Radar astronomy

Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the echoes. This research has been conducted for four decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an active one. Radar systems have been used for a wide range of solar system studies. The radar transmission may either be pulsed and continuous.

The strength of the radar return signal is proportional to the inverse fourth-power of the distance. Upgraded facilities, increased transceiver power, and improved apparatus have increased observational opportunities.

Radar techniques provide information unavailable by other means, such as testing general relativity by observing Mercury, and providing a refined value for the astronomical unit. Radar images provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques.

The extremely accurate astrometry provided by radar is critical in long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure very accurately where an object appears on the sky, but cannot measure the distance accurately at all. Radar, on the other hand, directly measures the distance to the object (and how fast it is changing). The combination of optical and radar observations normally allows the prediction of orbits at least decades, and sometimes centuries, into the future.

Advantages

• Control of attributes of the signal [i.e., the waveform's time/frequency modulation and polarization]
• Resolve objects spatially;
• Delay-Doppler measurement precision;
• Optically opaque penetration;
• Sensitive to high concentrations of metal or ice.

Disadvantages

• Signal strength drops off very steeply with distance to the target.
• Must have a relatively good ephemeris of the target before observing it.

Planetary

The following are 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 1960. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.

Moon - first detection in 1945 - Surface roughness, mapping of shadowed regions near the poles.

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.

Asteroids and comets

Radar provides the ability to study shape, size and spin state of asteroids and comets from the ground. Radar imaging has produced images with up to 7.5-m resolution. With sufficient data, the size, shape, spin and radar albedo of the target asteroids can be extracted.

Only a few comets have been studied by radar, including 73P/Schwassmann-Wachmann. There have been radar observations of more than 220 Near-Earth asteroids and over 100 Main belt asteroids.

See also

• Deep Space Network
• Radar
• Radio astronomy

External links

• Andrew J. Butrica NASA SP-4218: To See the Unseen - A History of Planetary Radar Astronomy. NASA. Retrieved on 2008-05-15..
• Planetary Radar at Arecibo Observatory. NAIC. Retrieved on 2008-05-15..
• Goldstone Solar System Radar. JPL. Retrieved on 2008-05-15..
• Dr. Steven J. Ostro and Dr. Lance A. M. Benner JPL Asteroid Radar Research. Caltech. Retrieved on 2008-05-15..
• Radar Astronomy and Space Radio Science. Retrieved on 2008-05-15..