I’m glad to see the widespread coverage of the DART mission results, both in terms of demonstrating to the public what is possible in terms of asteroid threat mitigation, and also of calming overblown fears that we have too little knowledge of where these objects are located. DART (Double Asteroid Redirection Test) was a surprisingly demonstrative success, shortening the orbit of the satellite asteroid Dimorphos by an unexpectedly large value of 33 minutes. The recoil effect from the ejection of asteroid material, perhaps as high as 0.5% of its total mass, accounts for the result.

Watching the ejecta evolve has been fascinating in its own right, as the interactions between the two elements of the binary asteroid come into play along with solar radiation pressure. Asteroids have previously been observed that displayed a sustained tail, as Dimorphos did after impact, and the DART results suggest that the hypothesis of similar impacts on these objects is correct. Thus we learn valuable lessons about how asteroids behave when impacted either by technologies or by natural objects. We can expect the study of ‘active asteroids’ to get a boost from the success of this mission.

The two images below are from the Hubble instrument, which observed the development of Dimorphos’ tail. Jian-Yang Li (Planetary Science Institute) is lead author of a recent paper in Nature on the evolution of the ejecta. Li comments on the interplay between the gravity of Dimorphos and parent asteroid Didymos as well as the pressure of sunlight in the first two and a half weeks after the impact. Bear in mind that an impact on a single as opposed to a binary asteroid would not display such complex effects. The presence of Didymos was indeed useful:

“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. In the meantime, the pressure from sunlight constantly pushes the dust in the ejecta towards the opposite direction of the Sun, and slowly modifies and finally destroys 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.”

Image: 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.

So we’ve learned that slamming a 570 kilogram spacecraft into this type of asteroid at something over 22,000 kilometers per hour can alter its orbital speed. Data from the Light Italian CubeSat for Imaging of Asteroids (LICIACube) is part of the current analysis, while we’ll learn yet more about the effects of the impact from the European Space Agency’s Hera mission, which will survey both Didymos and Diomorphos, focusing on the crater left by DART and the changes to the mass of the impacted asteroid.

The matter of locating those hazardous asteroids that have yet to be identified is now highlighted by what we can consider the next planetary defense mission, the Near-Earth Object Surveyor, planned for a 2028 launch. The mission will carry a 50 centimeter diameter telescope operating at two infrared wavelengths, conducting a multi-year survey in search of near-Earth objects larger than 140 meters. The goal is to find 90 percent of such objects coming within 48 million kilometers of Earth’s orbit. The observation strategy employed should allow accurate enough determination of asteroid orbits to allow them to be found again and their trajectories tracked.

Image: Near-Earth asteroids and the possibilities of impact. Credit: NASA.

The paper on DART, one of five papers recently published in Nature on the mission, is Jian-Yang Li et al., “Ejecta from the DART-produced active asteroid Dimorphos,” Nature 01 March 2023 (abstract).