Emily LakdawallaJul 29, 2014

Rosetta update: Long journey to a comet nearly complete

A journey of nearly a decade is almost over. Rosetta is making its final approach to comet 67P/Churyumov-Gerasimenko, and the comet's strange shape is beginning to come into focus. As of today, the spacecraft is only 2000 kilometers away from the comet, and 8 days away from arrival.

For a long time, the comet was a dot; then, it became a strange two-lobed shape inviting comparisons to boots or rubber ducks. Now, though, it's becoming a world with discernible surface features. In the most recent images available from the OSIRIS camera, taken on July 20, we can begin to see bright spots and dark spots and things that might possibly be craters.

Comet 67P/Churyumov-Gerasimenko on July 20, 2014
Comet 67P/Churyumov-Gerasimenko on July 20, 2014 Rosetta was only 5500 kilometers away from the comet when it took these three photos two hours apart on July 20, 2014. The original images have a resolution of about 100 meters per pixel; the views here have been enlarged by a factor of 8 and rotated by about 15 degrees for some reason.Image: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

They're close enough now that the comet is even beginning to take shape in NavCam images. Rosetta's NavCam has a resolution about 5 times lower than OSIRIS. They have started releasing daily NavCam images, which is awesome -- it's wonderful to tune in to their website each day and see the comet grow inexorably larger. (Note that they don't work weekends, so the NavCam images taken on Saturday and Sunday are released with the Monday one.) They are releasing the NavCam images with a factor-of-10 enlargement; I've reduced that to a more reasonable factor of 2 in the composite below. It looks like they didn't use quite the same interpolation algorithm on the first couple as they did the most recent three, so the first couple are blurrier than the rest.

Rosetta approaches Churyumov-Gerasimenko (5 NavCam views, July 23-27, 2014)
Rosetta approaches Churyumov-Gerasimenko (5 NavCam views, July 23-27, 2014) Each day, the comet grows in Rosetta's forward view. These five views were taken with the Navigation Camera over five days. They are enlarged by a factor of two from their original resolution.Image: ESA / Rosetta / NavCam

(Note: The Rosetta mission is inconsistent about its capitalization of NavCam, often writing "NAVCAM" instead. I think that people tend to write "NAVCAM" because it looks more like the acronym "OSIRIS" but since NavCam is an abbreviation of Navigation Camera, I'm choosing to use "NavCam.")

There has been a lot of discussion online about the Rosetta images and how they are being released. Privately, I've advocated for regular release of, if nothing else, the NavCam images, so I am really delighted to see the team choose to do that, and to do it in a timely fashion, even if it's only one per day. Releasing daily NavCam images accomplishes the goal of bringing the public along for the approach and arrival at the comet. It also helps me as a writer, allowing me to choose when to write an update about the mission, because I know I will always have an image to anchor it with, even if I don't write an update on a Thursday when their weekly OSIRIS release comes out.

One question I have had about Rosetta's image data concerns when all of the data will be made available through public archives. ESA missions, like NASA missions, are required to deliver scientific data to a publicly available archive; in fact, ESA planetary missions usually have to supply their data both to ESA's Planetary Science Archive and NASA's Planetary Data System. ESA has been putting the same kind of pressure on its mission scientists that NASA has, to accept shorter proprietary periods before public data release. Rosetta mission personnel have mentioned in the past a proprietary period of 6 months for the science team to analyze and calibrate the data before making it available. But that didn't tell me when the first release was going to come, so I inquired with the project. I got an answer to my question this morning from Maud Barthelemy, the Rosetta Archive Scientist at ESA. The six-month clock starts after the landing of Philae on the comet -- more precisely, it begins at the moment that the first science sequence executes after landing.

According to this calendar, all of the data gathered prior to landing will be part of the first release. If landing occurs as planned on November 19, then the first science data will be publicly released on May 19, 2015. But if landing is delayed, the first release will be delayed correspondingly. After that, releases will occur every three months, covering the data acquired from 6 to 9 months previously. This policy is similar to NASA Mars missions (and is a slightly more rapid release than Cassini, for which the quarterly releases include data acquired 9 to 12 months previously). So watch for the landing, and set your calendar reminders for six months after! Of course, it is almost universally true that the first science data releases from missions (NASA or ESA) don't come on time -- or they are only partially on time, with some data sets delivered and others not -- and I don't really expect any different from Rosetta. But I do hope that ESA will at least be able to get the NavCam data out on time.

The data from Rosetta's early OSIRIS views are already being put to good use. The Rosetta team is using it to create shape models of the comet -- 3D representations of its bizarre shape. For most missions, shape models are scientific products, and it usually takes quite a while to carefully process data to make them. But for Rosetta, the shape model is a crucially important data set necessary for mission success. They are going to have to put a lander down on this bizarre object somehow, and the shape model will help them find a location that they can reach. I don't envy the navigators that task. It's hard enough to rendezvous with another world in deep space. It's even harder when the world you're trying to connect with is actually two. I can't imagine what the gravitational field looks like around this thing. Where is "up" and where is "down" in the crazy neck between the two lobes?

Shape model of comet 67P/Churyumov-Gerasimenko based on data up to July 24, 2014
Shape model of comet 67P/Churyumov-Gerasimenko based on data up to July 24, 2014 This shape model of the comet was derived from Rosetta OSIRIS images taken from July 14 to 24. The mission is using it to begin to prepare for the landing of Philae.Image: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

There will be a day-long media event celebrating Rosetta's rendezvous at ESA's Operations Centre (ESOC) on August 6. I won't be there, but have asked Daniel Fischer to attend and file a report for The Planetary Society. The event includes a press briefing at 13:00 CEST (06:00 PDT) that will be livestreamed. But the ESA blog has already laid out what to expect next:

  • On August 6, the spacecraft will rendezvous, at a distance of 100 kilometers
  • By the end of the month, the spacecraft will have lowered its altitude to 50 kilometers
  • By the end of the month, the team will have selected five candidate landing sites
  • The spacecraft will transition to a 30-kilometer altitude and OSIRIS will image these sites at a resolution of 55 cm per pixel (comparable to Lunar Reconnaissance Orbiter's images of the Moon)
  • A primary landing site will be identified in mid-September
  • Detailed characterization will happen in September and October
  • Landing is currently planned for November 11 (yes, this differs from what Maud Barthelemy told me, but I imagine this date will be in flux for a while) (EDIT: project scientist Matt Taylor commented that "the 19th November date given by Maud is correct, it follows lander deployment, descent and the first science sequence, which is nominally initiated on 11th and ends on 19th."

As a final note, while preparing this post, I composed a list of all the image releases I could find so far. I figure this list may come in handy to some of you, so: enjoy! Keep in mind that unlike NASA images, ESA images are not in the public domain. Anybody can reuse them for educational or informational purposes, but must give proper credit. So I've included the proper credits for the photos with the lists.

OSIRIS images


NavCam images

Credit ESA/Rosetta/NavCam

Shape models


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