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The Rosetta/Philae mission to comet 67P/Churyumov-Gerasimenko

Michael Richmond
Dec 13, 2014
Mar 31, 2015
May 21, 2015

Much of the material herein is taken from the excellent work by Emily Lakdawalla and other writers for the Planetary Society Blog.

Movie courtesy of The ESA

The journey to Comet 67P

Comet 67P has an orbit that ranges from as far out as Jupiter to a bit closer than Mars. It is always more distant from the Sun than the Earth.

Right now, it is about half-way on the inward portion of its orbit, heading towards the warmth of the inner solar system.

Obviously, Rosetta is there, too. How did it reach this point in space? The answer: in a very roundabout way. Rosetta was launched way back in March, 2004, so it has been travelling for over 10 years.

Why has it taken so long? The European Space Agency team found a way to save lots of fuel by using four planetary flybys to give the spacecraft a series of boosts. This special path meant that the rocket used to lift Rosetta up off the Earth could be relatively small and inexpensive.

Click on the picture below to see a movie of Rosetta's journey.

Movie courtesy of The ESA

Rosetta and its instruments

Rosetta is a pretty big spacecraft -- its main body is about the size of a small truck:

.... but it grows much larger when it spreads out its solar panels.

The mass of the spacecraft started out at about 3000 kg, but a lot of that was its propellant. By the time it reached the comet, its mass was only about half its initial value.

Philae and its instruments

Riding aboard Rosetta is the little lander, called "Philae". It's about the size of a deluxe coffee maker:

Image courtesy of the BBC.

It has quite a few instruments of its own.

The landing -- what happened?

On Nov 5, 2014, cameras watched as the lander made its approach to the comet. Would it manage to land safely?

The plan was for

  1. Philae to separate from Rosetta
  2. spend several hours falling toward the comet's surface
  3. just as it reached the surface, striking at a speed of about 1 meter per second, gentle gas jets would push it INTO the surface for about 15 seconds so that ....
  4. .... the harpoons on Philae's legs would grab the surface

The first steps went smoothly:


And hours later, the lander touched down ----

---- or so it was thought.

Within a few hours after THAT, however, it became clear that things weren't as expected. After much confusion, it became clear that

the lander had bounced off the surface --- twice

What happened?

So, instead of hitting and sticking, the lander hit the surface and then bounced twice. One big bounce, lasting about two hours, and then a smaller bounce, lasting only about 20 minutes(?). Eventually, the lander settled down onto the surface, but not in a secure manner, and probably not solidly balanced on all three legs.

How could the lander fly so far up above the surface before falling down again? The key is the very, very weak gravity around Comet 67P. The surface gravity there is only about 0.002 percent the strength of Earth's gravity, so a typical adult human would weigh less than one ounce! If you were standing on the surface of the comet, and jump upwards at just about one mile per hour, you would go up -- up -- up -- and never fall back! The escape velocity from the surface is less than 1 meter per second.

Here's a shot of the surface of the initial landing site, taken just a few seconds before the lander touched down and bounced away again. The picture shows a region about 40-by-40 meters wide -- roughly half the size of a soccer field. The big rock at upper right is about 5 meters wide.

Image courtesy of ESA / Rosetta / Philae / ROLIS / DLR

After the bounces, when Philae had finally reached a resting position on the comet's surface, it took this picture with its CIVA camera. The metal structure near the top is one of Philae's feet.

Image courtesy of ESA/Rosetta/Philae/CIVA

The big problem with this new landing site is that it lies in a shadowy region, so that the spacecraft only gets sunlight for about 1.5 hours of each 12-hour comet day, instead of the roughly 6 hours which was expected. That is, unfortunately, not long enough for Philae's solar panels to recharge its batteries.

What's worse is that the batteries drain during the long night, as the spacecraft tries to keep itself warm.

The result: within just a few days, Philae ran out of power and shut down.

"Peanuts" comic strip by Charles Schulz

What had we learned as of Dec 2014?

Well, one of the first things we learned, long before Philae took off for the surface, was the mass and density of comet 67P. Rosetta was able to take pictures which can be used to determine the size and volume of the comet, and the subtle changes to its motions caused by the gravity of 67P can be used to estimate the mass of the comet. We now know that the comet has

This tells us that the comet is less like a ball of ice (which has density 1000 kg per cubic meter) than a ball of fluffy snow.

Now, as for the science results from the Philae lander, this talk is just about one week too early.

But some tidbits of information have come out.

But don't forget: the comet, and Rosetta, and Philae, are all heading toward the Sun: they will reach their closest approach in August, 2015.

As the Sun draws near, the temperature of the comet -- and the Philae lander -- will increase. In addition, the strength of sunlight striking the solar panels on Philae will also increase. It is possible that the lander will "wake up" at some point and start to communicate with Rosetta again. Scientists may yet have a chance to carry out some of the experiments they have been planning for years ....

What have we learned since then?

Movie copyright the ESA

In the five months since Philae landed on the comet, it has moved somewhat closer to the Sun.

Not close enough for the increased sunlight to wake up Philae -- not yet! Philae lander manager Stefan Ulamec is on record as stating that he thinks the lander might start working again some time after May. We'll just have to wait.

In January, 2015, the journal Science published one issue with a set of articles from the Rosetta teams.

But, something big DID happen recently, just two weeks ago. The 2015 Lunar and Planetary Science Conference was held in Houston, TX. Many of the Rosetta scientists released their first results since the landing at this meeting. You can read through the paper titles and abstracts yourself at

Let's take a peek at just a very few of the talks.

"wind-blown dunes"

Let's look again at an image of the area of the initial landing site. Pay special attention to the large boulder near the top, which is about 5 meters wide.

Members of the ROLIS camera team interpret the pattern of dust around boulder as a "dune", similar to the ones seen on planets with significant air-borne erosion properties. The boulder has a ramp of dust on one side, and a small "moat" or depression on the other (in shadow here).

There are additional examples of this phenomenon elsewhere on the surface, as in this photograph from the OSIRIS team.

Clearly, there's no atmosphere on comet 67P dense enough to support a wind. Instead, the ROLIS team suggests that a phenomenon called "splash saltation" could be responsible: basically, if a big particle crashes into the surface, it can send many smaller particles flying up and forward in the same general direction.

periodic structure in mass spectrometer results
The Ptolemy mass spectrometer team talked about their analysis of material which entered their instrument after the initial landing and bounce. The molecular weight of the material showed a somewhat periodic structure, which the team interprets as due to the presence of a polymer: a long, chain-like molecule built of blocks with identical size. In particular, the team suggests polyoxymethylene.

Figure 2 from Huebner et al., ESASP 278, 163 (1987)

Alas, I haven't been able to find any copies of the graphs they showed at the meeting, so we'll have to make do with illustrations from an old paper discussing the composition of a different comet: Comet Halley:

Figure 1 from Huebner et al., ESASP 278, 163 (1987)

The Ptolemy instrument was designed to measure samples of material taken directly from the surface, so it could do a much better job if the lander wakes up and can gather samples via its SD2 drill system.

a world made of pebbles
The CIVA camera team discussed various aspects of images taken by their instrument. One feature they noticed is ubiquity of pebble-like structures in and on the surface near the landing site, and the absence of dust.

But don't forget that there ARE regions of the comet where dust is common:

Figure 2 taken from Thomas et al., Science 347 (2015)

The pebbly nature of the surface could provide a clue to the manner in which comets formed in the early solar system. Dust particles clumped together to form centimeter-sized pebbles, and the pebbles then stuck to each other to build up larger and larger bodies. In the case of bodies like the Earth, these meter-to-kilometer conglomerations merged into objects large enough that their self-gravity compacted them, and heated them, destroying the original pebbles and creating large bulk quantities of homogenous minerals. But in the case of Comet 67P (and perhaps other small bodies), that merging/heating/melting process never happened.

As the comet and Rosetta and Philae all fall in toward the Sun, the temperature grows higher and higher. We can see increasing activity from the comet's surface, as in this image taken on Mar 22, 2015.

Will the increased sunlight eventually wake up the lander? I certainly hope so!

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Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.