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 race to find a safe and scientifically interesting landing site began as soon as Rosetta arrived at comet 67P/Churyumov-Gerasimenko on 6 August 2014. Only then will the experts be able to study the comet in sufficient detail to choose a landing site. By mid-October the site will have been selected.
Why the rush? The date chosen for landing is a delicate balance between competing issues. On one hand, the lander must receive enough sunlight to generate power with its solar cells and recharge its batteries, and thus the landing cannot take place when the comet is too far from the Sun. The surface must be a reasonable temperature to ensure lander survival as well.
On the other hand, landing cannot occur too close the Sun either, because as the comet heads into the inner Solar System, the heat of the Sun becomes an increasingly powerful influence on its surface. Ices in and under the surface will sublimate, turning straight into gas, while previously trapped gas escapes through fissures opened in the comet's rock-ice nucleus. This gas escapes from the surface, dragging dust grains with it.
As activity builds up, this makes for an increasingly hostile and risky environment in which to operate the orbiter and deploy the lander. Already, comet 67P/C-G has given hints of such activity with an apparent outburst between late April and early June this year, and such events will likely grow in frequency and intensity as the comet draws nearer to the Sun.
Taking all of these factors into account, the most suitable time for landing is thought to be when the comet is about 3 Astronomical Units (450 million km) from the Sun. This is where it will be by mid-November.
Between August and October, Rosetta will have edged closer to the comet. If the activity is not too high by mid-October, Rosetta will be orbiting the comet at an altitude of about 10 km. A series of small manoeuvres will then place 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. 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 could be as short as 2 hours or as long as 12 hours, depending on the particular landing scenario chosen to get to the selected landing site. The lander will touchdown somewhere inside a "landing ellipse", roughly a few hundred metres across. The landing ellipse will have been selected to be as free as possible of hazards such as large boulders, 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, although its attitude will be 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 cold gas thruster on top will 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 escort phase. 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, at 186 million kilometres. 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.