67P/Churyumov-Gerasimenko Archives

November 7, 2014December 23, 2015 by Tim Reyes

A Comet’s Tale – Rosetta’s Philae, Five Days from Touchdown

Rosetta, the scientific mission to explore a comet's surface. "Ambition", a short Sci-Fi film, set in the near future, and Rosetta, the children's fable, to encourage the next generations to undertake on the great adventures still to come. (Photo Credits: ESA, Platige Image, ESA Communications)

In the recently released Rosetta short film called “Ambition”, the master begins a story to his apprentice – “Once upon a time.” The apprentice immediately objects to his triteness. But he promises that it is worth the slight tribulation. Who could have imagined ten years ago that Rosetta would become so successful in two such contrasting approaches to telling a tale.

The Rosetta mission is part franchise and part scientific mission. In five days, Rosetta will reach a crossroad, a point of no return as epic as moments in Harry Potter or Lord of the Rings. A small mindless little probe called Philae will be released on a one-way trip to the surface of a comet. Win or lose, Philae will live on in the tale of a comet and a mission to uncover the mysteries of our planet’s formation.

ESA did not promise a good mission as Aidan Gillen promises a good story in Ambition. A space mission is never put in terms of a promise but rather it is thousands of requirements and constraints that formulate a mission plan and a spacecraft design. The European Space Agency put 1 billion Euros ($1.3 billion) to work and did so in what now looks like one of the greatest space missions of the first century of space exploration.

The Rosetta mission is actually two missions in one. There is the comet chaser, the orbiter – Rosetta and then the lander Philae. The design of Rosetta’s objectives is some part, probably in large part, was conceived by dismissing the presence of Philae. Make a space probe to a comet that just orbits the small body. Select your scientific instrumentations accordingly. Now add a small lander to the mission profile that will do something extraordinary – what Rosetta cannot do with its instrumentation. Finally, make sure that Rosetta has everything needed to support Philae’s landing on a comet.

Here is what they have as the game plan on November 12^th (the sequence of events begins while its still November 11^th in the Americas). These two times are absolutely non- trivial. They are finely tuned to a timepiece called 67P/Churyumov–Gerasimenko. If calculations were made in error, then Philae’s ultimate fate is unknown. Start exactly on time and Philae will be given the best chance at making a successful touchdown on the comet.

Separation of Philae from Rosetta: 09:03 GMT (10:03 CET)

Touchdown on the comet: 16:02 GMT (17:02 CET).

During this time, comet 67P/Churyumov–Gerasimenko will complete over half a rotation on its axis. To be exact, it will rotate 56.2977% of a full rotation. Comet 67P will have its back turned towards Rosetta as it holds the diminutive Philae for the last time and releases Philae for the first and only time.

Now that the ESA, with help from the graphic artists from Platige Image from Poland, has released something entertaining for the science fiction minded among us, they have again released a next episode in their children’s fable of Rosetta and Philae (video below). This cartoon of the final moments of Rosetta and Philae together preparing for the descent which could well be the final moments of Philae.

Philae could fail, crack like an egg on a sharp rock or topple over a cliff or into a crevasse on the surface of 67P. What happens to Philae will make for a Grimm’s fairy tale ending or something we would all prefer. In either case, the ESA is using graphic arts and storytelling to inspire the next generations to join in what our JFK called “great adventures of all time” [ref].

Through a contest something NASA and JPL have used several times to involve the public, the ESA asked the public to come up with a name for the landing site, site J. Out of the thousands of entries, 150 people suggested the name Agilkia [ref]. Alexandre Brouste from France, the designated winner, has been invited to watch the landing activities at Rosetta’s mission control in Darmstadt, Germany. It follows from the Eqyptian theme of the mission’s two probes. “Rosetta” comes from the clay tablet discovered in the 1800s that led to the deciphering of Egyptian hieroglyphics. Philae” is a island on the Nile which held magnificent Eqyptian temples. With the operation of the Aswan dam starting in 1902, the island of Philae was repeatedly flooded and the temple was at risk. UNESCO beginning in 1960 started a project to save the islands historic structures. They were all moved to a nearby Nile island called Agilkia [related U.T. article]. This becomes a part of the Rosetta story – a lander named Philae in reference to the obelisks used along with the Rosetta stone to decipher Eqyptian writings, departing its mother ship on a short but critical voyage to a final resting place, the landing site now called Agilkia.

Upon landing, a landing confirmation signal is expected from Philae via Rosetta at about 8:02 AM PST (11:02 AM EST, 17:02 Central European Time). Alexandre Brouste of France, the designated winner of the landing site naming contest will be in Darmstadt, Germany in mission control to watch the landing unfold with the Rosetta engineers and scientists. Surely, millions of citizens of the European Union and people worldwide will be watching via the World Wide Web.

The timeline and events to unfold as Philae, the lander is released from Rosetta, the comet orbiter. (Illustration Credit: ESA)

Previous Rosetta and Philae articles at Universe Today

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

Why Watch ESA Rosetta’s Movie ‘Ambition’? Because We Want to Know What is Possible

October 24, 2014December 23, 2015 by Elizabeth Howell

Stinky! Rosetta’s Comet Smells Like Rotten Eggs And Ammonia

A view of Comet 67P/Churyumov-Gerasimenko on Sept. 26, 2014 from the orbiting Rosetta spacecraft. Credit: ESA/Rosetta/NAVCAM

While you can’t smell in space — there is no medium to carry the molecules, the same reason you can’t hear things — you can certainly detect what molecules are emanating from comets and other solar system bodies. A new analysis of Comet 67P/Churyumov-Gerasimenko by the orbiting Rosetta spacecraft thus found a rather pungent chemistry combination.

The spacecraft detected hydrogen sulphide (the smell of rotten eggs), ammonia and formaldehyde with traces of hydrogen cyanide and methanol. But compared to the amounts of water and carbon monixide 67P has, these molecule concentrations are quite miniscule.

“This all makes a scientifically enormously interesting mixture in order to study the origin of our solar system material, the formation of our Earth and the origin of life,” stated the University of Bern’s Kathrin Altwegg, from the center of space and habitability.

“And after all: it seems like comet Churyumov was indeed attracted by comet Gerasimenko to form Churyumov-Gerasimenko, even though its perfume may not be Chanel No 5, but comets clearly have their own preferences.”

More seriously, astronomers do say that at three astronomical units (Earth-Sun distances) from the Sun, the comet is emitting more molecules than expected. The next step will be to compare Rosetta’s data with ground-based data of other comets to see if this is common.

Source: University of Bern

October 23, 2014December 23, 2015 by Elizabeth Howell

Rosetta’s Comet Springs Spectacular Leaks As It Gets Closer To The Sun

This Rosetta image of Comet 67P/Churyumov-Gerasimenko shows spectacular jets erupting from the small body on Sept. 10, 2014. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Jet! The comet that the Rosetta spacecraft is visiting is shedding more dust as machine and Solar System body get closer to the Sun.

While activity was first seen at the “neck” of the rubber-duckie shaped comet a few weeks ago, now scientists are seeing jets spring from across the comet.

This is just one signal of cometary activity picking up as 67P gets closer to the Sun. For the moment, it appears the prime landing site is still safe enough for Philae to land on Nov. 19, officials said, while noting there is a jet about a kilometer away that the lander can study when it gets there.

Jets spring from the "neck" area of Comet 67P/Churyumov-Gerasimenko. The smaller lobe is on the left, and the larger on the right. These images were taken about 7.2 kilometers (4.5 miles) from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA — Jets spring from the “neck” area of Comet 67P/Churyumov-Gerasimenko. The smaller lobe is on the left, and the larger on the right. These images were taken about 7.2 kilometers (4.5 miles) from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“At this point, we believe that a large fraction of the illuminated comet’s surface is displaying some level of activity,” stated Jean-Baptiste Vincent a scientist from the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) that took the pictures. He is with the Max Planck Institute for Solar System Research in Germany.

The comet is about 470 million kilometers (292 miles) from the Sun and will make its closest approach in 2015. Rosetta is the first mission to orbit a comet as it gets close to the Sun, and Philae (if successful) will make the first “soft” landing on a cometary surface.

Source: European Space Agency

October 15, 2014December 23, 2015 by Jason Major

Here’s a High-Res Look at Philae’s Landing Spot

Mosaic of OSIRIS images of landing site "J" on Comet 67P/CG. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The long-awaited deployment of the Philae lander, currently “piggybacked” aboard ESA’s Rosetta spacecraft orbiting the nucleus of Comet 67P/Churyumov-Gerasimenko, will occur in less than a month and we now have our best look yet at the area now green-lighted for touchdown. The picture above, made from two images acquired by Rosetta’s OSIRIS imaging instrument, shows a 500-meter circle centered on “Site J,” a spot on the comet’s “head” carefully chosen by mission scientists as the best place in which Philae should land, explore, and ultimately travel around the Sun for the rest of its days. And as of today, it’s a GO!

Site J was selected from among five other possible sites and was chosen because of the relative safety of its surface, its accessibility to consistent solar illumination, and the scientific and observational data it can make available to Philae’s suite of onboard instruments.

“None of the candidate landing sites met all of the operational criteria at the 100% level, but Site J is clearly the best solution,” said Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center.

Illustration of the Rosetta Missions Philae lander on final approach to a comet surface. The date is now set for landing, November 12. (Photo: ESA)

The mosaic above comprises two images taken by Rosetta’s OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) narrow-angle camera on Sept. 14 from a distance of about 30 km (18.6 miles). Image scale is 0.5 m/pixel.

As Comet 67P/CG continues toward perihelion its outgassing and sublimation jetting will undoubtedly increase, and Philae will be getting a front-row seat to the action.

“Site J is just 500-600 meters away from some pits and an area of comet outgassing activity,” said Holger Sierks, principal investigator for Rosetta’s OSIRIS camera from the Max Planck Institute for Solar System Research in Gottingen, Germany. “They will become more active as we get closer to the Sun.”

Watch “Landing on a Comet: the Trailer”

After completing a series of “Go/No-go” decisions by Rosetta’s flight dynamics team, Philae’s separation from Rosetta will occur on Nov. 12 at 08:35 GMT. It will land about seven hours later at around 15:30 GMT. Because of the distance to the comet and spacecraft — about 509 million km — confirmation of a successful touchdown won’t be received on Earth until 28 minutes and 20 seconds later. (And you thought Curiosity’s “seven minutes of terror” was nerve-wracking!)

Creepy Comet Looms In The Background Of Newest Philae Spacecraft Selfie

The Rosetta spacecraft takes a selfie Oct. 7 with its target, 67P/Churyumov–Gerasimenko, from an altitude of about 9.9 miles (16 kilometers). Credit: ESA/Rosetta/Philae/CIVA

So this spacecraft — taking this picture — is going to land on the surface of THAT comet. Doesn’t this give you a pit in your stomach? This is a selfie taken from the Philae spacecraft that, riding piggyback, captured the side of the Rosetta spacecraft orbiting Comet 67P/Churyumov-Gerasimenko.

The image is so close-up — just 9.9 miles (16 kilometers) from 67P’s surface — that mission planners can even spot Landing Site J on the comet’s smaller lobe.

“Two images, one with a short exposure time, one with a longer one, were combined to capture the whole dynamic range of the scene, from the bright parts of the solar arrays to the dark comet and the dark insulation cladding the Rosetta spacecraft,” the European Space Agency stated.

It’s quite the zoom-in after the last selfie that Philae produced for the public in September, which was taken from 31 miles (50 kilometers) away. The spacecraft is expected to make the first touchdown ever on a comet next month. Rosetta, meanwhile, will keep following 67P as it gets closest to the sun in 2015, between the orbits of Earth and Mars.

Tomorrow (Oct. 15), mission managers will announce if Site J is go or no go for a landing. More information is coming from Rosetta’s examination of the site from its new, lower altitude of 6.2 miles (10 kilometers).

Source: European Space Agency

October 3, 2014December 23, 2015 by Bob King

Rosetta’s Comet in Thrilling 3-D

3D view created by Mattias Malmer of the recent ESA image (below) showing multiple jets of gas and dust spraying from Comet 67P/Churyumov-Gerasimenko. Grab your red-blue plastic glasses and prepare to enter another dimension. Malmer created the view by draping a navigation camera image over a 3D model of the comet and then photographing it from two slightly different perspectives. Click for large version. Credit: ESA/Rosetta/NAVCAM/processing by Mattias Malmer

She’s gonna blow! Rosetta’s navigation camera recently grabbed our best view yet of the geyser-like jets spraying from the nucleus of Comet 67P/Churyumov-Gerasimenko. They were taken on September 26 as the spacecraft orbited the comet at a distance of just 16 miles (26 km) and show jets of water vapor and dust erupting from several discrete locations beneath the surface along the neck region of the comet’s nucleus. Mattias Malmer, a 3D technical director, created the spectacular 3D views by draping the navigation camera images over a 3D model of the comet and then photographing it from two slightly different perspectives.

Jets of gas and dust are seen escaping comet 67P/C-G on September 26 in this four-image mosaic. Click to enlarge. Credit: ESA/Rosetta/NAVCAM

Jets form when the sun warms the comet’s coal-black surface, causing ices beneath to sublimate or change directly from solid to gas without becoming liquid. This is possible because of the near-zero atmospheric pressure at the comet. Pressure builds in the pockets of gas until they find escape through cracks or pores as plume-like jets. Comet dust along with the gas fashions the coma and tail over time. Something similar happens when you shake up a bottle of champagne and then loosen the cork. Trapped carbon dioxide (what makes the “fizz”) blasts the cork across the room.

Comet Churyumov-Gerasimenko rotating from darkness into light. (Mattias Malmer)

If you liked the still images, check out these videos by Malmer. He used the same draping technique and then animated the stills. Be sure to stop by his Cascade of Light blog for more images and videos when you get a chance.

Comet Churyumov-Gerasimenko rotating in 3D (Mattias Malmer)

I saved the best for last. What majesty!

3D rotation of Comet 67P/C-G with jets (Mattias Malmer)

September 15, 2014December 23, 2015 by Ken Kremer

Comet’s Head Selected as Landing Site for Rosetta’s Historic Philae Lander

Context image showing the location of the primary landing site for Rosetta’s lander Philae. Site J is located on the head of Comet 67P/Churyumov–Gerasimenko. An inset showing a close up of the landing site is also shown. The inset image was taken by Rosetta’s OSIRIS narrow-angle camera on 20 August 2014 from a distance of about 67 km. The image scale is 1.2 metres/pixel. The background image was taken on 16 August from a distance of about 100 km. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The ‘head’ of the bizarre comet 67P/Churyumov-Gerasimenko has been selected as the primary landing site for the Rosetta spacecraft’s attached Philae lander, attempting mankind’s first ever landing on a comet in mid-November.

Scientists leading the European Space Agency’s Rosetta mission announced the primary landing site at a media briefing today, Sept. 15, at ESA headquarters.

After weeks of detailed study and debate focused on balancing scientific interest with finding a ‘technically feasible’ and safe Philae touchdown site, the team chose a target dubbed Site J as the primary landing site from among a list of five initially selected sites, said Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center, at the briefing.

“Site J is the primary landing site around the head of the comet,” Ulamec announced.

“Site C is the backup site on the body [near the bottom of the comet].”

“This was not an easy task. Site J is a mix of flat areas and rough terrain. It’s not a perfectly flat area. There is still risk with high slope areas.”

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

He also made clear that there is still some landing uncertainty with the targeting of the lander onto the comet.

Site J is an intriguing region on Comet 67P/Churyumov–Gerasimenko that offers unique scientific potential, with hints of activity nearby, and minimum risk to the lander compared to the other candidate sites, according to ESA.

“As we have seen from recent close-up images, the comet is a beautiful but dramatic world – it is scientifically exciting, but its shape makes it operationally challenging,” says Ulamec.

“None of the candidate landing sites met all of the operational criteria at the 100% level, but Site J is clearly the best solution.”

Philae’s history-making landing on comet 67P is currently scheduled for around Nov. 11, 2014, and will be entirely automatic. The 100 kg lander is equipped with 10 science instruments.

“All of Rosetta’s instruments are supporting the landing site selection,” said Holger Sierks, principal investigator for Rosetta’s OSIRIS camera from the Max Planck Institute for Solar System Research in Gottingen, Germany.

“Site J is just 500-600 meters away from some pits and an area of comet outgassing activity. They will become more active as we get closer to the sun.

The team is in a race against time to select a suitable landing zone quickly and develop the complex landing sequence since the comet warms up and the surface becomes ever more active as it swings in closer to the sun and makes the landing ever more hazardous.

Since the descent to the comet is passive it is only possible to predict that the landing point will place within a ‘landing ellipse’ typically a few hundred metres in size, the team elaborated.

The three-legged lander will fire two harpoons and use ice screws to anchor itself to the 4 kilometer (2.5 mile) wide comet’s surface. Philae will collect stereo and panoramic images and also drill 20 to 30 centimeters into and sample its incredibly varied surface.

“We will make the first ever in situ analysis of a comet at this site, giving us an unparalleled insight into the composition, structure and evolution of a comet,” says Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.

“Site J in particular offers us the chance to analyse pristine material, characterise the properties of the nucleus, and study the processes that drive its activity.”

“It’s amazing how much we have learned so far.”

“We are in a true revolution of how we think Planets form and evolve,” Bibring elaborated at the briefing.

“We will make many types of scientific measurements of the comet from the surface. We will get a complete panoramic view of the comet on the macroscopic and microscopic scale.”

Rosetta is currently orbiting the comet from a distance of 30 km, said ESA Rosetta flight director Andrea Accomazzo. He said it will likely go even closer to 20 km and perhaps 10 km.

Four-image photo mosaic comprising images taken by Rosetta's navigation camera on 2 September 2014 from a distance of 56 km from comet 67P/Churyumov-Gerasimenko. The mosaic has been contrast enhanced to bring out details of the coma, especially of jets of dust emanating from the neck region. Credits: ESA/Rosetta/NAVCAM/Marco Di Lorenzo/Ken Kremer - kenkremer.com — Four-image photo mosaic comprising images taken by Rosetta’s navigation camera on 2 September 2014 from a distance of 56 km from comet 67P/Churyumov-Gerasimenko. The mosaic has been contrast enhanced to bring out details of the coma, especially of jets of dust emanating from the neck region. Credits: ESA/Rosetta/NAVCAM/Marco Di Lorenzo/Ken Kremer – kenkremer.com

“Now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites,” says ESA Rosetta flight director Andrea Accomazzo.

“Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that’s what makes this a risky operation.”

Four-image photo mosaic comprising images taken by Rosetta's navigation camera on 31 August 2014 from a distance of 61 km from comet 67P/Churyumov-Gerasimenko. The mosaic has been rotated and contrast enhanced to bring out details. The comet nucleus is about 4 km across. Credits: ESA/Rosetta/NAVCAM/Ken Kremer/Marco Di Lorenzo — Four-image photo mosaic comprising images taken by Rosetta’s navigation camera on 31 August 2014 from a distance of 61 km from comet 67P/Churyumov-Gerasimenko. The mosaic has been rotated and contrast enhanced to bring out details. The comet nucleus is about 4 km across. Credits: ESA/Rosetta/NAVCAM/Ken Kremer/Marco Di Lorenzo

The final landing site selections were made at a meeting being held this weekend on 13 and 14 September 2014 between the Rosetta Lander Team and the Rosetta orbiter team at CNES in Toulouse, France.

“No one has ever attempted to land on a comet before, so it is a real challenge,” says Fred Jansen, ESA Rosetta mission manager.

“The complicated ‘double’ structure of the comet has had a considerable impact on the overall risks related to landing, but they are risks worth taking to have the chance of making the first ever soft landing on a comet.”

Five candidate sites were identified on Comet 67P/Churyumov-Gerasimenko for Rosetta’s Philae lander. The approximate locations of the five regions are marked on these OSIRIS narrow-angle camera images taken on 16 August 2014 from a distance of about 100 km. Enlarged insets below highlight 5 landing zones. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA Processing: Marco Di Lorenzo/Ken Kremer

Stay tuned here for Ken’s continuing Rosetta, Earth and Planetary science and human spaceflight news.

Ken Kremer

September 8, 2014December 23, 2015 by Elizabeth Howell

Rosetta’s Cloudy Comet Shroud Spotted From The Ground, While Spacecraft Picks Up Dust Grains

A composite image of Rosetta's target (Comet 67P/Churyumov–Gerasimenko) obtained by the Very Large Telescope. Credit: C. Snodgrass/ESO/ESA

This picture shows it is possible to look at Rosetta’s comet from Earth, but what a lot of work it requires! The picture you see above is a composite of 40 separate images taken by the Very Large Telescope (removing the background stars).

Despite the fact that Rosetta is right next to Comet 67P/Churyumov–Gerasimenko, ground-based observatories are still useful because they provide the “big picture” on what the comet looks like and how it is behaving. It’s an observational challenge, however, as the comet is still more than 500 million kilometers (310 million miles) from the Sun and hard to see.

On top of that, the European Space Agency says the comet is sitting in a spot in the sky where it is difficult to see it generally, as the Milky Way’s prominent starry band is just behind. But what can be seen is spectacular.

“Although faint, the comet is clearly active, revealing a dusty coma extending at least 19 000 km [11,800 miles] from the nucleus,” ESA stated. “The comet’s dusty veil is not symmetrical as the dust is swept away from the Sun – located beyond the lower-right corner of the image – to begin forming a tail.”

And that dust is beginning to show up in Rosetta’s grain collector, as you can see below!

Rosetta's dust collector, Cometary Secondary Ion Mass Analyser (COSIMA), collected its first grains from Comet 67P/Churyumov–Gerasimenko in August 2014. This image shows before and after images of the collection. Credit: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/ BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S — Rosetta’s dust collector, Cometary Secondary Ion Mass Analyser (COSIMA), collected its first grains from Comet 67P/Churyumov–Gerasimenko in August 2014. This image shows before and after images of the collection. Credit: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/
BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S

Rosetta’s Cometary Secondary Ion Mass Analyser (COSIMA) picked up several dust grains in August, which you can see in the image, and are now looking at the target plate more closely to figure out more about the dust grains.

“Some will be selected for further analysis: the target plate will be moved to place each chosen grain under an ion gun which will then ablate the grain layer by layer. The material is then analyzed in a secondary ion mass spectrometer to determine its composition,” ESA stated.

All of these results were presented today (Sept. 8) at the European Planetary Science Congress 2014.

September 5, 2014December 23, 2015 by Bob King

What Comets, Parking Lots and Charcoal Have in Common

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% range of most known comets.

Photo of Comet 67P/C-G taken by Rosetta on August 6, 2014. Credit: ESA — 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. The same charcoal, when viewed in normal light on Earth, appears black. Credit; Bob King — 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 — 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.

Hmmm … maybe it really is a giant cookie.

August 14, 2014December 23, 2015 by Elizabeth Howell

Gravity Isn’t The Only Thing Holding Asteroids Together: Study

Rubble piles are common among asteroids, as illustrated by this artist's conception of 2011 MD. Credit: NASA/JPL-Caltech

How do asteroids hold their rubble piles together? Previously, scientists said it was a combination of friction and gravity. But new observations of asteroid 1950 DA reveals something else is at work. The asteroid is rotating too quickly for gravity to keep it together, so what’s going on?

“We found that 1950 DA is rotating faster than the breakup limit for its density,” stated Ben Rozitis, a postdoctoral researcher at the University of Tennessee, Knoxville who led the research. “So if just gravity were holding this rubble pile together, as is generally assumed, it would fly apart. Therefore, interparticle cohesive forces must be holding it together.”

Cohesive forces refer to the act of individual molecules or particles sticking together. It’s the first time scientists have found this in action on an asteroid. Better yet, if confirmed in other asteroids this has implications for protecting Earth from a killer asteroid should one come our way.

If the threat turns out to be a loosely held together asteroid, an impact in just the right spot would break the single asteroid into many. (Of course, you’d want to make sure that the problem doesn’t end up turning into multiple smaller asteroids hitting Earth instead of a single large one.)

Now the researchers are interested in knowing if cohesive forces are also in action on Comet 67P/Churyumov–Gerasimenko — the comet being examined by Rosetta right now and in November, by the lander Philae.

The study was published in the journal Nature.

Source: The University of Tennessee, Knoxville