of-the-art, but today they are surpassed, at least in number of pixels, by most smartphones. However, besides hardware, image processing in software has advanced as well and the images may be enhanced to double their resolution.

CIVA-P will have the immediate task, as part of the initial autonomous command sequence, of surveying the complete landing site. It is critical to the deployment of other instruments. It will also utilize the Z-axis rotation of the Philae body to survey. CIVA-M/V is a microscopic 3-color imager (7 micron resolution) and CIVA-M/I is a near infra-red spectrometer (wavelength range of 1 to 4 microns) that will inspect each of the samples that is delivered to the COSAC & PTOLEMY ovens before the samples are heated.

The CIVA micro-camera. Mass: less than 100 grams, Power: less than 2 Watts, Minimum Operating Temperature: -120C (Credit:ESA, Philae Lander Fact Sheet)

ROLIS is a single camera, also with a 1024×1024 CCD detector, with the primary role of surveying the landing site during the descent phase. The camera is fixed and downward pointing with an f/5 (f-ratio) focus adjustable lens with a 57 degree field of view. During descent it is set to infinity and will take images every 5 seconds. Its electronics will compress the data to minimize the total data that must be stored and transmitted to Rosetta. Focus will adjust just prior to touchdown but thereafter, the camera functions in macro mode to spectroscopically survey the comet immediately underneath Philae. Rotation of the Philae body will create a “working circle” for ROLIS.

The multi-role design of ROLIS clearly shows how scientists and engineers worked together to overall reduce weight, volume, and power consumption, and make Philae possible and, together with Rosetta, fit within payload limits of the launch vehicle, power limitations of the solar cells and batteries, limitations of the command and data system and radio transmitters.

Philae's APXS - Alpha Proton X-Ray Spectrometer (Credit: Inst. for Inorganic Chemistry & Analytical Chemistry, Max-Planck Institute for Chemistry) — Philae’s APXS – Alpha Proton X-Ray Spectrometer (Credit: Inst. for Inorganic Chemistry & Analytical Chemistry, Max-Planck Institute for Chemistry)

APXS. This is a Alpha Proton X-ray Spectrometer. This is a near must-have instrument of the space scientist’s Swiss Army Knife. APXS spectrometers have become a common fixture on all Mars Rover missions and Philae’s is an upgraded version of Mars Pathfinder’s. The legacy of the APXS design is the early experiments by Ernest Rutherford and others that led to discovering the structure of the atom and the quantum nature of light and matter.

This instrument has a small source of Alpha particle emission (Curium 244) essential to its operation. The principles of Rutherford Back-scattering of Alpha particles is used to detect the presence of lighter elements such as Hydrogen or Beryllium (those close to an Alpha particle in mass, a Helium nucleus). The mass of such lighter elemental particles will absorb a measurable amount of energy from the Alpha particle during an elastic collision; as happens in Rutherford back-scattering near 180 degrees. However, some Alpha particles are absorbed rather than reflected by the nuclei of the material. Absorption of an Alpha particle causes emission of a proton with a measurable kinetic energy that is also unique to the elemental particle from which it came (in the cometary material); this is used to detect heavier elements such as magnesium or sulfur. Lastly, inner shell electrons in the material of interest can be expelled by Alpha particles. When electrons from outer shells replace these lost electrons, they emit an X-Ray of specific energy (quantum) that is unique to that elementary particle; thus, heavier elements such as Iron or Nickel are detectable. APXS is the embodiment of early 20th Century Particles Physics.

CONSERT. COmet Nucleus Sounding Experiment by Radio wave Transmission, as the name suggests, will transmit radio waves into the comet’s nucleus. The Rosetta orbiter transmits 90 MHz radio waves and simultaneously Philae stands on the surface to receive with the comet residing between them. Consequently, the time of travel through the comet and the remaining energy of the radio waves is a signature of the material through which it propagated. Many radio transmissions and receptions by CONSERT through a multitude of angles will be required to determine the interior structure of the comet. It is similar to how one might sense the shape of a shadowy object standing in front of you by panning one’s head left and right to watch how the silhouette changes; altogether your brain perceives the shape of the object. With CONSERT data, a complex deconvolution process using computers is necessary. The precision to which the comet’s interior is known improves with more measurements.

MUPUS. Multi-Purpose Sensor for Surface and Subsurface Science is a suite of detectors for measuring the energy balance, thermal and mechanical properties of the comet’s surface and subsurface down to a depth of 30 cm (1 foot). There are three major parts to MUPUS. There is the PEN which is the penetrator tube. PEN is attached to a hammering arm that extends up to 1.2 meters from the body. It deploys with sufficient downward force to penetrate and bury PEN below the surface; multiple hammer strokes are possible. At the tip, or anchor, of PEN (the penetrator tube) is an accelerometer and standard PT100 (Platinum Resistance Thermometer). Together, the anchor sensors will determine the hardness profile at the landing site and the thermal diffusivity at the final depth [ref]. As it penetrates the surfaces, more or less deceleration indicates harder or softer material. The PEN includes an array of 16 thermal detectors along its length to measure subsurface temperatures and thermal conductivity. The PEN also has a heat source to transmit heat to the cometary material and measure its thermal dynamics. With the heat source off, detectors in PEN will monitor the temperature and energy balance of the comet as it approaches the Sun and heats up. The second part is the MUPUS TM, a radiometer atop the PEN which will measure thermal dynamics of the surface. TM consists of four thermopile sensors with optical filters to cover a wavelength range from 6-25 µm.

SD2 Sample Drill and Distribution device will penetrate the surface and subsurface to a depth of 20 cm. Each retrieved sample will be a few cubic millimeters in volume and distributed to 26 ovens mounted on a carousel. The ovens heat the sample which creates a gas that is delivered to the gas chromatographs and mass spectrometers that are COSAC and PTOLEMY. Observations and analysis of APXS and ROLIS data will be used to determine the sampling locations all of which will be on a “working circle” from the rotation of Philae’s body about its Z-axis.

COSAC Cometary Sampling and Composition experiment. The first gas chromatograph (GC) I saw was in a college lab and was being used by the lab manager for forensic tests supporting the local police department. The intent of Philae is nothing less than to perform forensic tests on a comet hundred of million of miles from Earth. Philae is effectively Sherlock Holmes’ spy glass and Sherlock is all the researchers back on Earth. The COSAC gas chromatograph includes a mass spectrometer and will measure the quantities of elements and molecules, particularly complex organic molecules, making up comet material. While that first lab GC I saw was closer to the size of Philae, the two GCs in Philae are about the size of shoe boxes.

Philae's two Gas Chromatograph (GC). Left: COSAC, integrated into Philae, Right: PTOLEMY on an engineering lab. (Credit: ESA) — Philae’s two Gas Chromatographs (GC). Left: COSAC, integrated into Philae, Right: PTOLEMY in an engineering lab. (Credit: ESA)

PTOLEMY. An Evolved Gas Analyzer [ref], a different type of gas chromatograph. The purpose of Ptolemy is to measure the quantities of specific isotopes to derive the isotopic ratios, for example, 2 parts isotope C12 to one part C13. By definition, isotopes of an element have the same number of protons but different numbers of neutrons in their nuclei. One example is the 3 isotopes of Carbon, C12, C13 and C14; the numbers being the number of neutrons. Some isotopes are stable while others can be unstable – radioactive and decay into stable forms of the same element or into other elements. What is of interest to Ptolemy investigators is the ratio of stable isotopes (natural and not those affected by, or that result from, radioactive decay) for the elements H, C, N, O and S, but particularly Carbon. The ratios will be telltale indicators of where and how comets are created. Until now, spectroscopic measurements of comets to determine isotopic ratios have been from a distance and the accuracy has been inadequate for drawing firm conclusions about the origin of comets and how comets are linked to the creation of planets and the evolution of the Solar Nebula, the birthplace of our planetary system surrounding the Sun, our star. An evolved gas analyzer will heat up a sample (~1000 C) to transform the materials into a gaseous state which a spectrometer can very accurately measure quantities. A similar instrument, TEGA (Thermal Evolved Gas Analyzer) was an instrument on Mars Phoenix lander.

SESAME Surface Electrical Sounding and Acoustic Monitoring Experiment This instrument involves three unique detectors. The first is the SESAME/CASSE, the acoustic detector. Each landing foot of Philae has acoustic emitters and receivers. Each of the legs will take turns transmitting acoustic waves (100 Hertz to KiloHertz range) into the comet which the sensors of the other legs will measure. How that wave is attenuated, that is, weakened and transformed, by the cometary material it passes through, can be used along with other cometary properties gained from Philae instruments, to determine daily and seasonal variations in the comet’s structure to a depth of about 2 meters. Also, in a passive (listening) mode, CASSE will monitor sound waves from creaks, groans inside the comet caused potentially by stresses from Solar heating and venting gases.

Next is the SESAME/PP detector – the Permittivity Probe. Permittivity is the measure of the resistance a material has to electric fields. SESAME/PP will deliver an oscillating (sine wave) electric field into the comet. Philae’s feet carry the receivers – electrodes and AC sine generators to emit the electric field. The resistance of the cometary material to about a 2 meter depth is thus measured providing another essential property of the comet – the permittivity.

Philae SESAME/DIM, Dust Impact Monitor. The monitor can measure particle size and velocity. Later as comet P67's activity rises, it can continue to return total particle flux. (Credit: ESA) — Philae SESAME/DIM, Dust Impact Monitor. The monitor can measure particle size and velocity. Later as comet P67’s activity rises, it can continue to return total particle flux. (Credit: ESA)

The third detector is called SESAME/DIM. This is the comet dust counter. There were several references used to compile these instrument descriptions. For this instrument, there is, what I would call, a beautiful description which I will simply quote here with reference. “The Dust Impact Monitor (DIM) cube on top of the Lander balcony is a dust sensor with three active orthogonal (50 × 16) mm piezo sensors. From the measurement of the transient peak voltage and half contact duration, velocities and radii of impacting dust particles can be calculated. Particles with radii from about 0.5 µm to 3 mm and velocities from 0.025–0.25 m/s can be measured. If the background noise is very high, or the rate and/or the amplitudes of the burst signal are too high, the system automatically switches to the so called Average Continuous mode; i.e., only the average signal will be obtained, giving a measure of the dust flux.” [ref]

ROMAP Rosetta Lander Magnetometer and Plasma detector also includes a third detector, a pressure sensor. Several spacecraft have flown by comets and an intrinsic magnetic field, one created by the comet’s nucleus (the main body) has never been detected. If an intrinsic magnetic field exists, it is likely to be very weak and landing on the surface would be necessary. Finding one would be extraordinary and would turn theories regarding comets on their heads. Low and behold Philae has a fluxgate magnetometer.

Philae ROMAP, Tri-Axial Fluxgate magnetometer and Plasma Monitor (Credit: ESA/MPS) — Philae ROMAP, Tri-Axial Fluxgate Magnetometer and Plasma Monitor (Credit: ESA/MPS)

The Earth’s magnetic (B) field surrounding us is measured in the 10s of thousands of nano-Teslas (SI unit, billionth of a Tesla). Beyond Earth’s field, the planets, asteroids, and comets are all immersed in the Sun’s magnetic field which, near the Earth, is measured in single digits, 5 to 10 nano-Tesla. Philae’s detector has a range of +/- 2000 nanoTesla; a just in case range but one readily offered by fluxgates. It has a sensitivity of 1/100th of a nanoTesla. So, ESA and Rosetta came prepared. The magnetometer can detect a very minute field if it’s there. Now let’s consider the Plasma detector.

Much of the dynamics of the Universe involves the interaction of plasma – ionized gases (generally missing one or more electrons thus carrying a positive electric charge) with magnetic fields. Comets also involve such interactions and Philae carries a plasma detector to measure the energy, density and direction of electrons and of positively charged ions. Active comets are releasing essentially a neutral gas into space plus small solid (dust) particles. The Sun’s ultraviolet radiation partially ionizes the cometary gas of the comet’s tail, that is, creates a plasma. At some distance from the comet nucleus depending on how hot and dense that plasma is, there is a standoff between the Sun’s magnetic field and the plasma of the tail. The Sun’s B field drapes around the comet’s tail kind of like a white sheet draped over a Halloween trick-or-treater but without eye holes.

The structure of an active comet. In early 2015, 67P/Churyumov–Gerasimenko will wake-up. The heat of the Sun will increase gas and dust production which will interact with Solar UV radiation and the Solar Wind. The Sun's magnetic field will be draped around the coma and tail of the comet. (Photo: ESA) — The structure of an active comet. In early 2015, 67P/Churyumov–Gerasimenko will wake up. The heat of the Sun will increase gas and dust production which will interact with Solar UV radiation and the Solar Wind. The Sun’s magnetic field will be draped around the coma and tail of the comet. (Photo: ESA)

So at P67’s surface, Philae’s ROMAP/SPM detector, electrostatic analyzers and a Faraday Cup sensor will measure free electrons and ions in the not so empty space. A “cold” plasma surrounds the comet; SPM will detect ion kinetic energy in the range of 40 to 8000 electron-volts (eV) and electrons from 0.35 eV to 4200 eV. Last but not least, ROMAP includes a pressure sensor which can measure very low pressure – a millionth or a billionth or less than the air pressure we enjoy on Earth. A Penning Vacuum gauge is utilized which ionizes the primarily neutral gas near the surface and measures the current that is generated.

Philae will carry 10 instrument suites to the surface of 67P/Churyumov-Gerasimenko but altogether the ten represent 15 different types of detectors. Some are interdependent, that is, in order to derive certain properties, one needs multiple data sets. Landing Philae on the comet surface will provide the means to measure many properties of a comet for the fist time and others with significantly higher accuracy. Altogether, scientists will come closer to understanding the origins of comets and their contribution to the evolution of the Solar System.

September 10, 2014December 23, 2015 by Jason Major

Philae Snaps a Spacetastic Selfie

Image of Rosetta's solar array and comet 67P/C-G taken by Philae on Sept. 7, 2014 (ESA/Rosetta/Philae/CIVA)

Spacecraft “selfies” are always a treat and this one is doubly awesome: taken by the Philae lander piggybacked onto ESA’s Rosetta, it shows one of the spacecraft’s 14-meter-long (46-foot) solar arrays glinting with reflected sunlight while off in the distance is the double-lobed nucleus of Comet 67P/Churyumov-Gerasimenko!

Rosetta has been circling the comet for over a month now and returning some truly amazing images, but leave it to little Philae to put it all into perspective. Such a show-stealer! (Not that we mind, of course.)

The image above was acquired with Philae’s CIVA (Comet nucleus Infrared and Visible Analyzer) instrument on Sept. 7, 2014, from a distance of 50 km (31 miles) from Comet 67P/C-G. It’s actually a composite of two separate images made with different exposures adjusted for the lighting disparities between the spacecraft and comet.

Artist impression of Philae on the surface of comet 67P/Churyumov-Gerasimenko. Credit: ESA/ATG medialab

The Philae (say “FEE-lay”) lander itself weighs 100 kg (220 lbs) and is about a meter wide and 80 cm high (3.2 x 2.6 feet). The CIVA instrument, one of ten installed on the lander, is composed of seven miniature cameras that will take panoramic pictures of 67P’s surface and reconstruct its structure in 3D, as well as a microscope and a near-infrared imager to study its composition, texture, and reflectivity. (Source)

This is the second image from Philae this year to feature part of the Rosetta spacecraft (but the first to show the comet); the previous was taken in April 2014.

Back in 2007 Philae took a shot that showed Rosetta’s solar panel and Mars; check that one out here.

Currently Rosetta is being transitioned to its Global Mapping Phase (GMP). This is an incredibly intensive process that will determine how close the spacecraft will be able to get to the surface of the comet as engineers search for the best landing area to which to deploy Philae in November.

Learn more about the Rosetta mission and Comet 67P/C-G here.

Source: ESA

August 19, 2014December 23, 2015 by Bob King

Australian Amateur Terry Lovejoy Discovers New Comet

The small fuzzy potential comet is at center in this photo taken discovered by Terry Lovejoy. Credit: copyright Alain Maury and Joaquin Fabrega

It’s confirmed! Australian amateur astronomer Terry Lovejoy just discovered his fifth comet, C/2014 Q2 (Lovejoy). He found it August 17th using a Celestron C8 fitted with a CCD camera at his roll-off roof observatory in Brisbane, Australia.

Image triplet taken by Terry Lovejoy on which he discovered the comet. The comet moves slightly counterclockwise around the larger fuzzy spot. Credit: Terry Lovejoy — Image triplet taken by Terry Lovejoy of his comet discovery. The comet moves slightly counterclockwise around the larger fuzzy spot over the time frame. Credit: Terry Lovejoy

“I take large sets of image triplets, i.e 3 images per star field and use software to find moving objects,” said Lovejoy. “The software I use outputs suspects that I check manually by eye.”

Most of what pops up on the camera are asteroids, known comets, or false alarms but not this time. Lovejoy’s latest find is a faint, fuzzy object in the constellation Puppis in the morning sky.

Sky as seen from central South America showing the approximate location of the new comet on August 19 in Puppis near the bright star Canopus. Stellarium — Sky as seen from central South America showing the approximate location of the new comet (purple circle) on August 19 in Puppis near the bright star Canopus. The view shows the sky facing southeast just before the start of dawn. Stellarium

Glowing a dim magnitude +15, the new comet will be a southern sky object until later this fall when it swings quickly northward soon around the time of perihelion or closest approach to the sun. Lovejoy’s find needs more observations to better refine its orbit, but based on preliminary data, Maik Meyer, founder of the Comets Mailing List, calculates a January 2, 2015 perihelion.

Another photo of C/2014 Q2 taken on August 19, 2014. Credit: Jean-François and Alain Maury

On that date, it will be a healthy 84 million miles from the sun, but one month earlier on December 7, the comet could pass just 6.5 million miles from Earth and be well placed for viewing in amateur telescopes.

Everything’s still a little up in the air right now, so these times and distances are likely to change as fresh observations pour in. Take all predictions with a major grain of salt for the moment.

photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Credit: NASA — Comet Lovejoy (C/2011 W3) photographed by NASA astronaut Dan Burbank, onboard the International Space Station on Dec. 22, 2011 from 250 miles up. Credit: NASA

You might remember some of Terry’s earlier comets. Comet Lovejoy (C/2011 W3), a Kreutz sungrazer discovered in November 2011, passed just 87,000 miles above the sun’s surface. Many astronomers thought it wouldn’t survive the sun’s heat, yet amazingly, although much of its nucleus burned off, enough material survived to produce a spectacular tail.

More recently, Comet Lovejoy (C/2013 R1) thrilled observers as it climbed to naked eye brightness last November, managing to do the impossible at the time and draw our eyes away from Comet ISON.

Congratulations Terry on your new find! May it wax brightly this fall.

* Update: The latest orbit calculation from the Minor Planet Center based on 24 observations now puts perihelion at 164.6 million miles (265 million km) on February 14, 2015. Closest approach to Earth of 93.2 million miles (150 million km) will occur in January.

August 3, 2014December 23, 2015 by Bob King

Getting to Know Comet 67P/Churyumov-Gerasimenko

Comet 67P/Churyumov-Gerasimenko at 621 miles (1,000 km) on August 1. Wow! Look at that richly-textured surface. This photo has higher resolution than previous images because it was taken with Rosetta's narrow angle camera. The black spot is an artifact. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

We’re finally getting to know the icy nucleus behind comet 67P/Churyumov-Gerasimenko. For all the wonder that comets evoke, we on Earth never see directly what whips up the coma and tail. Even professional telescopes can’t burrow through the dust and vapor cloaking the nucleus to distinguish the clear outline of a comet’s heart. The only way to see one is to fly a camera there.

Asteroids we've seen up close show cratered surfaces similar to yet different from much of the cratering on comets. Credit: — Asteroids we’ve seen up close show cratered surfaces similar to yet different from much of the cratering so far seen on comets. Not to scale. Credit: NASA except for Steins (ESA)

Rosetta took 10 years to reach 67P/C-G, a craggy, boot-shaped body that resembles an asteroid in appearance but with key differences. Asteroids shown in close up photos often display typical bowl-shaped impact craters. From the photos to date, 67P/C-G’s ‘craters’ look shallow and flat in comparison. Were they impacts smoothed by ice flows over time? Did some of the dust and vapor spewed by the comet settle back on the surface to partially bury and soften the landscape?

Comet 81P/Wild 2 photographed during the Stardust mission in 2004. Wild 2 measures 1.03 x 1.24 x 1.71 miles and goes around the sun once every 6.4 years. Its surfaced is riddled with flat-bottomed craters, some of which may also be gas vents from vaporized ice. Credit: NASA — Comet 81P/Wild 2 photographed during the Stardust mission in 2004. Wild 2 measures 1.03 x 1.24 x 1.71 miles and goes around the sun once every 6.4 years. Its surfaced is riddled with flat-bottomed depressions some of which may also vent gas from vaporizing ice. Click for more 81P/Wild 2 photos. Credit: NASA

While 67P is doubtless its own comet, it does share certain similarities with Comet 81P/Wild including at least a few crater-like depressions seen during NASA’s Stardust mission. In January 2004, the spacecraft gathered photos, measurements and dust samples during its brief flyby of the nucleus. Photos reveal pinnacles, flat-bottomed depressions and bright plumes or jets of vaporizing ice.

Some of the comets we've seen close up through the eyes of visiting spacecraft. Credit: NASA — Some of the comets we’ve seen close up through the eyes of visiting spacecraft. Credit: NASA

In a 2004 paper by Donald Brownlee and team, the group experimentally reproduced the flat-floored craters by firing projectiles into resin-coated sand baked a bit to make it cohere. Their results suggest the craters formed from impacts in loosely compacted material under the low-gravity conditions typical of small objects like comets. To quote the paper: “Most disrupted material stayed inside the cavity and formed a flat-floored deposit and steep cliffs formed the rim.” Icy materials mixed with dust may have also played a role in their appearance and other crater-like depressions called pit-halos.

Latest image of the comet taken by Rosetta's navigation camera on August 2, 2014. Credit: ESA/Rosetta/Navcam — Latest image of the comet taken by Rosetta’s navigation camera from a distance of only 311 miles (500 km) on August 2, 2014. The comet’s larger size in the field means fewer artifacts. Credit: ESA/Rosetta/Navcam

Speculation isn’t science, so I’ll stop here. So much more data will be streaming in soon, we’ll have our hands full. On Wednesday, August 6th, Rosetta will enter orbit around the nucleus and begin detailed studies that will continue through December 2015. Studying the new pictures now arriving daily, I’m struck by the dual nature of comets. We see an ancient landscape and yet one that looks strangely contemporary as the sun vaporizes ice, reworking the terrain like a child molding clay.

Comet 67P/Churyumov-Gerasimenko is well-placed in the mid-summer sky in Sagittarius but impossibly faint to see visually. Dave Herald's photo taken on August 21, 2014 shows only a tiny fuzz of magnitude +21. Credits: Background: Stellarium; David Herald — Comet 67P/Churyumov-Gerasimenko is well-placed in the mid-summer sky in Sagittarius but impossibly faint visually. Dave Herald’s photo taken on August 21, 2014 shows only a tiny fuzz of magnitude +21. Credits: Dave Herald; Stellarium

July 31, 2014December 23, 2015 by Jason Major

New Image of Rosetta’s Comet Reveals So Much More

Comet 67P/Churyumov-Gerasimenko imaged by OSIRIS on July 29, 2014

WOW! We’re really getting to the good stuff now! This is no computer-generated shape model, this is the real deal: the double-lobed nucleus of Comet 67P/C-G, as imaged by Rosetta’s OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) narrow-angle camera on Tuesday, July 29. At the time just about a week away from making its arrival, ESA’s spacecraft was 1,950 km (1,211 miles) from the comet when this image was taken. (That’s about the distance between Providence, Rhode Island and Miami, Florida… that’s one fancy zoom lens, Rosetta!)

Comet 67P/Churyumov-Gerasimenko was imaged on 14 July 2014 by OSIRIS, Rosetta’s scientific imaging system, from a distance of approximately 12 000 km. This movie uses a sequence of 36 interpolated images each separated by 20 minutes, providing a 360° preview of the complex shape of the comet. (ESA) — Comet 67P/Churyumov-Gerasimenko imaged on July 14, 2014 by OSIRIS from a distance of approximately 12,000 km. (ESA)

This latest image reveals some actual surface features of the 4-km-wide comet, from a few troughs and mounds to the previously-noted bright band around the “neck” connecting the two lobes. The resolution in the July 29 OSIRIS image is 37 meters per pixel.

Since Rosetta is quickly closing the gap between itself and the comet we can only expect better images to come in the days ahead, so stay tuned — this is going to be an exciting August!

Keep up with the latest news on ESA’s Rosetta blog here, and find out where exactly Rosetta and Comet 67P/C-G are in the Solar System here.

Watch: Once Upon a Time There Was a Spacecraft Called Rosetta

Image credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Source: ESA

July 31, 2014December 23, 2015 by Brian Koberlein

What Are Comet Tails?

The view of Comet PANSTARRS L4 on 03-22-2013 over Warrenton, Virginia. Modified Canon Rebel Xsi DSLR 30 second exposure, ISO 1600, University Optics 80mm F6 Refractor (600mm). Credit and copyright: John Chumack.

Comets are renowned for their big beautiful tails that stretch across the sky. But what’s in those things, anyway? And how can comets get multiple tails?

In the past, humans generally used one of two greetings for comets:
1. Dear God, what is that thing? Terrible omens! Surely we will all die in fire.
2. Dear God, what is that thing? Great omens! Surely we will all have a big party… where we all die in fire?

For example, the appearance of what came to be known as Halley’s comet in 1066 was seen as a bad omen for King Harold II. Conversely, it was a good omen for William the Conqueror.

Because of their tails and transitory nature, comets were long thought to be products of the Earth’s atmosphere. It wasn’t until the 1500s, when Tycho Brahe used parallax to determine a comet’s distance. He realized that they were Solar System objects, like planets.

So, good news, we no longer regard them as omens and everyone stopped panicking. Right? Wrong. When Comet Halley approached Earth in 1910, astronomers detected cyanide gas in its tail. French astronomer Camille Flammarion was quoted as saying the gas could “impregnate the atmosphere and possibly snuff out all life on the planet.” This caused a great deal of hysteria. Many bought gas masks and “comet pills” to protect themselves.

With the rise of photographic astronomy, it was found that comets often have two types of tails. A bright tail composed of ionized gas, and a dimmer one composed of dust particles. The ion tail always points away from the Sun. It’s actually being pushed away from the comet by the solar wind.

Comets often develop two tails as they near the sun - a curved dust tail and straight, ion tail. Credit: NASA — Comets often develop two tails as they near the sun – a curved dust tail and straight, ion tail. Credit: NASA

We now know that a comet’s ion tail contains “volatiles” such as water, methane, ammonia and carbon dioxide. These volatiles are frozen near the comet’s surface, and as they approach the Sun, they warm and become gaseous. This also causes dust on the comet’s surface to stream away. The heating of a comet by the Sun is not uniform.

Because of a comet’s irregular shape and rotation, some parts of the surface can be heated by sunlight, while other parts remain cold. In some cases this can mean that comets can have multiple tails, which creates amazing effects where different regions of a comet stream off volatiles.

Comet Lovejoy passing behind green oxygen and sodium airglow layers on December 22, 2011 seen from the space station. Credit: NASA/Dan Burbank

These ion tails can be quite large, and some have been observed to be nearly 4 times the distance of the Earth from the Sun. And even though they fill a great volume, they are also pretty diffuse. If you condensed a comet’s tail down to the density of water, it wouldn’t even fill a swimming pool.

We also now know that there isn’t a clear dividing line between comets and asteroids. It’s not the case the comets are dirty snowballs and asteroids are dry rocks. There is a range of variation, and asteroids can gain dusty or gaseous tails and take on a comet-like appearance. In addition, we’ve also found comets orbiting other stars, known as exocomets.

And finally one last fact, the term comet comes from the Latin cometa, which indicated a hairy star.

So, what’s your favorite comet? Tell us in the comets below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

July 28, 2014December 23, 2015 by Bob King

Observing Alert – Delta Aquarid Meteor Shower Peaks This Week

A bright meteor from September 21, 1994. Credit: John Chumack.

With the southern Delta Aquarid meteor shower peaking tomorrow morning, the summer meteor-watching season officially begins. While not a rich shower from mid-northern latitudes, pleasant weather and a chance to see the flaming remains of a comet seem motivation enough to go out for a look. With a rate 10-15 per meteors an hour you’re bound to catch a few.

The farther south you live, the better it gets. Observers in the southern hemisphere can expect double that number because the shower’s radiant will be much higher in the sky. Any meteors flashing south of the radiant won’t get cut off by the southern horizon like they do further north.

The annual shower gets its name from Delta Aquarii, a dim star in the dim zodiac constellation Aquarius. You don’t need to know the constellations to enjoy the show, but if you know the general direction of the radiant you’ll be able to tell shower members from the nightly sprinkle of random meteors called sporadics. If you can trace the path of a meteor backward toward Aquarius, chances are it’s an Aquarid.

Delta Aquarid Meteors (2) on 07-30-2013 — A Southern Delta Aquarid meteor captured on July 30, 2013. Credit: John Chumack

There are actually two meteor showers in Aquarius active this time of year – the northern and southern Delta Aquarids. The northern version sprinkles fewer meteors and peaks in mid-August.

The Southern Deltas peak over the next two mornings – July 29 and 30 – but will be out all week. Both serve as a warm-up for the upcoming Perseid meteor shower that climaxes on August 12.

Tonight’s shower will suffer no interference from moonlight, making for ideal meteor watching. Unfortunately, Perseid rates will be reduced by a bright waning gibbous moon.

Don’t be surprised if you see a few Perseids anyway. The shower’s just becoming active. If you can draw a meteor’s trail back to the northeastern sky, it just might be a member. Read more about Perseid prospects from our own David Dickinson.

Meteors from Delta Aquarid meteor shower radiate from near the star Delta Aquarii not far from the bright star Fomalhaut in the Southern Fish low in the south before dawn. Stellarium

Nearly all meteor showers originate from clouds of sand to seed-sized bits of debris spewed by vaporizing comet ice as they swing near the sun. The Delta Aquarids may trace its origin to dust boiled off Comet 96P/Machholz.

The best time to watch the shower is in the early morning hours before dawn when the radiant rises in the south-southeastern sky above the bright star Fomalhaut. Try to get away from city lights. Point your lawn chair south and spend some time in heavenly contemplation as you wait for Aquarius to toss a few javelins of light your way.

July 26, 2014December 23, 2015 by Bob King

NASA Preps for Nail-biting Comet Flyby of Mars

This graphic depicts the orbit of comet C/2013 A1 Siding Spring as it swings around the sun in 2014. On Oct. 19, the comet will have a very close pass at Mars. Its nucleus will miss Mars by about 82,000 miles (132,000 kilometers). The comet's trail of dust particles shed by the nucleus might be wide enough to reach Mars or might also miss it. Credit: NASA/JPL

As Comet C/2013 A1 Siding Spring inches closer to the Red Planet, NASA’s taking steps to protect its fleet of orbiting Mars spacecraft. On October 19, the comet’s icy nucleus will miss the planet by just 82,000 miles (132,000 km). That’s 17 times closer than the closest recorded Earth-approaching comet, Lexell’s Comet in 1770.

Comet C/2013 A1 (Siding Spring) on July 11, 2014. The comet, discovered by comet hunter Rob McNaught from Siding Spring Observatory in New South Wales, Australia on January 3, 2013, shows a bright coma and well-developed tail. Credit: Joseph Brimacombe — Comet C/2013 A1 (Siding Spring) on July 11, 2014. The comet, discovered by comet hunter Robert McNaught from Siding Spring Observatory in New South Wales, Australia on January 3, 2013, shows a bright coma and well-developed tail. Credit: Joseph Brimacombe

No one’s worried about the tiny nucleus doing any damage. It’ll zip right by. Rather it’s dust particles embedded in vaporizing ice that concern NASA planners. Dust spreads into a broad tail that could potentially brush Mars’ upper atmosphere and strike an orbiter. A single particle of debris half a millimeter across may not seem like your mortal enemy, but when it’s traveling at 35 miles (56 km) per second relative to the spacecraft, one hit could spell trouble.

“Three expert teams have modeled this comet for NASA and provided forecasts for its flyby of Mars,” explained Rich Zurek, chief scientist for the Mars Exploration Program at NASA’s Jet Propulsion Laboratory in Pasadena, California. “The hazard is not an impact of the comet nucleus, but the trail of debris coming from it. Using constraints provided by Earth-based observations, the modeling results indicate that the hazard is not as great as first anticipated. Mars will be right at the edge of the debris cloud, so it might encounter some of the particles — or it might not.”

The agency’s taking a prudent approach. NASA currently operates the Mars Reconnaissance Orbiter (MRO) and Mars Odyssey spacecraft with a third orbiter, MAVEN, currently on its way to the planet and expected to settle into orbit a month before the comet flyby. Teams operating the orbiters plan to have all spacecraft positioned on the opposite side of Mars when the comet is most likely to pass by.

Already, mission planners tweaked MRO’s orbit on July 2 to move it toward a safe position with a second maneuver to follow on August 27. A similar adjustment is planned for Mars Odyssey on August 5 and October 9 for the Mars Atmosphere and Volatile Evolution (MAVEN) probe. The time of greatest risk to the spacecraft is brief – about 20 minutes – when the widest part of the comet’s tail passes closest to the planet.

Will dust shed by the comet streak as meteors in the Martian sky on October 19? The rovers will be watching. Credit: NASA/JPL

One question I’m always asked is whether the Mars rovers are in any danger of dust-producing meteors in the comet’s wake. While the planet might get peppered with a meteor shower, its atmosphere is thick enough to incinerate cometary dust particles before they reach the surface, not unlike what happens during a typical meteor shower here on Earth. Rover cameras may be used to photograph the comet before the flyby and to capture meteors during the comet’s closest approach.

Despite concerns about dust, NASA knows a good opportunity when it sees one. In the days before and after the flyby, all three orbiters will conduct studies on the comet.

According to a recent NASA press release, instruments on MRO and Odyssey will examine the nucleus, coma and tail and possible effects on the Martian atmosphere:

Comet Siding Spring observed by the Spitzer Space Telescope in two wavelengths of infrared light in March 2014. The hint of blue-white corresponds to dust, red-orange to gas. Credit: NASA/JPL-Caltech/M. Kelley (Univ. Maryland)

“Odyssey will study thermal and spectral properties of the comet’s coma and tail. MRO will monitor Mars’ atmosphere for possible temperature increases and cloud formation, as well as changes in electron density at high altitudes and MAVEN will study gases coming off the comet’s nucleus as it’s warmed by the sun. The team anticipates this event will yield detailed views of the comet’s nucleus and potentially reveal its rotation rate and surface features.”

This is Comet Siding Spring’s first trip to the inner solar system. Expect exciting news as we peer up close at pristine ices and dust that have been locked in deep freeze since the time the planets formed.

For more information on the event, check out this NASA website devoted to the comet.

July 21, 2014December 23, 2015 by Jason Major

Once Upon a Time There Was a Spacecraft Called Rosetta…

The road to a comet isn't an easy one! Luckily Rosetta and Philae have a lot of help.

…and that time is now! ESA’s Rosetta spacecraft is just over a mere two weeks away from its arrival at Comet 67P/Churyumov-Gerasimenko (which has recently surprised everyone with its binary “rubber duckie” shape) and the excitement continues to grow — and rightfully so, since after ten years traveling through the Solar System Rosetta is finally going to achieve its goal of being the first spacecraft to orbit a comet!

As part of the “Are We There Yet” campaign to encourage public participation in this historic space exploration event, ESA has released the next installment of Rosetta’s story in adorable animated format. Check it out above, and feel free to fall in love with a solar-powered spacecraft.

Keep up with Rosetta’s journey on the ESA website here, and enter the #RosettaAreWeThereYet contest by sharing your photos here (you could win a trip to ESA’s Operations Center in Darmstadt, Germany in November for Philae’s landing party!)

Video: ESA

July 17, 2014December 23, 2015 by Bob King

Rosetta Zooms Toward an Extraordinary Comet

Comet 67P/C-G photographed on July 14, 2014 from a distance of approximately 12 000 km. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

I thought the photos earlier this week were amazing. This little movie, made of 36 ‘smoothed’ or interpolated images of Comet 67P/Churyumov-Gerasimenko, takes it to the next level, showing the comet’s complex shape even more clearly as Rosetta nudges ever closer to its target. Some have likened it to a duck, a boot and even a baby’s foot. The original photos used for the animation were more pixelated, but a technique known as “sub-sampling by interpolation” was used to smooth out the pixels for a more natural look. Be aware that because of processing, 67P C-G appears smoother than it might be. While the surface looks textured, including what appears to be a small crater atop the duck’s head, we have to be careful at this stage not to over-interpret – some of the details are artifacts.

Raw pixelated image of the comet (left) and after smoothing. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

No one knows yet how such an unusual shape formed in the first place. Possibly the comet is a ‘contact binary’ made of two separate comets or two parts of larger, shattered comet that stuck together during a low-velocity collision. This may have happened more 4 billion years ago when the icy building blocks of the planets and comets were numerous and collisions far more frequent than they are today. Contact binaries aren’t uncommon; we see them in asteroids and comets alike.

The Rosetta blog lists other intriguing scenarios:

* The comet may have once been a more spherical object but after many trips around the sun developed an asymmetrical shape from ice vaporization and outgassing.

* A near-catastrophic impact blasted away a huge chunk of comet ice.

* The strong gravitational pull experienced during a close pass of a large planet like Jupiter or Saturn may have pulled it into an irregular shape.

* A large outburst could have weakened a region on the comet’s surface that later crumbled away.

Detailed view of the likely contact binary asteroid 25143 Itokawa visited by the Japanese spacecraft Hayabusa in 2005. Credit: JAXA

“We will need to perform detailed analyses and modelling of the shape of the comet to determine how best we can fly around such a uniquely shaped body, taking into account flight control and astrodynamics, the science requirements of the mission, and the landing-related elements like landing site analysis and lander-to-orbiter visibility,” said Rosetta Mission Manager Fred Jansen. ” But with fewer than 10,000 km to go before the August 6th rendezvous, our open questions will soon be answered.”

In the meantime, keep the photos and movies coming. We can’t get enough.