MIRO is the first microwave instrument sent into space to study a solar system body. As a combined spectrometer and radiometer, MIRO can sense temperature and identify chemicals. Rosetta scientists will use it to determine how different materials in the comet change from ice to gas, and to observe how much comet C-G changes in temperature as it approaches the sun.
When Rosetta reaches comet C-G, MIRO will study four of the ten most abundant molecules usually present in a comet’s nucleus— water, carbon monoxide, methanol, and ammonia. Water and carbon monoxide are key parent gases for molecules that wind up in the comet's coma and tail. Methanol is a key hydrocarbon. Ammonia is one of the most abundant gases in the outer solar system.
MIRO will also detect the ratio of isotopes in the comet so that Rosetta scientists can determine the conditions in which the comet was formed. Different conditions, such as a shock wave triggered by a supernova moving through a molecular cloud, can create different isotopes. Isotopes are different versions of atoms, having different numbers of neutrons in their nuclei. For example, MIRO will detect the ratio of one oxygen isotope (16O) to two others (17O and 18O). These isotope ratios will not provide scientists with conclusive evidence of the comet’s origins, but they will provide crucial clues to that mystery. Rosetta scientists are like CSI detectives collecting forensic evidence from the scene of a crime, except in this case, they are trying to determine not a crime, but the origin of the comet itself.
MIRO will be able to determine the surface and subsurface temperatures of comet C-G's nucleus. This will be particularly important as the comet begins its active phase and jets of gas open up on the nucleus surface. MIRO will measure that rate at which the comet jets gas from the nucleus. Combined with detection of the comet’s surface-and-subsurface will these outgassing rates will help scientists understand the rate at which heat from the sun is conducted to the comet’s interior, which will provide them with more clues about the internal structure of the comet's nucleus. Scientists will also be looking for evidence of pockets or surface features, such as valleys, in the nucleus that are more likely to produce jets of gas. MIRO will also track the speed of these gases using Doppler shift, which is like tracking the movement of a train by listening to the changing pitch of its whistle.
Before arriving at comet C-G, MIRO will have studied Mars and the two asteroids, Steins and Lutetia. It was part of a suite of instruments that observed NASA's Deep Impact spacecraft as it sent a projectile into the path of comet Tempel-1 on July 4, 2005. MIRO detected water in the scattered light from exploding dust as it spewed from the comet in the weeks after the collision.