Rosetta will collect science data until the very end of its descent on Friday. The opportunity to study a comet at such close proximity makes the descent phase one of the most exciting of the entire mission.
A summary of the goals of the instruments that are operating during the descent are provided below:
During the descent to the small comet lobe from an initial starting point of an altitude of around 19 km, the OSIRIS cameras (both narrow- and wide-angle) will first image the regions of the large lobe that it passes over. As the spacecraft approaches the small lobe, the cameras will target the walls of the Ma’at pits. The very high resolution data of these features will provide important information for our understanding of how activity is driven on the comet and maybe how the comet was formed in the first place.
In order to downlink the highest possible number of images in the limited time available before the spacecraft impacts the surface, especially in the final stages of the descent, the images will be highly compressed, up to 20 times compared with ‘normal’ images. On top of compressing, the images will also be downsized, such that instead of a handful of full-frame 2048 x 2048 pixel images, we will be able to receive many more smaller images in the region of 1000 x 1000 pixels down to 480 x 480 pixels.
It is also important to remember that the cameras were not designed to image the comet from this close proximity: NAC becomes out of focus from an altitude of around 1 km and less, and WAC from below 200-300 m. Below 200-300m the images will be increasingly blurred and thus not as ‘crisp’ as we have been accustomed to.
Select OSIRIS images from the descent will be presented by the instrument Principal Investigator, Holger Sierks, at various points during our live broadcast, and published in parallel on our ESA channels. This is expected to include the very last image downloaded from the spacecraft, anticipated to be available within about 10 minutes of the confirmation of end of mission.
ROSINA will collect unique data on the density of gas around the comet and its composition. It is expected to provide readings down to the Knudsen Layer, where the sublimation of the gases actually happens.
MIRO will complement OSIRIS and ROSINA measurements by measuring the surface temperature.
GIADA will measure the dust density and the way the dust grains are accelerated away from the comet.
RPC’s instrument suite will monitor the plasma environment, and also the smallest dust particles. This will give a unique close-up look at the interaction between the solar wind and the surface of the comet, and it will sample levitating charged grains.
Alice will get its highest resolution ultraviolet spectra of the surface of the whole mission and provide complementary measurements to some of the RPC data.
RSI will get the most accurate measurements of the gravity field during descent.
The spacecraft’s Navigation Camera will also play a role in the final sequence of data collected, albeit very early in the descent. Shortly after the spacecraft has been set on a collision course for the comet, the NAVCAM will take five images. These are due to be downloaded in the early hours of 30 September, and will be used by the flight team to predict the impact time to within a four-minute window.
How much data?
Many of the science instruments are expected to return their last data from between 20 m and 5 m above the surface.
The bit rate for the Madrid Deep Space Network ground station during the descent is 45760 bps and the expected data downlinked between the end of the collision manoeuvre on 29 September and impact on 30 September is 1558 Mbits (which equates to 195 MB of science and instrument-related house-keeping data).
The break-down per instrument is expected as follows:
Alice: 37 Mbit
GIADA: 3 Mbit
MIRO: 6 Mbits
NAVCAM: 64 Mbit
OSIRIS: 1177 Mbits
ROSINA: 49 Mbits
RPC: 128 Mbit
Instrument House Keeping: 93 Mbit
Why are not all instruments operating?
Because Rosetta is so far from the Sun now it cannot generate enough power from its solar panels to keep everything running, so not all instruments will be able to operate in the final phase of the mission. Indeed, after careful consideration, balancing science operations constraints – some instruments require quite some time to obtain their measurements – and the potential science return, the timeline prepared by the science operations team at ESAC, with the Rosetta Science Working Team, sees MIDAS, COSIMA and VIRTIS turned off. The SREM will also be off.
After Rosetta has touched down, it will not be possible to collect or return any additional data. The unique measurements obtained during this final descent will be a fitting closing chapter to Rosetta’s time spent living with this comet.