As you may know, ESA’s Rosetta spacecraft arrived at Comet 67P/Churyumov-Gerasimenko on 6 August 2014, and the current plan is to drop the Philae lander onto the comet’s surface on 12 November. Between those two dates, however, a huge programme of orbit manoeuvres has been / is to be undertaken, keeping the operations boys and girls at the European Space Operations Centre (ESOC) very busy. I’d like to catch-up a bit, by giving a flavour of all this activity leading up to the comet landing in November.
On arrival, the spacecraft entered a rather weird triangular ‘orbit’ around the nucleus in a plane in front of the comet relative to the Sun. See Figure 1. Initially the average distance from the comet was about 60 km, and the probe’s speed relative to the nucleus was around 60 cm/sec. Clearly, this ‘orbit’ was not bound gravitationally to the comet as the escape speed from the very weak gravity field at this distance is very much slower than 60 cm/sec. So the triangular track was achieved by ‘free-flying’ a hyperbolic trajectory along the triangle’s sides, and performing small thruster firings (< 1 m/sec of delta-V) at the triangle’s corners (see How Spacecraft Fly pages 69-78 for explanation of hyperbolic orbits, and page 173 for information about delta-Vs). Consequently the triangular sides are not straight lines, but slightly curved due to the gravitational influence of the comet. By carefully measuring the degree of curvature of the sides of the triangle, an estimate can be made of the total mass of the comet nucleus. This operation was repeated with a smaller triangular orbit, around 50 km from the comet, in order to refine the mass estimate. As an outcome of this exercise, the comet’s mass was estimated to be around 10 billion tonnes (1 with 10 zeros) or 10 thousand billion kilograms (1 with 13 zeros). This was a first step in determining the characteristics of the comet’s gravity field, and an assessment of the volume of the nucleus also gave a density estimate of the order of 300 kg per cubic metre – about 30% of that of water.
One of the principal constraints on this exercise was to keep the orbit plane perpendicular to the Sun direction to minimise the ‘drag’ effects on the spacecraft due to outgassing of material from the nucleus. The 64 square metre solar array would produce the greatest contribution to this perturbation, but the orbit constraint ensured that the array was ‘edge-on’ to any radial flow of material from the comet.
Further manoeuvres are planned to successively lower the orbit – a 20 km by 10 km elliptical orbit, followed by a 10 km by 10 km close orbit in which the orbital speed is around 26 cm/sec. It is expected that the spacecraft will briefly ‘dive’ to 5 km to drop the lander in November.
If you can’t quite get your head round the orbit strategy, a video presentation by Frank Budnik (a member of the flight dynamics team at ESOC) can be found at:
This was part of the ESOC press presentation on Arrival Day back on 6 August 2014. The relevant talk is the second in a sequence of presentations.