Beyond the significant first rendezvous with and escort of a comet, Rosetta will attempt another first: landing on a comet.
Landing on a comet
The Rosetta orbiter will release the Philae lander at 08:35 UTC/09:35 CET on 12 November at a distance of approximately 22.5 km from the centre of comet 67P/Churyumov-Gerasimenko. About seven hours later Philae should touch down on the landing site, known as Agilkia. With a one-way signal travel time between Rosetta and Earth on 12 November of 28 minutes 20 seconds, that means that confirmation of separation will arrive on Earth ground stations at 09:03 UTC/10:03 CET and of touchdown at around 16:00 UTC/17:00 CET.
On 6 August, Rosetta arrived at a distance of 100 km from the comet. Between then and mid-October, the spacecraft edged closer to the comet approaching to within 10 km of the centre.
By the end of October, a series of small manoeuvres had placed the spacecraft into the pre-delivery orbit. The delivery manoeuvre requires extremely high precision, so Rosetta will remain in this orbit for a few days to give the spacecraft operators time to verify the position and velocity of the spacecraft with great accuracy.
The first in a series of Go/NoGo decisions will be taken on 11 November, prior to separation, with a confirmation from the flight dynamics team that Rosetta is on the right trajectory ahead of lander delivery. Further Go/NoGo decisions will be made during the night of 11–12 November concerning readiness of the orbiter and uplink of commands, and confirmation of the lander, culminating in confirmation of the lander readiness for separation. Once this is done, the landing attempt can begin.
From the pre-delivery orbit, Rosetta will manoeuvre to a hyperbolic trajectory flying in front of the comet, on the Sun side. Two hours later, the lander will be automatically released. It can be pushed away from the orbiter at a selectable speed of between 0.05 m/s and 0.51 m/s. The exact value will depend on the properties of the comet, and on the chosen separation and descent scenarios. If the initial deployment is not successful, a backup spring mechanism will ensure that the lander is released at a speed of about 0.18 m/s. For obvious reasons then, a separation and descent strategy with this value is favoured.
Once released, Philae is on its own. A signal will take about 30 minutes to cross the distance between Earth and the comet at that point, which is far too long to allow any kind of manual intervention.
The descent to the landing site, Agilkia, will take about seven hours. The lander will touch down somewhere inside a “landing ellipse”, roughly a few hundred metres across. The landing ellipse was selected to be as free as possible of hazards such as large boulders and to avoid, as much as possible, slopes exceeding 30 degrees, but there will nevertheless be a degree of risk involved.
As Philae descends it will fall slowly without propulsion or guidance, gradually gathering speed in the comet’s weak gravitational field, with its attitude stabilised via an internal flywheel.
During the descent, images will be recorded with the downward looking camera and some of the science experiments on the lander will be active too. Meanwhile, the orbiter will continue on its trajectory away from the comet’s nucleus. A small manoeuvre will allow it to look back and monitor Philae’s descent using cameras. This manoeuvre also ensures that there can be communication between the orbiter and lander during the descent and up to 90 minutes after landing.
Philae will reach the surface at roughly walking pace, around 1 m/s. That may not sound like much, but as the comet’s surface gravity is roughly one hundred thousand times weaker than Earth’s, a sophisticated system must be used to prevent it from rebounding into space. The three-legged landing gear will absorb the momentum and use it to drive an ice screw in each foot into the surface. At the same time, two harpoons will fire to lock the probe onto the surface, and a small thruster on top may be used to counteract the recoil of the harpoon.
Once anchored to the nucleus, Philae's primary science mission will begin and must happen quickly. Its initial battery life is only 64 hours, and while it also has solar cells with which to recharge the batteries and extend its lifetime, this will depend on the landing site location and illumination, and how much dust collects on the panels.
Philae will take panoramic images of its surroundings, with a section in 3D, and high-resolution images of the surface immediately underneath it. It will perform on-the-spot analysis of the composition of the comet’s ices and organic material, and a drill will take samples from a depth of 23 cm and feed them to the on-board laboratory for analysis. The lander will also make measurements of the electrical and mechanical characteristics of the nucleus surface.
The data will be relayed to the orbiter, ready for transmission back to Earth at the next period of contact with a ground station. For the first five Earth days, there will be regular contact between the orbiter and lander when the two can see each other as the comet rotates with its 12.4 hour period. In addition, low-frequency radio signals will be beamed between Philae and the orbiter through the nucleus, to probe its internal structure.
The detailed in-situ surface measurements that Philae makes at its landing site will be used to complement and calibrate the extensive remote observations made by the orbiter covering the whole comet. Once the primary science mission has been completed, the lander will continue to monitor the physical and chemical properties of the comet’s surface as it continues on its journey towards the Sun and for as long as the batteries are able to recharge.
In the meantime, Rosetta will begin the next major part of its mission: the orbiter will continue to manoeuvre around the comet at walking pace, collecting dust and gas samples and making remote sensing observations as the comet warms up and the nucleus and its environment evolve. The comet will reach its closest point to the Sun (perihelion) in August 2015. Rosetta will then track the waning of activity as the comet heads back towards the outer Solar System, at least until the end of 2015.