Hubble Space Telescope detects boulders knocked from the surface of Dimorphos.

The Hubble Space Telescope has detected a cluster of boulders knocked from the surface of Dimorphos (officially S/2003 (65803) 1 Didymos B) when NASA's DART probe deliberately impacted the body on 22 September 2022. The objects were discovered in an image made of the asteroid on 19 December 2022, three months after the impact. These boulders, which are estimated to be between 1 m and 6.7 m in diameter, are some of the faintest Solar System bodies ever discovered by Hubble, and are drifting away from their parent body at speeds of about 1 km per hour.

Image of Dimorphos captured on 19 December 2022 by the Hubble Space Telescope. The bright white object at lower left is the asteroid Dimorphos. It has a blue dust tail extending diagonally to the upper right. A cluster of blue dots surrounds the asteroid. These are boulders that were knocked off the asteroid when, on 26 September 2022, NASA deliberately slammed the half-tonne DART impactor spacecraft into the asteroid as a test of what it would take to deflect some future asteroid from hitting Earth. European Space Agency.

Dimorphos is a natural moon of near-Earth asteroid 1996 GT (65803) Didymos. It was discovered on 20 November 2003 by Czech astronomer Petr Pravec, and is thought to have formed when (65803) 1 Didymos shed some of its mass, due to its high rotation rate. This phenomenon, known as the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect, is thought to be caused by the absorbance of solar radiation by a body, which then re-emits the energy as photons. Since photons have mass, they exert a tiny amount of energy on the body emitting altering its spin rate by a tiny amount. Over time, this can cause bodies to spin faster and faster, until they are pulled apart by the centrifugal forces generated.

A spherical asteroid with two wedge-shaped projections. Re-radiated light from the 'B' fin has the same magnitude as the 'A' fin, but is not parallel to the incoming light. This produces a torque on the object. Wikimedia Commons.

Dimorphos is (or was before the impact) about 177 m in diameter, with an oblate spheroid shape and a surface covered in loose boulders with no visible craters. It orbited 1996 GT (65803) Didymos at an average distance of 1.206 km, completing one orbit every 11.9 hours. The impact is estimated to have knocked 1000 tonnes of material from the body's surface, altered it's orbital period by 32 minutes, and caused it to develop a dust tail 10 000 km long.

Animation of DART around Didymos and impact on Dimorphos.   Horizons/JPL/CalTech/NASA/Wikimedia Commons.

The parent body to Dimorphos, asteroid 1996 GT (65803) Didymos is calculated to have a 768.9 day (2.105 year) orbital period, with an elliptical orbit tilted at an angle of 3.14° to the plain of the Solar System which takes in to 1.01 AU from the Sun (101% of the distance at which the Earth orbits the Sun) and out to 2.27 AU (2.27 times the distance at which the Earth orbits the Sun, considerably more than the distance at which the planet Mars orbits). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). 

The orbit and current position of asteroid 1996 GT (65803) Didymos. JPL Small Body Database.

This means that 1996 GT (65803) Didymos has occasional close encounters with the Earth, with the last having happened in October 2022, and the next predicted for September 2041. The asteroid also has regular close encounters with the planet Mars, with the last having happened in July 1989, and the next predicted for February 2042.

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