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 six 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 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 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.
Famous quotes containing the words radar and/or astronomy:
“So I begin to understand why my mothers radar is so sensitive to criticism. She still treads the well-worn ruts of her youth, when her impression of mother was of a woman hard to please, frequently negative, and rarely satisfied with anyoneleast of all herself.”
—Melinda M. Marshall (20th century)
“Awareness of the stars and their light pervades the Koran, which reflects the brightness of the heavenly bodies in many verses. The blossoming of mathematics and astronomy was a natural consequence of this awareness. Understanding the cosmos and the movements of the stars means understanding the marvels created by Allah. There would be no persecuted Galileo in Islam, because Islam, unlike Christianity, did not force people to believe in a fixed heaven.”
—Fatima Mernissi, Moroccan sociologist. Islam and Democracy, ch. 9, Addison-Wesley Publishing Co. (Trans. 1992)