Posted on 15/07/2014 by emily
The third ‘fat’ burn
We’re heading for the final 10,000 km between Rosetta and comet 67P/C-G today, with four of ten rendezvous burns left to complete before arriving on 6 August.
Tomorrow’s burn is the third of four so-called ‘Far Approach Trajectory’ (FAT) burns that are being conducted once a week in the period 2–23 July.
The burn will get underway on 16 July at 13:36 CEST, and will reduce Rosetta’s speed by 11 m/s relative to the comet. It will take about 26 minutes. The one-way signal travel time between the spacecraft and Earth tomorrow will be 22 minutes 34 seconds. (Ed - we’ll post an update as a comment to this blog entry with the outcome – please allow a little extra time on top for the spacecraft operators to check the data and relay that information to us!)
As reported last week, the 9 July burn was successfully completed; lasting 46 minutes 2 seconds, it achieved a change in velocity of 25.7 m/s. Once tomorrow's burn is complete, there is one more ‘FAT’ burn scheduled for 23 July, which will reduce the velocity by 5 m/s; then a pair of “Close Approach Trajectory” (CAT) burns on 3 and 6 August will will impart changes in velocity of about 3 and 1 m/s, respectively.
Thus, in 22 days time, on 6 August, we will have covered 99% of the remaining distance to the comet, rendezvousing with the comet at a distance of 100 km. Eventually, the plan is to come to within just 10 km altitude, although there are many unknowns including the comet’s mass, gravitational field, and activity which need to be determined and understood before Rosetta can move in that close.
What will orbiting at an altitude of 10 km really look like? By now, you are used to seeing artist impressions of the spacecraft around the comet, but of course these are usually from a perspective that shows the spacecraft and comet at roughly the same size, with the former in the foreground and the latter in the background. In reality, Rosetta with its 32-metre solar array “wingspan” is less than 1% of the size of the comet, which is roughly 4 km across at its widest point.
To get a better sense of the relative size of Rosetta and 67P/C-G, the image included in this blog post shows Rosetta and the comet to scale, with the spacecraft at a close orbit altitude of 10 km. This graphic still uses an artist impression of the comet based on a shape of derived from ground-based observations made some years ago, but in a matter of weeks we’ll be able to replace it with what comet 67P/C-G actually looks like.
Indeed, we’ll get an update on that in our regular image release here on the blog on Thursday.