As most people know, the Rosetta spacecraft will be ending its mission to comet 67P/CG this month. If all goes as planned, Rosetta will brake and execute a slow dive, shooting back as much data as possible before impact.
Rosetta has been circling in, closer and closer to the surface, but on 29 September it will make its final monuevre (ESA prefers the British spelling), initiating a “free-fall slowly towards the comet” for about 20 km.
The project team has chosen to aim for one of the “active pits”, identified as a source of gas and dust. The final descent will pick up very close range observations of one such pit, to learn as much as possible.
This should be an exciting and fitting ending for Rosetta.
The Rosetta mission to comet 67P/CG is nearing the end. On September 30 the spacecraft will dive to the surface, collecting as much close up data as possible on the way. This planned crash will end communication with the spacecraft, and terminate the mission. Rosetta will go out with a splash (though on a comet, a “splash” is a slow, cold event!)
This week Rosetta also brought a close to the dramatic story of the plucky little lander, Philae. The Philae lander failed to grapple on landing as intended (with almost no gravity, “landing” required grabbing on), bounced wildly, and ended up lost in a shady crevasse, where it never could recharge its batteries to stay alive.
As Rosetta orbits closer and closer to the comet, it has been grabbing higher and higher resolution imagery. Currently at a mere 2.7 km from the surface, Rosetta can image with a resolution of 5cm per pixel. Aided by the shifting angle of the sun as the comet loops outward, the camera has finally caught a clear image of the lander, lying on its side in the dark. (This identification is aided by the fact that there can’t be anything else even remotely resembling Philae on the surface of this comet!)
As ESA says, the image makes clear why communication and recharging were so difficult: it is on its side, and nestled in a crevasse. Confirmation of the location and orientation of the lander will solidify understanding of the limited data that was returned from the surface.
So, in a couple of weeks, it will be “adieu” to Rosetta. The orbiter and the lander will remain on the surface of comet 67P/CG, frozen and inert, until the comet breaks up (or until we send another mission there and retrieve them).
Juno is an interesting and ambitions mission, because it aims to get close to Jupiter, inside the intense radiation and magnetic fields. The spacecraft and instruments had to be really, really rugged and radiation hardened. The spacecraft has entered an elliptical polar orbit, which is designed to swoop in via the reduced radiation, and back out to a “cooler” distance.
Assuming that the spacecraft and instruments survive, which is far from certain, Juno will collect some of the first data about the structure, composition, and dynamics of Jupiter. Jupiter is huge, darn near a star, and it has been around since the solar system condensed. So we will get some more insight into the early solar system, including more information about where the water we find everywhere came from.
Fingers crossed, Juno will do 37 orbits over the next 20 months. When the mission is over, the spacecraft will be deliberately crashed into Jupiter. Unlike Rosetta’s planned crash into 67P/CG, which will collect data, Juno’s dive is intended to destroy any possible biological contamination from Earth. This precaution recognizes the possibility of habitable (if not necessarily inhabited) moons in the area. Future expeditions do not want to find “life” on Europa that somehow managed to ride there on Juno. However unlikely this scenario, Juno will burn up to avert it.
As I already noted, it was one year ago that the Philae lander tumbled to a landing on 67P/CG. If you haven’t looked at it, check out the reconstruction and review on the Rosetta blog, “Reconstructing Philae’s flight across the comet”.
No project is ever perfect, and no project is ever finished. But all projects come to an end. The Rosetta mission will end in about 10 months.
As 67P/CG continues out away from the sun, the amount of solar power available for the orbiter decreases. In addition, dust has degraded the solar cells.
Power is life, and Rosetta is running out. In a few months, it will be impossible to operate all the instruments at the same time, and eventually the transmitter will go out.
She is also running out of fuel for maneuvering. And, in fact, everything is aging and will start failing.
And finally, the farther she is, the harder it is to receive her signals, and the less data that can come back, even if it can be collected.
In short, it’s game over.
Facing these facts, the Rosetta team plans to do a final dive, circling in to a crash landing while the spacecraft still has power and capability. On this final swoop, she will suck up as much data as possible, from closer and closer, blasting it back to Earth. Pretty much a triumphant, screaming, last hurrah.
At the end, the probe will crash into the surface and that will be that.
Someday, someone might visit 67P/CG and find the frozen corpses of Rosetta and Philae. But we’ll hear no more from them in our lifetimes.
With the unplanned and uncontrolled crash landing, Philae completed only the first few hours of exploration before running out of power and settling into hibernation. No effort to contact Philae has succeeded, but now, as the comet recedes from the sun, Rosetta is closing in on the rapidly cooling comet for one last close up look and a final deliberate plunge to the surface http://blogs.esa.int/rosetta/2015/11/12/from-one-comet-landing-to-another-planning-rosettas-grand-finale/. If Philae has survived the cold and then the heat and turmoil of perihelion, we may get one last bit of information about her.
As noted last week, a special issue of Astronomy & Astrophysics is devoted to reports of results from the Rosetta mission to 67P/CG. Many of these papers are based on data collected as Rosetta approached 67P/CG last spring, so there will be many more reports from the close observations this summer.
In the Thomas et al. paper the most interesting thing for me, is how mundane but alien the low gravity, thin atmosphere, very cold surface is. The imagery shows, for instance, ripples that resemble wind blown dust or sand on Earth. In this, we recognized that 67P/CG is a world just like where we live.
But the analysis indicates that these familiar looking formations probably formed through different processes than where we live. The spewing gasses from the boiling comet are very thin, but may reach 500m/s: powerful enough to ‘blow around” dust, at least close up.
Even more interesting, the investigators hypothesis that “On Earth, the major force to overcome when sculpting wind-blown ripples is gravity holding the grains in place. On the comet, where gravity is minuscule, the major hurdle is the cohesive forces between the dust grains holding them together.” We see something familiar, but the physics is completely different.
See the paper for more details on the careful thought, the methods and evidence, and discussion of uncertainties. It’s actually really cool. (You local library can help you get access to the paper.)
Thomas, N., B. Davidsson, M. R. El-Maarry, S. Fornasier, L. Giacomini, A. G. Gracia-Berná, S. F. Hviid, W. H. Ip, L. Jorda, H. U. Keller, J. Knollenberg, E. Kührt, F. La Forgia, I. L. Lai, Y. Liao, R. Marschall, M. Massironi, S. Mottola, M. Pajola, O. Poch, A. Pommerol, F. Preusker, F. Scholten, C. C. Su, J. S. Wu, J. B. Vincent, H. Sierks, C. Barbieri, P. L. Lamy, R. Rodrigo, D. Koschny, H. Rickman, M. F. A’Hearn, M. A. Barucci, J. L. Bertaux, I. Bertini, G. Cremonese, V. Da Deppo, S. Debei, M. de Cecco, M. Fulle, O. Groussin, P. J. Gutierrez, J. R. Kramm, M. Küppers, L. M. Lara, M. Lazzarin, J. J. Lopez Moreno, F. Marzari, H. Michalik, G. Naletto, J. Agarwal, C. Güttler, N. Oklay, and C. Tubiana, Redistribution of particles across the nucleus of comet 67P/Churyumov-Gerasimenko. A&A, 583 11// 2015. http://dx.doi.org/10.1051/0004-6361/201526049
One of the surprises of the Rosetta mission to comet 67P/CG was the discovery of the unusual double lobed shape of the comet. Distant observations of a small body could never give enough detail to reveal this, only by visiting it could we see the shape.
This discovery posed a question for geologists: how did the comet attain this configuration. There are several candidate explanations, formation from two blobs, erosion of one body, or a collision of two bodies (in something like that order of “a priori” likelihood according to earlier understanding).
To decipher this question, the team examined the detailed imagery and data from reported by Rosetta. This data is very good, with a resolution up to .1m per pixel, and includes three dimensional shape maps and measurements of local gravity and other phenomena. From this data, it was possible to identify and map strata that reveal the geological history of the comet.
The team concluded that 67P/CG was once two separate (more or less spherical) bodies, each with “onion layers” of strata. At some point in time, the two bodies collided at low speed, mooshing together without smashing one or both.
This conclusion specifically rules out the idea that there was one larger comet that has eroded or distorted over time, which was probably the first guess of most observers.
This conclusion is supported by considerable evidence and some very cool (if complicate) computational modelling, as explained in a letter published in Nature (ref 1 below, full text available from most major libraries). Notably, one source of insight is the local gravity vectors, which are consistent with two different sets of strata. Interesting.
These results were described in an ESA blog, which generated considerable commentary disputing the conclusion and arguing for the hypothesis that 67P/CG was a single body, distorted and eroded to the current shape.
This alternative interpretation was mainly based on feature matching from the Rosetta imagery, which are seen by some to suggest continuous features that extend across the central fissure.
This controversy was addressed in another blog entry by Dr Matteo Massironi of the University of Padova, Italy who led the published study. His remarks are mostly patient and polite, if not especially respectful. He stands by the expert analysis, and points out that the blog commentary relied on faulty data, incorrect interpretation of imagery, and questionable arguments. Massironi restrained himself from openly saying, “please read the damn paper”, although he did advise that one should learn some damn science.
Massironi points out that the Rosetta team was well aware of the alternative explanations, and because of the controversial conclusions they marshaled multiple sources of evidence.
“Due to the controversial implications that the onion-like contact binary raises, we tried to find other lines of evidence that might undermine what was apparent from the former observations. This is why, from the best fitting planes, we passed to the geological sections and afterwards worked on the angular relationships between strata and the local gravity vectors. All these independent observations based on primary structures support the view in which the comet derives from a contact binary of two comets with an onion-like interior.”
I don’t know if this satisfies the blogosphere, but it gives me some confidence.
Again, we see here the difference between real science and Hollywood/Internet science. You can’t just look at some pictures and tell a story. You need to actually learn some science, and do some serious work with the data.
Mateo Massironi, Emanuele Simioni, Francesco Marzari, Gabriele Cremonese, Lorenza Giacomini, Maurizio Pajola, Laurent Jorda, Giampiero Naletto, Stephen Lowry, Mohamed Ramy El-Maarry, Frank Preusker, Frank Scholten, Holger Sierks, Cesare Barbieri, Philippe Lamy, Rafael Rodrigo, Detlef Koschny, Hans Rickman, Horst Uwe Keller, Michael F. A/’Hearn, Jessica Agarwal, Anne-Therese Auger, M. Antonella Barucci, Jean-Loup Bertaux, Ivano Bertini, Sebastien Besse, Dennis Bodewits, Claire Capanna, Vania Da Deppo, Bjorn Davidsson, Stefano Debei, Mariolino De Cecco, Francesca Ferri, Sonia Fornasier, Marco Fulle, Robert Gaskell, Olivier Groussin, Pedro J. Gutierrez, Carsten Guttler, Stubbe F. Hviid, Wing-Huen Ip, Jorg Knollenberg, Gabor Kovacs, Rainer Kramm, Ekkehard Kuhrt, Michael Kuppers, Fiorangela La Forgia, Luisa M. Lara, Monica Lazzarin, Zhong-Yi Lin, Jose J. Lopez Moreno, Sara Magrin, Harald Michalik, Stefano Mottola, Nilda Oklay, Antoine Pommerol, Nicolas Thomas, Cecilia Tubiana, and Jean-Baptiste Vincent, Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature, advance online publication 09/28/online 2015. http://dx.doi.org/10.1038/nature15511
As I commented earlier, we can expect a flood of science results from Rosetta this fall and winter. Unlike Hollywood, we don’t get back instant results, nor is every discovery a photogenic movie. We got back data that must be carefully analyzed and then reported in conferences and articles. That takes time, but we know when the conferences are going to happen so we know when results will appear. (The raw date will be available for everyone as the publications come out.)
Indeed, we have seen the first of these publications in the last few weeks, too many for me to keep up with, including, “How Rosetta’s comet got its shape” (spoiler alert: there was a low speed collision of two icy balls).
Skimming through the blog, I picked one to look at in more detail. One of the more interesting things about visiting a comet is how it changes as it approached (and leaves) perihelion. Several papers will discuss the atmosphere and magnetosphere of 67P/CG, but we also have months worth of images of the surface, which reveal the rapid change as things heated up.
As reported in a letter in Astronomy & Astrophysics , the optical imaging detected the rapid development and growth of surface features (craters? crevasses? Terran terminology probably doesn’t apply). This is one of the first and certainly the most detailed observation of such processes ever achieved.
The changes are consistent with collapsing materials, presumably as underlying ice and other volatile material heats in the sun, melts and boils. The changes appear to expose other materials including what may be ice. They proceed in an organized wave that moves as much as 10 cm per day (which is pretty fast for erosion!)
The investigation is incomplete. No jets of particles was spotted by the optical imagery, which leaves open the question of where the material went. The phenomena were only observed in smooth regions, raising questions about the different processes that might have occurred in the rougher regions.
These studies will incorporate data from the other instruments to fill out the picture and perhaps answer some of the questions.
“The dramatic changes observed on Imhotep are a spectacular event, unique to comets, with a currently unpredictable end state.” Rosetta has given us a unique opportunity to learn something about this “spectacular event”, close up, and in detail.
Barbieri, P. Lamy, R. Rodrigo, D. Koschn, H. Rickman, H. U. Keller, M. F. A’Hearn, A.-T. Auger, M. A. Barucci, J.-L. Bertaux, I. Bertini, S. Bess, G. Cremonese, V. Da Deppo, B. Davidsson, S. Debei, M. De Cecco, M. R. El-Maarry, S. Fornasier, M. Fulle, P. J. Gutiérrez, C. Güttler, S. Hviid, W.-H Ip, L. Jorda, J. Knollenberg, G. Kovacs, J. R. Kramm, E. Kührt, M. Küppers, L. M. Lara, M. Lazzarin, J. J. Lopez Moreno, S. Lowry, S. Marchi, F. Marzari, M. Massironi, S. Mottola, G. Naletto, N. Oklay, M. Pajola, A. Pommerol, N. Thomas, I. Toth, C. Tubiana, and J.-B. Vincent, Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko.Astronomy & Astrophysics, (to appear) 2015. http://dx.doi.org/10.1051/0004-6361/201527020