Landing on a Comet

What Will we Learn?

Now that the European Space Agency has chosen the name Agilkia (pronounced ah-ZHEEL-kia) for the next week’s landing spot on Comet 67P/Churyumov-Gerasimenko, our attention turns to the science that will be done by the Philae lander. What will we learn as this tiny probe settles down onto the icy surface of the comet?

Its stated goals are simply to transmit information about the surface, using data gathered by ten different scientific instruments. At the end of the lander’s mission (which could last several months), astronomers will have the most complete understanding of a comet’s nucleus to date.

Among other things, the lander will provide data that will reveal the age of the comet’s nucleus, what it’s made of, the mix of elements and compounds throughout the comet, the comet’s internal structure, and its interaction with the solar wind from the viewpoint of the surface.

Since we already know that comets are among the oldest objects in the solar system, this data will be like looking at conditions in the solar nebula before and during the Sun and planets were forming. Think of it as a window to the past, when all the building blocks of our solar system were finally falling into place.

Here’s a list of the instruments Philae will use to do all this amazing work:

• the Alpha Proton X-ray Spectrometer (APXS), which will detect alpha particles and x-rays and from those detections give information about the elements in the comet’s surface ices;
• the ÇIVA (Comet Nucleus Infrared and Visible Analyzer). It will take panoramic images of the surface, and use a spectrometer to measure the composition of the comet, its surface texture and reflectivity (the albedo) of surface samples;
• CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission). This is a radar instrument that will essentially do a tomographic scan of the comet’s nucleus. This “CAT” scan-type study will get done by measuring electromagnetic wave propagation from the Rosetta orbiter through the nucleus. The result will be a high resolution scan of what the comet is like inside the nucleus;
• COSAC (COmetary SAmpling and Composition). This is the instrument that will use study samples of the comet a gas chromatograph and time-of-flight mass spectrometer. These components study the chemical elements in the sample and will tell scientists the volatile components of the comet. Volatiles are elements that have low boiling points. At the comet these could include water, methane, nitrogen, carbon dioxide, and ammonia;
• MUPUS (MUlti-PUrpose Sensors for Surface and Sub-Surface Science). This instrument uses sensors on the lander to measure various aspects of the surface, including the density of its materials, its temperature, and other properties;
• Ptolemy will stable isotope ratios of key volatiles on the comet’s nucleus;
• ROLIS (Rosetta Lander Imaging System) is the heart of the lander’s imaging system. It will return high-resolution images of the comet as Philae approaches and lands on the comet, and then provide stereo panoramic images as other instruments study the surface;
• ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) is a magnetometer and plasma monitor to study the nucleus’ magnetic field and its interactions with the solar wind;
• SESAME (Surface Electric Sounding and Acoustic Monitoring Experiments) is a unique way to measure properties of the comet’s upper layers. It uses three instruments: the Cometary Acoustic Sounding Surface Experiment (CASSE), which measures the way in which sound travels through the surface; the Permittivity Probe (PP) investigates the surface’s electrical characteristics, and the Dust Impact Monitor (DIM) measures dust falling back to the surface.
• SD2 (Drill, Sample, and Distribution subsystem). This is the field geologist of the lander. It will drill into the surface and retrieve samples down to depths of 0 to 230 millimeters (0.0 to 9.1 inches). The retrieved samples will be distributed to the Ptolemy, COSAC, and ÇIVA subsystems for analyses. In addition to the drill, SD2 also has a carousel, 26 platinum ovens that will heat samples to different temperatures, and a volume checker.

This incredibly versatile little geology sampling mission is scheduled to land on the comet nucleus on November 12, 2014. It will detach from Rosetta at 08:35 UTC, and if all goes well, it will land seven hours later. You can follow the mission at ESA’s Rosetta site, as well as at NASA/JPL’s Rosetta site (NASA is involved with the mission; its contributions include three instruments) and I’m sure the Twitter feed will go crazy about it. I, for one, can’t wait to see what Philae finds at the comet!