| MIRO (Microwave Instrument for the Rosetta Orbiter) |
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Of the ten most abundant molecules known to be present in a comet nucleus, MIRO will study four of them up close and personal when it visits comet C-G. Those molecules are water and carbon monoxide, both key parent gases for molecules that wind up in the comet's coma and tail; methanol, a key hydrocarbon; and ammonia, a gas that is one of the most abundant in the outer solar system.
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MIRO, the first microwave instrument sent into space to study a solar system body, will also collect such clues as the ratio of one oxygen isotope (16O) to two others (17O and 18O). (These isotopes are versions of oxygen atoms with different amounts of neutrons in their nuclei.)
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Isotopic ratios such as these may provide clues about the circumstances under which the comet formed, since different conditions (a shock wave, triggered by a supernova, moving through a molecular cloud for example) create different isotopes. Determining comet C-G's isotopic ratios won't provide clinching evidence of the comet's origins -- not yet -- but scientists believe that collecting the evidence is worthwhile, somewhat like a detective collecting forensic evidence at the scene of a crime.
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As a combination spectrometer and radiometer, MIRO is capable of sensing temperature as well as identifying chemicals. Part of its task will be 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 open up on the nucleus surface.
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MIRO's measurements of outgassing rates, coupled with surface-and-subsurface temperatures, will provide clues about the internal structure of the comet's nucleus by helping scientists understand the rate at which heat energy from the sun is conducted to the interior of the comet. Such techniques may also help us determine whether the nucleus has any pockets, or surface features such valleys, that may be more prone to producing jets than other regions of the surface. MIRO will further monitor the evolution of outgassing material by using its ability to track speeds via Doppler shift (like tracking the movement of a train by the changing pitch of its whistle).
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Before arriving at comet C-G, the instrument will apply its talents to studies of Mars and the two asteroids Rosetta will pass.
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MIRO will also be used to observe NASA's Deep Impact spacecraft blasting a crater in comet Tempel 1 on July 4, 2005. If there is a lot of scattered light from dust during the experiment, MIRO may be one of only two instruments able to record the presence of water on the comet (the other being SWAS, the Submillimeter Wave Astronomy Satellite in low Earth orbit), and the only one able to observe nearly continuously.
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Making it possible for MIRO to fly aboard Rosetta required slimming the device down to just 10% of the mass its predecessors have had. While comparable Earth-based instruments weigh as much as 200 kg (440 lbs), which is more than Rosetta's entire payload, MIRO weighs only about 20 kg (44 lbs).
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The device consists of two heterodyne radiometers, one operating at millimeter wavelengths (190 GHz, ~1.6 mm) and one operating at submillimeter wavelengths (562 GHz, ~0.5 mm). Both are configured with a broadband continuum detector to determine the brightness temperature of the comet nucleus and the asteroids. The submillimeter receiver is also configured as a very high-resolution spectrometer.
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MIRO's Principle Investigator is Samual Gulkis of NASA's Jet Propulsion Laboratory in Pasadena, California. The science and engineering teams are from the U.S., France, and Germany.
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Please note that as of this writing, the website at this link contains some outdated references.
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