Last September, humanity conducted its first real planetary defense test. NASA’s DART mission collided with Dimorphos, the smaller companion of the asteroid Didymos, altering its orbit. Immediate analysis indicated that it was a success, and follow-up research published today in the journal Nature confirms that it exceeded all expectations.
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DART stands for Double Asteroid Redirection Test and is a kinetic impactor. This approach to planetary defense assumes that a small but fast spacecraft crashes into a “dangerous” object. The impact imparts momentum to the celestial body and shifts its orbit. Dimorphos wasn’t dangerous, but since it orbited Didymus, it was a great testing ground for how much its orbit could be shifted inward if slowed down. The impact alone was expected to shorten its duration by seven minutes. The final result was about 33 minutes, and this was due to the ejected cloud of debris.
“People might think of the DART mission as a pretty simple experiment, like playing pool in space — one solid spacecraft hitting one solid asteroid. However, asteroids are much more complex than just solid rock, in fact most asteroids are what we think of as piles of rubble,” said Dr. Christina Thomas of Northern Arizona University.
Thomas is one of the lead authors of five papers on the impact (literal and metaphorical) of the DART mission. One article was devoted to the changes in the Dimorphos period. Together, this study gives us a better understanding of the test and provides insight into other bodies and planetary defenses. The momentum imparted was estimated in another article, suggesting that more momentum came from the ejected material than from the impact alone.
“If you hit a pile of rubble by a spacecraft, a lot of material will be ejected and fly away from the object. We see this in our early post-impact images. This ejected material carries momentum. The change in period that we observe is not only the result of the momentum transfer from the colliding spacecraft, but also the additional momentum caused by the movement of the ejected material,” explains Thomas.
“For a perfectly inelastic collision — one solid spacecraft directly colliding with an asteroid without ejecting material — the change in orbital period is estimated at seven minutes. When we included the ejected material in the pre-impact analysis, we had a wide range of predictions for period changes up to just over 40 minutes.”
Scientists estimate that the DART impact occurred between two boulders, one of which hit the spacecraft as it crashed into the asteroid. The event was monitored by a small spacecraft called LICIACube, as well as several telescopes in space and on Earth.
Among them were civilian scientists using Unistellar telescopes who were able to trace the formation and evolution of the debris cloud. Some were even able to see the collision live.
Scientists have measured the brightness (or magnitude) of a binary asteroid system, allowing them to evaluate many properties of the object. The event created an artificial “comet” or active asteroid. This actually gives insight into the study of active asteroids. Unistellar estimated that the ejected material was between 0.3 and 0.5 percent of Dimorphos’ mass.
“Based on the magnitude and making assumptions about the grain size from active observations of main belt comets, we got the mass loss in the collision. This is the key to our paper, we basically treated the collision as the creation of a comet, to be able to get meaningful data that eventually agrees with data obtained using other methods, such as changing the period of Dimorphos, ”said one of the authors Dr. Frank Marquis.
Marfisi also highlights the ability of Unistellar telescopes to see in color. They saw that the ejection had a pronounced red tint. It is currently unclear if this was the effect of light through the dust or true color. The European Space Agency’s Hera mission, which launches next year, will reach asteroids in late 2026 and provide more information about it.
Not a single asteroid is known to pose a threat to Earth for at least the next 100 years. However, our catalog of smaller objects that can cause regional devastation is not yet fully completed. Although with the hope that they will never have to be used, planetary defense techniques are important. And DART shows that kinetic impactors work, as reported in another article in this issue.
“If we were to use a kinetic striker, we could expect more deflection from the striker than with a simple collision. This means that we can change the asteroid’s trajectory with less warning time. This fact would be incredibly important if we needed to reject the real target, ”notes Thomas.