Emily LakdawallaJun 10, 2014

Rosetta update: Both "big burns" completed successfully

Rosetta is now in the final phase of its approach to comet 67P/Churyumov-Gerasimenko after a decade-long journey. Usually, when you steer a spacecraft to a planet, you can use the planet's gravity to help capture you into orbit. But a comet is so lacking in mass that it provides no help to your navigation. In order for Rosetta to enter orbit, the spacecraft has to do all the work to precisely match the size, shape, and orientation of its orbit to the comet's. The final phase of the approach includes a series of rocket burns to brake Rosetta to the right speed. The two largest burns have now succeeded, and Rosetta reported this morning via Twitter that the second burn was close to perfect.

Midpoint of burn
ROS/67P dist
Relative velocity
May 7202016:451,918,449775.1
May 21290.89289.5919:081,005,056754.1
June 4269.5269.4917:48425,250463.0
June 1890.76 14:31194,846192.1
July 258.80 12:5751,707101.3
July 924.91 11:5722,31443.0
July 1610.65 11:129,59018.4
July 234.62 10:304,1267.9

Rosetta is now closing on the comet at a rate of about 17,000 kilometers per day, and is less than 300,000 kilometers away. The comet started showing activity, growing a coma, in April, so I had hoped to accompany this update with some new photos of the increasingly large coma. Alas, ESA has released no images; their media relations team didn't respond to a question I sent them yesterday about when we might expect more. So the only photos I have to show you are the ones I posted a month ago. I know they have images, because they use them for optical navigation.

Comet Churyumov-Gerasimenko develops a coma, March 27 to May 4, 2014
Comet Churyumov-Gerasimenko develops a coma, March 27 to May 4, 2014 A sequence of images showing comet 67P/Churyumov–Gerasimenko moving against a background star field in the constellation Ophiuchus between 27 March and 4 May 2014, as the distance between the spacecraft and comet closed from around 5 million to 2 million kilometers. The comet (and Rosetta) were between 640 million km and 610 million km from the Sun during the sequence. The comet is seen to develop a dust coma as the sequence progresses, with clear activity already visible in late April. Exposure times are 720 seconds for each image, taken with the OSIRIS Narrow Angle Camera. The globular cluster M107 is also clearly visible in the field of view.Image: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

When should Rosetta start actually resolving the comet as a disk? The Rosetta OSIRIS Narrow Angle Camera has pixels that are 0.0186 milliradians across. The comet has a nucleus about 4 kilometers across. The nucleus should subtend one pixel when Rosetta is about 200,000 kilometers away. Rosetta will close to that distance at about the same time as the next rocket burn on June 18. It takes more than one pixel to start resolving details on the nucleus, though. Maybe when it's about 10 pixels across we'll start discerning a lump here or a bump there. That won't happen until the second week of July. I can't wait to see what it looks like! I hope they show us some photos.

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