The European Space Agency spacecraft Rosetta arrived at Comet 67P/Churyumov-Gerasimenko on 6 August 2014, and has since been in orbit surveying the comet nucleus at close quarters prior to the landing attempt scheduled for 12 November 2014.

The comet is around 4 km (2.5 miles) in size, and is essentially a 'dirty snowball' comprised of water ice and dust.  The spacecraft has an array of cameras which have been busy producing  high-res images of this amazing object, and pictures taken in September show that the comet is beginning to produce active outgassing/jetting as it responds to the increasing heat of the sun. 

The content of the majority of this post can be found in the 'Picture gallery' page of this website, which shows 12 recent images from the Rosetta spacecraft showing a variety of features of the comet nucleus.  During mid-September images were taken for the purpose of selecting the primary landing site for the Philae lander, and as an outcome of the process 'Site J' was selected as the primary landing site (see Images 4 and 9), and 'Site C' and the backup (see Image 5).  The criteria for choosing Site J were to minimise risk to the lander, while still being scientifically interesting - for example images have shown signs of outgassing activity nearby.  In terms of minimising risk, the majority of slopes in the neighbourhood are less than 30 degrees, and there are few boulders.  The site also provides sufficient solar illumination to provide necessary electrical to the lander's systems.

Finally I should of course acknowledge that all images are courtesy of the European Space Agency.

Quite a lot of things have happened since I last blogged about Rosetta, so I need to catch up a bit – please see last post below for my excuses and apology!

 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.


Thereafter the spacecraft entered a circular 30 km by 30 km ‘global mapping orbit’ to further refine knowledge of the gravity field, but also to begin the process of mapping the surface of the nucleus.  The orbital speed in this orbit was around 15 cm/sec.   The transition from the triangular orbit to the mapping orbit was fairly complex – see Figure 2 – but a more helpful insight is perhaps provided by a video animation to be found at:
http://www.esa.int/spaceinvideos/Videos/2014/01/Rosetta_s_orbit_around_the_comet


  

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:

http://www.esa.int/spaceinvideos/Videos/2014/08/Rosetta_at_comet_First_images_science_results

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.