The moon is comparatively close and was detected by radar, soon after the invention of the technique, in 1946. Measurements included surface roughness and later mapping of shadowed regions near the poles.
The next easiest target is Venus. This was a target of great scientific value, since it could provide an unambiguous way to measure the size of the astronomical unit, which was needed for the nascent field of interplanetary spacecraft. In addition such technical prowess had great public relations value, and was an excellent demonstration to funding agencies. So there was considerable pressure to squeeze a scientific result from weak and noisy data, which was accomplished by heavy post-processing of the results, utilizing the expected value to tell where to look. This led to early claims (from Lincoln Laboratory, Jodrell Bank, and Vladimir A. Kotelnikov of the USSR) which are now known to be incorrect. All of these agreed with each other and the conventional value of AU at the time.
The first un-ambiguous detection of Venus was made by Jet Propulsion Laboratory (JPL) on 10 March 1961. A correct measurement of the AU soon followed. Once the correct value was known, other groups found echos in their archived data that agreed with these results.
The following is a list of planetary bodies that have been observed by this means:
- Mars - Mapping of surface roughness from Arecibo Observatory. The Mars Express mission carries a ground-penetrating radar.
- Mercury - Improved value for the distance from the earth observed (GR test). Rotational period, libration, surface mapping, esp. of polar regions.
- Venus - first radar detection in 1961. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.
- Jupiter System - Galilean satellites
- Saturn System - Rings and Titan from Arecibo Observatory, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.
- Earth - numerous airborne and spacecraft radars have mapped the entire planet, for various purposes. One example is the Shuttle Radar Topography Mission, which mapped the entire Earth at 30 m resolution.
Read more about this topic: Radar Astronomy
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“Books of natural history aim commonly to be hasty schedules, or inventories of Gods property, by some clerk. They do not in the least teach the divine view of nature, but the popular view, or rather the popular method of studying nature, and make haste to conduct the persevering pupil only into that dilemma where the professors always dwell.”
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