The Very Large Telescope sights Rosetta's comet target, sees activity beginning
The European Southern Observatory has just released the first photo of Rosetta's cometary target, 67/P Churyumov-Gerasimenko, since the comet's passage behind the Sun as seen from Earth. It's not a very impressive photo; it is more important for what it portends than for what it is.
ESO/C. Snodgrass (Max Planck Institute for Solar System Research, Germany) & O. Hainaut (ESO)
Comet Churyumov-Gerasimenko on February 28, 2014
The Very Large Telescope took this photo of Rosetta's target comet about 8 weeks after the comet passed into solar conjunction. It was 4.9 AU from Earth at the time, and also about 4.4 AU from the Sun, just inside the orbit of Jupiter. The comet was 50% brighter than anticipated, suggesting that cometary activity has started as the comet travelsd closer to the Sun on the inbound leg of its orbit. The comet was moving toward an August 2015 perihelion. The left-hand view is a stack of many exposures aligned on the comet, in which background stars appear as streaks. In the right-hand image, the background stars have been subtracted, revealing the tiny dot of the comet.
The photo is a followup to this equally tiny dot of a detection captured last October by the same astronomers using the same instrument.
ESO/C. Snodgrass (Max Planck Institute for Solar System Research, Germany)
Comet Churyumov-Gerasimenko on October 5, 2013
The Very Large Telescope took this photo of Rosetta's target comet about 6 weeks before the comet passed into solar conjunction. It was 5.3 AU from Earth at the time, and also about 5 AU from the Sun, just inside the orbit of Jupiter. At the time, it was not showing any cometary activity. The right-hand image is a stack of all the observations aligned on the stars, with a streak drawn on it to denote the comet's moving position. The left-hand image is the same set of observations aligned on the comet, with background stars subtracted.
This work is made challenging, observer Colin Snodgrass remarked in a paper about his past observations of the comet, because of its location in the sky: "A considerable difficulty in observing 67P during the past years has been its position against crowded fields towards the Galactic centre for much of the time. The 2007/8 data presented here were particularly difficult, and the comet will once again be badly placed for Earth-based observations in 2014/5." However, by using difference image analysis of the sort illustrated here, they were able to obtain a good lightcurve for the comet and predict that it would begin activity around March of 2014.
In fact, that's what Snodgrass and coworkers found. The news, in the new photo, is that cometary activity has already started; it is 50% brighter than predicted from its decreasing distance to the Sun and Earth alone. It's so nice when predictions actually work!
We don't have pictures of the comet from the newly-awake Rosetta yet, because the science instruments haven't been turned on yet and the navigational camera hasn't been pointed in that direction yet; they're still working on bringing the spacecraft gently out of its long hibernation. Anyway, Rosetta's first images will be equally unimpressive, showing the comet as a faintly fuzzy dot against a background of bright stars. But Rosetta will be drawing inexorably closer to the comet over the next several months, reaching it in August of this year.
Here is an overview of upcoming activities, with the ever-present proviso that dates, times and events may change due to operational requirements:
17 March – Switch on the OSIRIS imaging system; all other instruments will be switched on in the following approximately 6 weeks
24 March – Pending successful re-activation, OSIRIS will take a first look in the direction of the comet. The comet will be too far away (around 5 million kilometres) to resolve in these first images and its light will be seen in just a couple of pixels. These images will be acquired regularly for navigation purposes and to already start planning the trajectory corrections planned for May.
In the meantime, Rosetta's NavCam has been briefly switched on for a check-out; NavCam imaging for operational purposes will start in May alongside OSIRIS.
(The first Navcam image was pointed in a direction opposite to that of the comet.)
What's next for Rosetta? Again, according to the status update:
May will see the start of a critical series of manoeuvres that will steadily bring Rosetta in line with the comet. Currently Rosetta is on a trajectory that would, if unchanged, take it past the comet at a distance of approximately 50 000 km and at a relative velocity of 800 m/s. The aim of the manoeuvres is to reduce Rosetta’s relative velocity to 1 m/s and bring it to within 100 km distance of the comet by 6 August. The manoeuvres will be planned for every second Wednesday (fortnightly), starting with the following:
Around 7 May - First 'test' manoeuvre to decrease Rosetta’s relative velocity to the comet by 20 m/s
21 May – reduce relative velocity by 290 m/s
4 June - reduce relative velocity by 270 m/s
18 June - reduce relative velocity by 90 m/s
After that, according to the latest post on the Philae blog, Rosetta's main task for the next several months will be to gather information about the comet nucleus in order to prepare for the November landing of Philae.
I didn't have a good sense of the comet's path around the Sun compared to Earth's, so I visited JPL's Horizons system to get myself a table of data describing that. I asked it to show me comet 67P/Churyumov-Gerasimenko's range with respect to the Sun and Earth every Monday for about two years, and also asked for its elongation (the angle from Sun, to comet, to Earth). Here are the results. You can see how when Rosetta arrives at the comet in August of this year, it will still be more than 3 AU from the Sun, inbound toward its perihelion, which will come a year later. The Rosetta mission hopes to land Philae on the comet in November.