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DART Mission Offers First View of Asteroid Impact Ejecta Tail Formation

March 1, 2023
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Ejecta from Dimorphos 8 hours after the DART impact as observed by HST on September 27, 2022. DART impacted Dimorphos from the 10 o’clock direction. The ejecta formed an overall cone-like shape that opened towards the DART incoming direction and viewed from the side. The stream at 2 o’clock direction is the tail.

Credit: NASA, ESA, STScI, Jian-Yang Li (PSI), Image Processing: Joseph DePasquale.

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Ejecta from Dimorphos 1.7 days after impact taken on September 28, 2022. The base of the ejecta cone started to be twisted by the gravity of Didymos, rotating in clockwise direction, forming two curved streams.

Credit: NASA, ESA, STScI, Jian-Yang Li (PSI), Image Processing: Joseph DePasquale.

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Ejecta from Dimorphos 4.7 days (above) and 8.8 days (below) after impact, taken on October 1 and October 5, 2022, respectively. The Sun is at the 8 o’clock direction. The ejecta is pushed by the sunlight towards the 2 o’clock direction and increasingly stretched to form streaks.

Credit: NASA, ESA, STScI, Jian-Yang Li (PSI), Image Processing: Joseph DePasquale.

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For the first time scientists have observed how the debris excavated by an impact into an asteroid disperses into space and how a tail can form from such an event.

NASA’s Double Asteroid Redirection Test (DART) mission on Sept. 26, 2022 crashed a spacecraft into asteroid Dimorphos, the minor-planet moon orbiting the asteroid Didymos, and altered the orbital period of the moon by 33 minutes. The project was to determine if an asteroid threatening Earth could be potentially effectively diverted by such an impact.

“We have observed a number of active asteroids that displayed tails.  Some hypotheses have been postulated for what caused the formation of the tails, one hypothesis being due to impact,” said Jian-Yang Li, Planetary Science Institute Senior Scientist and lead author of the paper “Ejecta from the DART-produced active asteroid Dimorphos” that appears in Nature. PSI’s Stephen Schwartz and Jordan Steckloff are co-authors.

“The DART impact and the subsequent tail formation definitively demonstrates that an asteroid tail can origin from an impact, and provides the details of the ejecta evolution and the tail formation that will form the direct observational basis for the interpretations of past and future observations of active asteroids,” Li said.

NASA’s Hubble Space Telescope observed the beginning and development of Dimorphos’ ejecta. “Our observations reveal the complex ejecta evolution in the first two and a half weeks under the influence of the gravity of Didymos and the pressure of sunlight, said Li, a DART Investigation Team member, and the lead of the HST observations of the impact ejecta.

“A simple way to visualize the evolution of the ejecta is to imagine a cone-shaped ejecta curtain coming out from Dimorphos, which is orbiting Didymos.  After about a day, the base of the cone is slowly distorted by the gravity of Didymos first, forming a curved or twisted funnel in two to three days,” Li said. “In the meantime, the pressure from sunlight constantly pushes the dust in the ejecta towards the opposite direction of the Sun, and slowly modify and finally destroy the cone shape. This effect becomes apparent after about three days. Because small particles are pushed faster than large particles, the ejecta was stretched towards the anti-solar direction, forming streaks in the ejecta.”

“What’s unique about the evolution of this particular ejecta is the distortion of the ejecta shape by the gravity of Didymos. This happens only because the ejecta comes out from the moon in a binary asteroid system. Had the impact happened on a single asteroid, such a complex evolution would not have happened, and this is really interesting,” Li said.

Li’s work was funded by a NASA grant through the Space Science Telescope Institute to support the analysis of this observation.


Alan Fischer
Public Information Officer

Jian-Yang Li
Senior Scientist

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