For the last two weeks, Rosetta has been orbiting comet 67P/C-G at a distance of about 30 km on the “Global Mapping Phase” (GMP), and is now set to go even lower.
The aim of the GMP was to gather high-resolution science data to help characterise the potential landing sites for Philae, while also continuing to monitor how the spacecraft responds to the environment of an active comet, before getting closer still. See the table below for a recap of the manoeuvres conducted this month, starting with the transfer to the global mapping phase (TGM), and refer to the animation to help visualise these changes in trajectory.
|Burn||Date||DV (m/s)||Duration (min:sec)|
|GMP slot 1||14/09||0.025||00:32|
|GMP slot 2||21/09||0.018||00:25|
(DV = delta v; TGM = transfer to global mapping; GMP = global mapping phase; two more burns scheduled this month for 24/09 and 29/09)
In fact, as we discussed in an earlier blog post, rather than fly one complete orbit, the spacecraft conducted two seven-day-long half orbits at about 30 km, in different planes. That is, on 10 September, the spacecraft was at the terminator plane (the boundary between day and night, which is itself the 06:00/18:00 plane), and performed a thruster burn to insert onto the 30-km circular orbit. The orbital plane was then 60 degrees away from the Sun’s direction, such that the spacecraft orbited over areas of the comet in their ‘morning’ hours. Seven days later, when the spacecraft was again on the terminator plane, it conducted another thruster burn to change the orbital plane such that it had the same characteristics as the previous orbit – but instead was flying over ‘afternoon’ areas of the comet. Thus, from 18 September, the spacecraft was in a 28 km x 29 km orbit around the comet with an orbital period of 13 days 14 hours 59 minutes.
That brings us to today, when the spacecraft moves onto the night side of the comet to allow the instruments to look in more detail at the thermal characteristics of the comet. Note that by ‘night’ we are not actually referring to the spacecraft being in the dark, it remains illuminated by the Sun, but rather the ground track of the spacecraft’s instruments on the comet surface is on the night side. That is, the spacecraft flies along a 04:00 am arc, 30 degrees before the terminator plane.
Just before entering the ‘night’ arc, Rosetta will perform a very small manoeuvre to lower the orbit such that when it completes the arc, on 29 September, it will be at just 20 km from the comet. The command for this manoeuvre is already on board and executes today at 11:00 CEST (09:00 UTC) . At the same time Rosetta will begin the night excursion and start descending.
On Monday, another manoeuvre will circularise the orbit at 20 km along the terminator plane.After a week at 20 km, a decision will be made as to whether it is safe to proceed to just 10 km.