In its initial version SIM (Space Interferometry Mission) was the American equivalent of GAIA. Unlike GAIA, SIM was an astrometric interferometer which would function in targeted mode, capable of observing a far more limited number of objects (several thousand at most) but with a much better accuracy than GAIA (intended accuracy: 1 |as). Such an accuracy would have allowed the detection of giant planets but was not sufficient to detect Earth-like planets at distances comparable to Earth's from the parent star. The astrometric performance proposed for SIM would be obtained by the use of an astrometric interferometer, in conjunction with two guide interferometers. The principal difficulty with the mission was the necessity of measuring and permanently controlling the geometry of the astrometric interferometer to an accuracy of a few tens of picometers over the baseline.
Because of funding difficulties, the initial SIM program was postponed sine die in 2006.
The SIM-LITE (Fig. 8.6) concept proposed in 2007, is a revision of initial SM concept that is devoted, almost exclusively to the detection of extrasolar planets.
The SIM-LITE concept is a response to one of the recommendations of the American roadmap towards direct detection and description of terrestrial-type extrasolar planets. The concept behind the instrument was revised to reduce the number of targets to a few tens of nearby stars but also to increase the accuracy of the astrometric mode to one tenth of microarcsecond. Such an accuracy clearly permits the detection of Earthlike planets at orbital distances comparable to that of the Earth - the astrometric signal equals 3 |as for an Earth at 1 AU at 10 pc -and the identification of the targets for future missions determining characteristics directly. At the same time, the reduction of initial costs was possible by limiting the instrument's capability to a narrow field, and thus reducing the interferometric loops.
The technology required to reach the sub-microarcsecond scales (picometer metrology mentioned above) is already available. At present, the SIM team is attempting to find international partners to warrant funding of the project.
As mentioned in Chap. 2, radial-velocity measurements lead only to a rough estimation of the mass of the candidate because of the lack of information on the inclination of the system (one measures M.sin(i)). To estimate the value of sin(i) and discriminate between a planetary candidate with a small inclination and a brown dwarf or a low mass star with a high inclination, astrometric data, when available, are often used.
The Hipparcos mission data have a mean accuracy of several milliarcseconds. This is not sufficient to detect a planetary candidate but it does allow the detection of low-mass stars and heavy brown dwarfs.
The HST Fine Guidance Sensor has also been used for astrometry and the determination of characteristics of planetary systems (specifically, the inclination). Combined with Hipparcos data and with radial velocity measurements, it is a powerful tool to differentiate between stars and planets.
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