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)

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

 

Rosetta’s Last Hurrah

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.

Rosetta’s last week at the comet. ESA
A simplified overview of Rosetta’s last week of manoeuvres at Comet 67P/Churyumov–Gerasimenko (comet rotation is not considered). After 24 September the spacecraft will leave the flyover orbits and transfer towards an initial point of a 16 x 23 km orbit that will be used to prepare for the final descent. The collision course manoeuvre will take place in the evening of 29 September, initiating the descent from an altitude of about 20 km. The impact is expected to occur at 10:40 GMT (±20 minutes) at the comet, which taking into account the 40 minute signal travel time between Rosetta and Earth on 30 September, means the confirmation would be expected at mission control at 11:20 GMT / 13:20 CEST (±20 minutes).

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.

 

Space Saturday

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

Old Space Probes Fade, New Probes Scream Into View

As our old friends Rosetta and New Horizons head out of the Solar System, and Dawn continues to image Ceres, we have a new spacecraft to watch: Juno arrived at Jupiter this week. (Some PR genius at NASA managed to fiddle the trajectory to arrive on the US Independence Day holiday—give that person a cookie!)

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.

Animation of Juno 14-day orbits starting in late 2016. Credits: NASA/JPL-Caltech

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.

Let’s stay tuned!

 

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.)

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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

Controversy About How 67P Got Its Shape?

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.

Title The comet’s two lobes Released 28/09/2015 3:00 pm Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Massironi et al (2015)

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1. 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

 

Space Saturday

Rosetta Science Results Coming Fast

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 [1], 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.

Cool!

Sequence of ten images showing changes in the Imhotep region on Comet 67P/C-G. The images were taken with the OSIRIS narrow-angle camera on Rosetta between 24 May and 11 July 2015. The individual images are also available separately. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Same sequence as above, with indication of dates and location of the morphological changes. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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1. 1. 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

 

Space Saturday

The Story Of The Philae Lander Wakeup

As everyone recalls, last November the Philae Lander made an uncontrolled and unplanned arrival on 67P/CG, ending up in a shady spot. With insufficient sunlight, the lander shutdown, hoping that it would awaken as the comet approached the sun.

Indeed, Philae did wake up, and sporadic contact was established in July,

Last week the Philae team posted a detailed recount of the contacts with Philae.

We now know that the lander is definitely askew, on its side and possibly blocked by surface features. This has made it difficult for the orbiter to “see” the signals from the lander. In addition, as the sunlight increased approaching perihelion (good for the lander) the gas and dust emissions increased sharply, forcing the orbiter to pull higher.

The post tells an interesting story of how the limited data was interpreted in the face of sporadic contact and failing equipment. This is fascinating real life engineering, though unfortunately the results have been disappointing.

Maneuvering the Rosetta orbiter to try to contact Philae precluded other scientific observations, and by July 25 it was necessary to break off and take up important data collection around perihelion. At the same time, the orbiter has moved higher to stay clear of the dust.

At this point, there is no chance of contacting the lander. There will be one last opportunity late this year, and the team is working to prepare a strategy based on what can be gleaned from the data returned so far.

This is an amazing story, even if there is no Hollywood ending. Philae was an awesome machine, and kudos to the folks who have been working so hard to learn what happened to it.

 

 

Space Saturday

Solar System Science Pouring In

After Perihelion (which would be a good name for a band), the Rosetta team continues to release new studies of 67P/CG. To date, most of the analysis is based on data from the first months in orbit, the perihelion will come out later (probably in time for big meetings in December and January).

One study examined fissuring on the surface of the comet, published in Geophysical Research Letters. Now that we have close up views, we can see many different fissures and cracks in the surface, which gives us basis for inferring some of the erosion that occurs on the comet. Of course, it will be really interesting to compare the “before” images to similar images collected after perihelion, which should show many changes which will tell us what happened.

The Rosetta team also has examined the magnetic fluctuations, dubbed “singing”, observed last year. These waves are different from what has been observed in other comets, and the team theorizes that they are due to the behavior of the plasma field at the time when 67P/CG was still cold and the plasma cloud small. The observations show the unexpected “singing” abated around February, as the comet came closer to the sun. In the near future we should have observations from perihelion, which may well show the development of the “song” like those observed in other comets.

it’s difficult to keep up with all the reports, and we haven’t even got to perihelion yet.  (“We haen’t even got to perihelion yet” would be a great title for a novel.)

Meanwhile, the Dawn spacecraft is comfortably orbiting Ceres at its final altitude of 1470 km. Each 11 days, Dawn completes a map of the surface of Ceres at three times the resolution of any previous imaging. This spacecraft is an awesome piece of engineering to last this long and collect so much data.

OK, Ceres looks kind of like the moon, all grey and cratered. But now we have really, really detailed maps and, with multiple overflights, 3D reconstructions.

Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA http://www.nasa.gov/jpl/pia19631/the-lonely-mountain

 

A BBC report reminded me that the Cassini spacecraft is Still Out at Saturn (yet another good name for a band). Cassini  launched in 1997(!), and has been touring the neighborhood of Saturn, observing the planet, rings, and many moons, and dropping the Huygens lander onto the surface (or at least methane lakes) of Titan. Awesome!

The probe is now preparing for its last loop away from Saturn before a final funeral plunge into Saturn in 2017.

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1. El-Maarry, M. R., N. Thomas, A. Gracia-Berná, R. Marschall, A. T. Auger, O. Groussin, S. Mottola, M. Pajola, M. Massironi, S. Marchi, S. Höfner, F. Preusker, F. Scholten, L. Jorda, E. Kührt, H. U. Keller, H. Sierks, M. F. A’Hearn, C. Barbieri, M. A. Barucci, J. L. Bertaux, I. Bertini, G. Cremonese, V. Da Deppo, B. Davidsson, S. Debei, M. De Cecco, J. Deller, C. Güttler, S. Fornasier, M. Fulle, P. J. Gutierrez, M. Hofmann, S. F. Hviid, W. H. Ip, J. Knollenberg, D. Koschny, G. Kovacs, J. R. Kramm, M. Küppers, P. L. Lamy, L. M. Lara, M. Lazzarin, J. J. Lopez Moreno, F. Marzari, H. Michalik, G. Naletto, N. Oklay, A. Pommerol, H. Rickman, R. Rodrigo, C. Tubiana, and J. B. Vincent, Fractures on comet 67P/Churyumov-Gerasimenko observed by Rosetta/OSIRIS. Geophysical Research Letters, 42 (13):5170-5178, 2015. http://dx.doi.org/10.1002/2015GL064500
2. Richter, I., C. Koenders, H. U. Auster, D. Frühauff, C. Götz, P. Heinisch, C. Perschke, U. Motschmann, B. Stoll, K. Altwegg, J. Burch, C. Carr, E. Cupido, A. Eriksson, P. Henri, R. Goldstein, J. P. Lebreton, P. Mokashi, Z. Nemeth, H. Nilsson, M. Rubin, K. Szegö, B. T. Tsurutani, C. Vallat, M. Volwerk, and K. H. Glassmeier, Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko. Ann. Geophys., 33 (8):1031-1036, 2015. http://www.ann-geophys.net/33/1031/2015/

 

Space Saturday