23 October 2008

The Science of Chandrayaan

Posted by Ryan Anderson

By now you’ve probably heard that India successfully launched its first moon mission yesterday (Oct. 22, 2008). But what will it do at the moon? Let’s take a look at the scientific payload and find out! This will be a 2-part post, starting with the five instruments made by the Indian space agency (ISRO), followed by six more made by countries around the world. Most of my information comes from the Chandrayaan Payload page, but I’ve tried to explain points that might not be clear based on their descriptions.

Terrain Mapping Camera (TMC) – It may surprise you to find out that this is a camera for mapping the lunar terrain! It takes images with a resolution of about 5m, and is a “pushbroom” imager, meaning it takes very long, narrow (20 km wide) images. It has two fields of view, both pointing 25 degrees away from straight down. One field of view looks ahead of the spacecraft and one looks behind. By combining the two views, scientists will be able to make a 3D map of the surface, the same way your brain uses the two slightly different fields of view from your eyes to construct a 3D model of the world around you.

HyperSpectral Imager (HySI) – This instrument will map the composition of the lunar surface. It takes images at 80m per pixel in 64 different wavelengths of light between 0.4 and 0.95 microns (blue to near-infrared). So, in a given image from HySI, you could choose a pixel and plot the brightness of the lunar surface at that location at 64 different wavelengths to get a spectrum. Based on which colors are bright and which are dark, scientists will be able to tell what minerals are present. The OMEGA and CRISM instruments at Mars work the same way.

Lunar Laser Ranging Instrument (LLRI) – All science is better when it involves lasers, right? This instrument has an infrared laser that pulses ten times per second. The instrument measures how long it takes for the light to travel from the spacecraft, down to the surface, and back again. Since the speed of light is pretty darn constant, the travel time can be converted into a distance between the spacecraft and the surface. If you know how far the spacecraft is from the surface, and how far it is from the moon’s center of mass (which you figure out based on its orbit), then you know how far the surface is from the center of mass. In other words: you get the topography! LLRI will measure the moon’s topography to within 5m. The topographic maps from LLRI (and TMC) will allow for all sorts of studies of the surface morphology and interior, including measuring the stress and strain on the moon’s crust, and the density distribution near the surface.

High Energy X-Ray Spectrometer (HEX) – Another instrument with a pretty straightforward name… if you know what a high energy x-ray spectrometer does. This instrument detects x-rays from radioactive elements on the lunar surface and from cosmic rays hitting the moon. This is useful for a number of things. One that I thought was pretty clever is that, by detecting x-rays from the decay of radioactive radon gas into lead, scientists will be able to trace how gases move around on the moon’s surface. That’s a handy bit of information to know if you are looking for places where water vapor might collect. Also, the lunar x-ray spectrum will tell scientists what the radioactive elements in the lunar crust are, and those elements can be used to trace overall composition. Finally, a thick ice deposit would absorb x-rays that normally would be emitted to space, so by measuring changes in x-ray emission, HEX might be able to detect water ice.

Moon Impact Probe (MIP) – Ever since the smashing success that was Deep Impact, it seems like everyone wants to crash things into solar system bodies. (For science, of course) MIP is mostly a technology demonstration which will be used to refine systems that could be used for soft landings. On the impactor, there is a radar ranging device, a camera, and a mass spectrometer. The mass spectrometer will be able to measure the extremely diffuse gases of the moon’s exosphere (gas that is blasted off the lunar surface by the solar wind). Of course, the impactor should also make a nice little crater that can be observed by the cameras on Chandrayaan as well as other missions.