From one comet landing to another: planning Rosetta’s grand finale

As announced in June along with confirmation of the mission’s extension, Rosetta teams are planning to end the operational phase of the mission in a controlled impact of the orbiter on the surface of Comet 67P/Churyumov-Gerasimenko at the end of September 2016. While the specific details of the trajectories and impact site are still under discussion, ESA’s Rosetta Spacecraft Operations Manager Sylvain Lodiot, Project Scientist Matt Taylor, and mission manager Patrick Martin, share some background information on the planning of this dramatic mission finale.

Why will the mission come to an end?
Following perihelion in August, Comet 67P/C-G and Rosetta are now moving away from the Sun and back out towards the orbit of Jupiter. This creates a number of challenges:

Reduced solar power
As the comet recedes from the Sun, the amount of sunlight falling on Rosetta’s solar panels will drop significantly. As the power drops, it will not be possible to operate all of the scientific instruments at the same time. Ultimately, the point will be reached where Rosetta would have to be put back in hibernation, as it was for the 31 months leading up to January 2014. However, this time Rosetta will follow the comet out beyond the orbit of Jupiter on its 6.5-year journey around the Sun, further from its source of power than ever before.

On top of this, the spacecraft will have been in space for over 12 years, two of them spent in the comet’s dusty environment, with degradation of the solar arrays expected to further reduce the available power.

Solar conjunction
By September-October 2016, Rosetta and the comet will appear very close to the Sun as seen from the Earth, making the uplinking of operational commands and the downlinking of scientific data extremely challenging. On 1 October 2016, the spacecraft will enter a period of conjunction, i.e. it will be behind the Sun as seen from Earth.

Reduced data rates far from Earth
The increased distance from Earth and apparent proximity to the Sun lead to a significant drop in data rates. By early July 2016, we should be getting 91kbps via NASA’s DSN 70m ground stations and only 22 kbps via ESA’s ESTRACK 35m stations. By comparison, by mid September 2016, these rates will drop to 57 kbps via DSN and only 14 kbps via ESTRACK, similar to a 1995 era dial-up modem. Access to NASA’s bigger dishes is not always guaranteed, due to the high demand for their use by other missions.

These low data rates, combined with reduced power availability, will lead to difficulties starting in August 2016, as the instrument teams and science operations planning teams at ESAC work to deal with power-sharing issues and a much-reduced science data download capacity.

Why can’t you put the spacecraft back into hibernation?
In principle, Rosetta could be put back into hibernation and awakened several years later as the comet begins to approach the Sun again. In practice, however, this does not appear possible.

First and foremost, the comet’s aphelion – maximum distance from the Sun – is further than Rosetta experienced during its previous hibernation between 2011 and 2014. There will not be enough power to control the spacecraft, including, for example, the thermal control of the spacecraft – meaning it could freeze – and not be able to come out of hibernation at all.

Second, Rosetta relies on propellant to manoeuvre around the comet as it carries out its scientific measurements, and this is ever depleting, limiting any renewed post-hibernation operations.

Third, the spacecraft and science instruments are aging and will be well beyond their nominal operating lifetime post-hibernation.

These constraints led the Science Working Team to consider scenarios that would see the mission end in September 2016. Taking into account the available resources, the SWT decided that the ultimate end would be to first make a slow approach to the comet, getting back to within 10 km and even closer. Getting this close has not been possible for much of the past year due to the high activity of the comet, and there is much to be gained scientifically in investigating the low altitude region of the comet’s coma and studying its post-perihelion surface in great detail. Then Rosetta would begin a slow descent towards the surface, taking scientific data at very low altitudes, and ultimately leading to a controlled impact, ending this landmark scientific mission with Rosetta joining Philae on the surface of the comet.

In the meantime, the upcoming months of Rosetta operations will see a renewed focus on close proximity to the comet as its activity continues to drop, as well as investigations of previously uncharted territories at larger distances around the comet, including the tail region.

What does the landing scenario currently look like?
We are still discussing the sequence of events that will take place in Rosetta’s final weeks. Operations very close to the comet will be very complex and challenging, even more so than the trajectory planned by the flight dynamics team in 2014 to deliver Philae to the surface.

The key reason is that the closer Rosetta gets to the comet, the more important its non-uniform gravitational potential will become. This will have a significant impact on its trajectory, with huge perturbations in its apogee height – the furthest point from the comet on an elliptical orbit – expected. This will require much more control on the trajectory and therefore many more manoeuvres – our planning cycles will be reduced significantly.

The broad plan is to fly Rosetta in bound, highly-elliptical bound orbits that will take us as close as possible to the comet in the last two months of the mission, with flyby distances less than 1 km from the surface towards the end: the instruments will be able to collect great scientific data, including incredible images. In the last days of the mission, Rosetta will be on ever-closer bound elliptical orbits. We will then perform a final manoeuvre to put Rosetta on a controlled, slow collision course with Comet 67P/C-G. Subject to possible changes based on final flight dynamics analyses, the controlled impact is foreseen to take place on 30 September 2016.

Will we be able to communicate with Rosetta during the final descent?
During the final ‘collision trajectory’, the spacecraft’s high-gain antenna will be Earth-pointing, making it possible to get back telemetry and scientific data all the way down to contact. However, once impact has occurred, it is highly unlikely that any further communication with the Rosetta will occur.

Why can’t we remain in contact with Rosetta on the surface?
There are two parts to this question: will Rosetta continue to function on the surface, and if so, will we be able to receive any information from it?

Rosetta was not designed for a landing. Even under a slow impact, the very large solar panels may be damaged, and some of the instrument booms sticking out from the body of the spacecraft may buckle or snap off. Under the very low gravity of the comet, the spacecraft may tumble, further damaging it.

Furthermore, as Rosetta is solar-powered, to be able to operate it would have to land in a fully-illuminated part of the comet. As the comet rotates during its 12.4-hour day, the arrays would likely point away from the Sun, reducing the power below the operational threshold.

Critically, even if Rosetta were to function on the surface for a while, it would be extremely difficult to communicate. If the high-gain antenna points away from the Earth just half a degree, we would lose line-of-sight contact with ground stations on Earth. In addition, the spacecraft orientation (for example for high gain antenna and solar array pointing) relies on operational star trackers; it could be that Rosetta lands ‘face down’ with the star trackers pointing into the surface and the instruments pointing ‘up’ into space.

All in all, it is foreseen that Rosetta’s mission will end at the point of contact on to the surface of Comet 67P/C-G, a symbolic finale to an epic journey spanning almost 20 years of planning and preparation, and 12 years in space.

More details on the end of mission scenario will be provided when they are known.

Note: on the nominal mission end date of 30 September 2016, Rosetta and the comet will be 573 million km (3.8 AU) from the Sun and 720 million km (4.8 AU) from Earth. The one-way signal travel time will be approximately 40 minutes.