Tag Archives: Philae lander

Looking Back At Philae

It’s been four years now since plucky little Philae attempted to land on comet 67P/CG.  It was a thrilling event, with an inexplicable failure that resulted in a slow motion crash landing and eventual loss of the lander.

This year engineer András Balázs reviews what happened to Philae.  As he remarks dryly, “not everything went as planned.” ([1], p. 90)

I’m sure we are all shocked, shocked! to hear that software was involved….

Like all spacecraft, Philae was designed to be fault tolerant, at least for certain values of “fault”.  The system included redundant processors and other equipment, software and communications protocols with error detection and recovery features, and fall back recovery processes.  For instance, the system had “triple-redundant emergency telecommand decoders”—four duplicate message decoders to assure correct results even in the face of multiple problems.

Nevertheless, she failed.

Balázs six lessons learned are a litany of humbling epiphanies.  From not being prepared for “unconceivable” problems to not being prepared for “conceivable” problems.  “When Even Redundanccy is Useless”.  And, of course, there were tradeoffs between science and safety, many missed opportunities and just plain poor decisions.  Millions of kilometers from home, every human error is unfixable, and every lost opportunity is lost forever.

“The Philae mission was a jump into the unknown.” ([1], p. 93)

Not only was Philae a bold and risky leap into the unknown, and not only was it, like all space missions, beyond our ability to reach it for repairs if needed, it was the first and only mission.  No engineer can hope that the first try will work—that’s called a “throw away”, intended only to teach us how to build a successful version.

But there was no option to do a throw away trial.  There seldom is in space missions.

“The software community could benefit from more such evaluations of the problems that so frequently occur in projects. —Michiel van Genuchten and Les Hatton” ([1], p. 90)

The editors of IEEE Software praised Balázs’ frank assessment of the failures of the project. I’ll second the praise. It’s not pleasant to look back at one’s own mistakes and failures.

I have to say that I am disappointed that we still don’t know why the both of the redundant harpoons and the hold down thruster failed to anchor the lander.  Suspicion falls on the software that was supposed to detect the impact and trigger the anchoring.  (In my head, I’m seeing the software taking too long, the spacecraft bouncing and then shooting the harpoons into nothing….)

But we don’t really know.  Sigh.

The photo released by European Space Agency ESA on Monday, Sept. 5, 2016 shows the comet lander Philae (circled) in a crack on the right side of a photo taken by Rosetta’s OSIRIS narrow-angle camera on Sept. 2, 2016 from a distance of 2.7 km of the Comet 67P/Churyumov–Gerasimenko. Philae was last seen when it first touched down at Agilkia, bounced and then flew for another two hours before ending up at a location later named Abydos, on the comet’s smaller lobe. (ESA/Rosetta/MPS for OSIRIS via AP)

  1. András Balázs A Comet Revisited: Lessons Learned from Philaes Landing. IEEE Software, 35 (4):89-93, 2018.


Rosetta Finale This Month

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!)

Philae close-up, labelled ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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).


Space Saturday

How Rosetta Will Wind Up

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.

The blog explains the situation, “From one comet landing to another: planning Rosetta’s grand finale“.

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.


Space Saturday

Comet Science From Rosetta

This week was the first anniversary of the Philae landing on 67P/Churyumov–Gerasimenko! Has it been a year already? ESA has an interesting postmortem on the Philae landing misfire (literally, a failure to fire)., with a video reconstruction of the cometfall.

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.

The Rosetta blog discusses two of the papers which explore surface features, boulders and dust, seen in imagery from the OSIRIS instrument. OSIRIS is two cameras (wide and narrow) that can capture visible, near IR and new UV light.

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.

Ripples in the Hapi (neck) region are attributed to a phenomenon known as airfall. Image credit: see below.

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.)


  1. 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


Space Saturday

Are You Ready For Perihelion?

After a year on station at comet 67P/CG, Rosetta will fly along past perihelion on August 13. Over the past few months, Rosetta has observed the changes as the comet heats up, spewing gas and dust. This process will reach a climax in the next month or two, as the comet swoops by and heads out away from the sun again.

ESA has a nice FAQ on this especially interesting part of the mission.

67P/CG doesn’t come very near the Sun (at the closest, not as close as Earth’s orbit), so it isn’t likely to fall apart or explode or anything really exciting. But there will certainly be changes as the surface heats and boils away. Rosetta will be the first close up of this process ever, so who knows what we might see and learn?

For perspective, I note the sort of data we have had before this, from telescope observations, such as:

Recent image from the 2 m Liverpool Telescope, taken on the morning of 19 July 2015. It comprises 10 x 20s r-band images. The length of the tail visible in the image in the twilight sky is approximately 120,000 km. Credit: Colin Snodgrass / Geraint Jones / Liverpool Telecope

For comparison, recent images alone are zillions of times more detailed:

Comet 67P/C-G on 20 July 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The instruments are generating a lot of detailed science, with many results just being published now. (No, I have not read these reports!)


  1.  Bibring, J. P., M. G. G. T. Taylor, C. Alexander, U. Auster, J. Biele, A. Ercoli Finzi, F. Goesmann, G. Klingelhoefer, W. Kofman, S. Mottola, K. J. Seidensticker, T. Spohn, and I. Wright, Philae’s First Days on the Comet. Science, 349 (6247):493-493,  2015. http://www.sciencemag.org/content/349/6247/493.short


Space Saturday


Screaming Pluto Flyby

… isn’t a bad name for a band…

NASA’s New Horizons is screaming in (well, it would be screaming if there was some air) on Pluto this week. No brakes!  Can’t stop!

This is a historic first, the closest any probe has come to Pluto, and checks the last box on the “visit the planets” dance card. (From Sputnik to now, it took just under 58 years.)

If nothing goes wrong, we’ll get some pretty good images, snapping away like mad.

New Horizons Dashboard

In contrast to the quick and “whatever we can grab” Pluto flyby, Dawn is doing a slow and very thorough survey of Ceres. Currently in a 4400 km orbit, Dawn is finding a lot of interesting geography, which will have to be puzzled through in the ensuing years.

Marc Raymon gives us a nice run down on some of the high lights of the observations, and the implications. He also tells us a little about communication with Earth from that distance. (Hint to NASA PR people: this post is way more interesting than the continuing hoo-haw over “guess what the bright spots are”.)

Now Dawn will spiral yet lower, to gather much more high resolution data. This ion drive thing is awesome!

And out at 67P/CG, Philae seems to be alive, but communication to the orbiter is spotty.  It’s great to hear that the lander has survived (seriously good engineering job), though it isn’t clear we’ll be able to get much information back.


Space Saturday

Philae Contact Troubles

ESA has a nice post, walking through the challenges in trying to get data back from Philae.

After the uncontrolled landing on 67P/CG last fall, the Philae lander ended up in a shady spot and had to shutdown for lack of power. As the comet has closed on the Sun, Philae now has recharged the batteries and contacted the Rosetta orbiter—but only for brief periods.

Responding to this, the Rosetta team has decided to sacrifice some activities to maneuver the orbiter to try to get a good signal from Philae. To date, this is a gamble that has not paid off, since not much has been received from Philae.

In any project like this, the mission is working with extremely tight budgets of energy, bandwidth, and memory; not to mention orbital physics and severe time limits. Furthermore, we have limited information about what is going on, especially if we can’t get data via radio. Getting a link is the basic first step, which is why these problems are so frustrating and tense.

All that said, ESA’s post “Rosetta and Philae: Searching for a good signal” was very helpful. They lay out the basic problems, hypotheses about what may be going on, and what actions are possible. This helps us all understand what they are trying to do and why it is difficult. It also helps us understand the risks and tradeoffs.

It is very nice to get this sort of grown up information from ESA, without the shallow Hollywood stuff that some think the public wants. (I’m talking to you, NASA PR flacks.)


Space Saturday