Rosetta -- Europe's comet-chasing spacecraft -- is in the middle of a three-day series of rocket firings that are setting the geometry for its rendezvous with comet 67/P Churyumov-Gerasimenko (which I have heard called "Cherry Gerry," for short). These aren't just any deep space maneuvers; between last night and January 23, Rosetta will have fired its rockets for more than seventeen hours.
ESA / AOES Medialab
Rosetta and Philae
The Philae lander is the small blue object perched on the side of the Rosetta spacecraft.
Seventeen hours! That's a lot!!
When all's done, four major rocket burns will have imparted a total velocity change of 778 meters per second to Rosetta. That is also quite a lot; it's as much as is usually required to put a spacecraft in orbit around a planet. For comparison, when Cassini-Huygens fired its main engine to enter orbit at Saturn, it changed its velocity by only 626 meters per second; MESSENGER's high-speed orbit insertion will require more than 800 meters per second. Of course those orbit burns are all done at once; Rosetta's big velocity change is happening over a space of a week or so. And Rosetta isn't entering orbit yet. Far from it; more than three years remain until it finally gets to the comet in May 2014.
Rosetta's biggest burn was completed successfully last night, according to the Rosetta blog. Another should have happened today, and two more are planned for the coming days:
Date/Time Start (UTC)
Jan 17 19:03
Jan 18 [time not specified]
Jan 18 18:59:31
Jan 19 01:13:00
Jan 21 18:49:07
Jan 23 18:42:05
There may be one or two small burns after these big ones for final fine-tuning. In July of this year, Rosetta will be put into a hibernation mode, sleeping until shortly before its arrival at the comet three years from now. That's necessary because the solar-powered Rosetta is traveling so very far from the Sun, as you can see in this diagram (the gridlines are measured in AU):
ESA / DLR
Rosetta Flugbahn (flight path)
Rosetta's circuitous path through the solar system. Gridlines are astronomical units (average Earth distance from the Sun). The green circle is Earth's orbit ("Erde," in German); blue, Mars; red, comet Churymov-Gerasimenko, Rosetta's eventual target, whose elliptical orbit it must match in order to be able to rendezvous with and land on it in 2014. The dotted gray line is Rosetta's path. Only small portions of the orbits of the two asteroids that Rosetta flew past (Steins and Lutetia) are shown in orange and yellow.