Landing on a comet! That’s the goal of the European Space Agency’s Rosetta probe, which for ten years (three of them spent dormant) has been speeding toward the Comet 67P, which comes around the sun once every 6.5 years. Once it’s near the comet, and in orbit around it, the probe will launch a lander which, for the very first time, will enable humans to sample the surface of a comet. To show the magnitude of this achievement, the BBC reports that the chase has involved 6 billion km, and the comet that the probe has now rendezvoused with is travelling 55,000 km per hour (34,175 mph). That a species can do this defies the imagination.
Here’s what the comet looks like:
How big is it? Not too large:
A YouTube video (notes below) show that the probe’s approach to the comet was via a series of triangular motions, and the Economist excerpt below that shows why they did it this way:
(YouTube): After a ten year journey through space, ESA’s Rosetta spacecraft will reach comet 67P/Churyumov-Gerasimenko in August 2014 [JAC: it did that on Aug. 6]. After catching up with the comet Rosetta will slightly overtake and enter orbit from the ‘front’ of the comet as both the spacecraft and 67P/CG move along their orbits around the Sun. Rosetta will carry out a complex series of manoeuvres to reduce the separation between the spacecraft and comet from around 100 km to 25-30 km. From this close orbit, detailed mapping will allow scientists to determine the landing site for the mission’s Philae lander. Immediately prior to the deployment of Philae in November, Rosetta will come to within just 2.5 km of the comet’s nucleus.
This animation is not to scale; Rosetta’s solar arrays span 32 m, and the comet is approximately 4 km wide.
From The Economist:
So how does Rosetta move in a triangle? Essentially, by cheating. Every few days it fires its thrusters to execute the turn at each corner of the triangle. Rosetta will remain 100km from the comet for a couple of weeks, before closing to 70km. The long sides of the triangle, and the amount of fuel burn required to execute the turns at its corners, will allow the probe’s controllers to observe the effect of the comet’s gravity, and thus determine its mass. To complicate matters, the comet is oddly shaped, which makes its gravitational field irregular. Next month, once the comet’s mass has been established and its gravity field is understood, Rosetta will go into a circular orbit at a distance of 30km. After making further observations, it will then shift to an elliptical orbit in which it passes 10km from the comet at its closest point.
In November, Rosetta will eject the robotic lander Philae, which is small (total mass 100 kg, payload 27 kg). Here it is:
According to Wikipedia, the small sampling probe contains the following instruments.
- APXS (Alpha Proton X-ray Spectrometer) APXS analyzes the chemical element composition of the surface below the lander. The instrument is an improved version of the APXS of the Mars Pathfinder.
- COSAC (COmetary SAmpling and Composition) The combined gas chromatograph and time-of-flight mass spectrometer perform analysis of soil samples and determine the content of volatile components.
- Ptolemy an instrument measuring stable isotopic ratios of key volatiles on the comet’s nucleus
- ÇIVA (Comet Nucleus Infrared and Visible Analyzer)
- ROLIS (Rosetta Lander Imaging System)
- CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission). The CONSERT radar will perform the tomography of the nucleus by measuring electromagnetic wave propagation from Philae andRosetta throughout the comet nucleus in order to determine its internal structures and to deduce information on its composition.
- MUPUS (MUlti-PUrpose Sensors for Surface and Sub-Surface Science)
- ROMAP (Rosetta Lander Magnetometer and Plasma Monitor)
- SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment)
- SD2 (Drill, Sample, and Distribution subsystem) Obtains soil samples from the comet at depths of 0 to 230 millimetres (0.0 to 9.1 in) and distributes them to the Ptolmy, COSAC, and Civa subsystems for analysis. The system contains four types of subsystem: drill, carousel, ovens, and volume checker. There are a total of 26 platinum ovens to heat samples—10 medium temperature 180 °C (356 °F) and 16 high temperature 800 °C (1,470 °F)—and one oven to clear the drill bit for reuse
Finally, although I thought comets were made of ice, this one apparently isn’t. While still releasing water from its surface, it’s rocks, or rather two rocks that may have formed when two comets collided.
I had no idea this was going on, though you space buffs surely did, but the idea of landing a probe on a comet only a few km across, and travelling 55,000 km per hour, is stunning. They’re almost there, and I have little doubt that the probe landing will be successful. Stay tuned.