Comet Rendezvous and Landing


Comet Rendezvous and Landing: Rosetta and Philae  
A Guest Post by Joseph Friedlander
The Rosetta spacecraft is in the complex process of being captured by a low-mass body, Comet 
67P/Churyumov–Gerasimenko.  (This comet orbits between Mars and Jupiter and was discovered 
in 1969 by Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko).
Mass of the Rosetta was around 3000 kg at launch, including the 100 kg lander and 165 kg of 
scientific instruments.  Most of the mission delta V has been supplied by three Earth flybys 
(gravity assists) but a delta v budget of around 2200 m/sec was onboard. The craft is now 
considerably lighter than at launch, having spent quite a bit of this.  Dry mass
of the orbiter alone is 1,230 kg. Power at 3.4 AU is around 850 watts. 

Data on Comet 67P/Churyumov–Gerasimenko
  • Year: 6.45 earth years
  • Idealized radius: (if spherical–it ain’t–) around 2-2.2 km (mean density about one tenth to one-ninth that of water–amazingly low)
This is the first body visited between Earth and Jupiter whose density virtually guarantees 
a majority of the body mass being water ice (snow) and carbonaceous matter.
As of June the body was outgassing accurately at around a kilogram a second of water. This 
is 90 tons a day, 30,000 tons plus a year. And peak outgassing season (perihelion and after) 
isn’t yet.
  • Escape velocity .46 meter (not kilometer!) a second. (under half a meter a second)
  • 12.7 hour day, 6.35 hours sunrise to sunset
  • Distance from sun– Aphelion 5.6839 AU (850,300,000 km) Perihelion 1.2429 AU 
  • (185,940,000 km)
  • Mass of Comet 67P/Churyumov–Gerasimenko is around 3.14 gigatons, 3140 megatons–
So the comet weighs over a billion times the now much lighter (after delta-v burns)
spacecraft and the Rosetta is trying to get captured.
In May the spacecraft encounter velocity was round 775 meters per second, comparable to a 
SR-71 still picking up speed.  Now it has been cancelled down to around a meter a second (a 
leisurely walk pace) and the spacecraft is now associated with Comet 
67P/Churyumov–Gerasimenko but not yet orbiting it. It’s complex path is a dance whose 
segments are hyperbolic escape trajectories, each cancelled out in turn by delta-v 
subtracting thruster burns.
http://en.wikipedia.org/wiki/Rosetta_(spacecraft)#mediaviewer/File:Rosetta_111106.jpg
Where things are at launch, now and in the coming year. 
The ESA Giotto spacecraft in 1986 captured this picture of Halley’s comet in full outgassing 
glory which was considered amazing at the time:


http://en.wikipedia.org/wiki/Halley%27s_Comet#mediaviewer/File:Halley_Giotto.jpg
But a new generation of instruments has arisen which now yield pictures of the (still cold–(Next  perihelion is on 13 August 2015))  Comet 67P/Churyumov–Gerasimenko with this level of detail from 285 km away (5.3 meters/pixel):
Credit & Copyright: ESA 
/ Rosetta / MPS for OSIRIS Team MPS / UPD / LAM /IAA / SSO / INTA / UPM / DASP / IDA.
Capture radius (for orbit) should be somewhere between 40 and 25 kilometers, probably 
somewhere in the low 30s. As Rosetta flies lower, it will pick up speed, do minor but 
complex delta V maneuvers to  be captured in the first orbital mission around a comet.
When orbital coverage has been sufficient a landing site selection process will ensue. The 
Philae lander weighs under 100 KG. 


http://sci.esa.int/science-e-media/img/3e/Philae_instruments_grey_background_625.jpg

It will not be the first landing on a small body but the first on a known active comet. 
November 2014 is the target landing date.  The pictures should be even more amazing.
This next year will also see a flyby of Pluto at 14 July 2015 (the NEW HORIZONS probe) and a 
orbiter of Ceres some months earlier. (the DAWN mission).
This is the first body visited between Mars and Jupiter whose density virtually guarantees a 
majority of the body mass being water ice (snow) and carbonaceous matter.  If this was not a 
one ton probe but a thousand ton one-way do or die manned colonizing effort with 30 people) 
(or a million ton colonizing effort with 30000 people,) the greatest reality TV show in 
history would be about to begin.
This is one of the most recent pictures, from a distance of 130 km and the image resolution is 2.4 metres per pixel. The large boulders at lower right are perhaps the size of a small school or commercial building.
http://blogs.esa.int/rosetta/files/2014/08/Comet_details_node_full_image_2.png
How would you approach the problem of colonizing a body between Mars and Jupiter with this 
gravity (1/10000th Earth) this orbital and escape velocity (under half a meter a second) 
this ‘geostationary height’ (around 2-3 km radius) and this distance from sun– 
Aphelion
5.6839 AU (850,300,000 km)
Perihelion 1.2429 AU (185,940,000 km)? (Next  perihelion is on 13 August 2015)
At the very least that implies a regular schedule for expanding and shrinking solar concentrators made of thin foils. And remember sunrise to sunset is under 7 hours. (If you are planning on a mirror system in orbit to relay constant sunlight, remember outgassing season). Many colonization architectures are possible. But would you bet your life on them?
  Reflections and comments welcome below.
————————————————————
Links of interest
http://en.wikipedia.org/wiki/Philae_(spacecraft) (The lander)
http://en.wikipedia.org/wiki/Rosetta_(spacecraft) (The orbiter)
http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko (The comet)
http://www.wired.com/2014/08/comet-walk/ (Why Clark Kent might have problems walking on 
Earth–your muscles can reach escape velocity too easily 🙂
The Rosetta Blog
http://blogs.esa.int/rosetta/
It is controlled from ESA’s Operation Centre in Darmstadt, Germany, The ‘Houston’ of ESA. 

http://blogs.esa.int/rosetta/files/2014/06/Tracking_network_control_room-350×232.jpg

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