65803 Didymos
Binary asteroid system targeted by NASA's DART impact mission.
65803 Didymos is a sub-kilometer binary asteroid and a near-Earth object of the Apollo group, also classified as potentially hazardous. It was first spotted in 1996 by the Spacewatch survey at Kitt Peak. Its small moon, about 160 meters across, was found in 2003. Because the system has two bodies, the asteroid was named Didymos, Greek for "twin." That moon, Dimorphos, became the target of NASA's DART mission, designed to see if a spacecraft impact could change an asteroid's orbit. The collision was observed by the Italian Space Agency's LICIACube, a flyby CubeSat.
The asteroid was discovered on April 11, 1996, using the 0.9-meter telescope at Kitt Peak National Observatory in Arizona. Signs of a companion first appeared in Goldstone radar echoes, and binarity was confirmed by optical lightcurve analysis and Arecibo radar imaging on November 23, 2003.
Didymos orbits the Sun between 1.0 and 2.3 AU every 770 days, with an eccentricity of 0.38 and an inclination of 3° to the ecliptic. Its closest approach to Earth's orbit is currently 0.04 AU (6.0 million km), though this changes over time. In November 2003 it passed 7.18 million km from Earth; the next similar approach will be in November 2123 at 5.86 million km. It will fly by Mars at 4.68 million km in July 2144. The Earth approach in October 2184, at 8.6 million km, has an uncertainty of about ±105 km. About a third of the time, Didymos travels far enough from the Sun to cross the inner main asteroid belt, where collisions are more likely. Roughly every 73,000 to 84,000 years, an impact hits it with energy equal to the DART spacecraft. Over its typical 8 to 10 million year lifetime as a near-Earth asteroid, Didymos has likely been struck several dozen times.
In the SMASS classification, Didymos is an Xk-type, bridging X-type and rare K-type asteroids. Visible and near-infrared spectroscopy show it is silicate-rich, making it a stony S-type. It rotates quickly, once every 2.26 hours, with a low brightness variation of 0.08 magnitude, suggesting a nearly spheroidal shape. Radar observations confirm it is oblate due to its rapid spin.
Dimorphos orbits Didymos in a mostly circular retrograde path with an 11.9-hour period. It is about 160 meters across, while the primary is 780 meters, a diameter ratio of 0.22. Before being named, it was provisionally called S/2003 (65803) 1 and informally known as "Didymoon"
Quick Facts
- Minorplanet
- yes
- Background
- #FFC2E0
- Discovered
- 11 April 1996
- Discoverer
- Spacewatch
- Discovery Site
- Kitt Peak National Observatory
- Mpc Name
- (65803) Didymos
- Pronounced
- ˈ · d · ɪ · d · ᵻ · m · ɒ · s
- Named After
- Greek word for "Twin"
- Epoch
- 21 January 2022 (JD 2459600.5)
- Uncertainty
- 0
- Observation Arc
- 24.82 years (9,066 days)
- Aphelion
- 2.2753 AU
Facts from the source article.
Lore & Background
Didymos was discovered on 11 April 1996 by the University of Arizona Steward Observatory's and Lunar and Planetary Laboratory's Spacewatch survey using its 0.9-meter telescope at Kitt Peak National Observatory. Its binary nature was first suspected from Goldstone delay-Doppler echoes and confirmed with optical lightcurve analysis and Arecibo radar imaging on 23 November 2003. The asteroid was named Didymos, Greek for 'twin', due to its binary nature, a name suggested by discoverer Joseph L. Montani.
The primary body is classified as an Xk-type in the SMASS classification, transitioning from X-type to rare K-type, but subsequent spectroscopy showed it to be silicate in nature, qualifying it as a stony S-type. It rotates rapidly with a period of 2.26 hours and low brightness variation, indicating a nearly spheroidal shape confirmed by radar observations showing it to be oblate. Its moon Dimorphos, approximately 160 meters in diameter, moves in a mostly circular retrograde orbit with an orbital period of 11.9 hours. The name Dimorphos, Greek for 'having two forms', was suggested by planetary scientist Kleomenis Tsiganis, representing how the moon's orbit would change after the DART collision.
Reader's Guide
Didymos's significance lies primarily in its role as the target of the DART mission, the first spacecraft to intentionally target an asteroid known to have a minor-planet moon. The DART spacecraft launched on 24 November 2021 and impacted Dimorphos on 26 September 2022, accompanied by the Italian Space Agency's LICIACube flyby CubeSat released 15 days before impact. The collision shortened Dimorphos's orbital period around Didymos by 32 minutes, far exceeding the minimum requirement of 73 seconds and the success benchmark of 10 minutes, with an uncertainty of ±2 minutes. Didymos is the most easily reachable asteroid of 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. The ESA's Hera mission, launched in October 2024, is planning to survey the dynamical effects of the DART impact and measure the crater's characteristics, with arrival expected in 2026. Two boulders on the system have been given names of traditional drums.
Did You Know?
- Its moon Dimorphos is about 160 meters in diameter, while the primary is 780 meters.
- The DART impact shortened Dimorphos's orbital period by 32 minutes.
- Didymos orbits the Sun at a distance of 1.0–2.3 AU every 770 days.
Frequently Asked Questions
Who is 65803 Didymos?
65803 Didymos is a sub-kilometer binary asteroid in the Apollo group of near-Earth objects, roughly 780 meters across, with a small companion moon called Dimorphos. It is classified as potentially hazardous because its orbit brings it close to Earth's path.
What role does 65803 Didymos play in space exploration?
The system served as the live test target for NASA's DART mission, which deliberately crashed a spacecraft into Dimorphos to demonstrate whether a kinetic impact could shift an asteroid's trajectory. The Italian Space Agency's LICIACube CubeSat flew by to capture imagery of the collision in real time.
Why is 65803 Didymos important?
It became the first real-world proof that a spacecraft impact can nudge an asteroid off its original path, validating a key planetary-defense concept. The name, Greek for "twin," reflects its two-body structure and made it a natural choice for a deflection experiment.
Where and when was 65803 Didymos discovered?
The asteroid was first spotted on 11 April 1996 by the Spacewatch survey at Kitt Peak National Observatory in Arizona, earning the provisional designation 1996 GT. Its companion moon Dimorphos was not identified until 2003.
More in Notable Asteroids, Part 4 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced