Жақын ғарыш объектілерін микротолқындарды кері шағылыстыру арқылы зерттеу

Observing nearby astronomical objects by analyzing reflected microwaves

Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from radio astronomy in that the latter is a passive observation (i. e., receiving only) and the former an active one (transmitting and receiving). Radar systems have been conducted for six decades applied to a wide range of Solar System studies. The radar transmission may either be pulsed or 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 astronomy is able to provide extremely accurate astrometric information on the structure, composition and movement of Solar System objects. This aids in forming long term predictions of asteroid Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure where an object appears in the sky, but cannot measure the distance with great accuracy (relying on parallax becomes more difficult when objects are small or poorly illuminated). 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. In August 2020 the Arecibo Observatory (Arecibo Planetary Radar) suffered a structural cable failure, leading to the collapse of the main telescope in December of that year. There is one remaining radar astronomy facility in regular use, the Goldstone Solar System Radar.

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.

The maximum range of astronomy by radar is very limited, and is confined to the Solar System. This is because the signal strength drops off very steeply with distance to the target, the small fraction of incident flux that is reflected by the target, and the limited strength of transmitters. The distance to which the radar can detect an object is proportional to the square root of the object's size, due to the one over distance to the fourth dependence of echo strength. Radar could detect something ~1 km across a large fraction of an AU away, but at 8 10 AU, the distance to Saturn, we need targets at least hundreds of kilometers wide. It is also necessary to have a relatively good ephemeris of the target before observing it.

Radar provides the ability to study the shape, size and spin state of asteroids and comets from the ground. Radar imaging has produced images with up to 7.5 meter resolution. With sufficient data, the size, shape, spin and radar albedo of the target asteroids can be extracted. Only 19 comets have been studied by radar, By 2018, this had grown to 138 Main Belt Asteroids, 789 Near Earth Asteroids, also at that time 20 comets had been observed.