65803 Didymos

65803 Didymos (1996 GT) is an Apollo asteroid discovered on April 11, 1996 by Joe Montani at Spacewatch at Kitt Peak. It has a moon, whence the appellation "Didymos", meaning "twin". The primary is about 800 m in diameter and the moon 150 m in diameter. The moon is in an orbit about 1.1 km from the primary and with an orbital period of 11.9 hours. Didymos is the most easily reachable asteroid its size from Earth, requiring a delta-v of only 5.1 km/s for a spacecraft to rendezvous, compared to 6.0 km/s to reach the Moon. It is the target of the proposed AIDA spacecraft, an unmanned mission that would test the possibility of changing an asteroid's orbit via impacting its surface.

Discovery and naming

Didymos was discovered by Joseph L. Montani using the Spacewatch 0.9-m telescope in 1996. The binary nature of the asteroid was discovered by others; suspicions of binarity first arose in Goldstone delay-Doppler echoes, and these were confirmed with an optical light-curve analysis, along with Arecibo radar imaging on November 23, 2003. It has been informally named "Didymoon".

Didymos

Didymos Greek: Δίδυμος) was an ancient Greek music theorist in the last century before the common era. He was a predecessor of Ptolemy at the library at Alexandria.

Theory

We know of his theory only indirectly from the works of Porphyry and Ptolemy. There one finds examples of his tetrachords as measured string lengths from which the following proportions can be calculated:

Like Archytas he used a major third, but appears to have been the first to use it in the diatonic as sum of the large and small whole tones as for the proportions (10:9)•(9:8)=5:4 obtains.The difference of these whole tones is the so-called syntonic comma, also referred to as Didymos' comma

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