In September of last year, after years of careful planning and development, NASA crashed a spacecraft smack into a rock drifting through the Solar System, just minding its own business.
It wasn't for the sheer hatred of space rocks, or the joy of collisions; the motive behind this exercise was to test our ability to knock an asteroid off-course, in the interest of Earth's safety. And now we know we're onto something. The measurements have come in, and the rock's course changed by significantly more than expected.
A series of five papers describing this course deflection, and the mechanisms behind it, have been published in Nature.
Right now, the planet beneath our feet appears to be sailing serenely through empty space. But there happen to be a lot of big space rocks out there, and if one were to hit us, we'd be in for a rough time. Just ask the dinosaurs.
One way we might deflect any large asteroids coming our way is by smashing into approaching rocks with a speeding spacecraft. The transfer of momentum from the spacecraft to the asteroid could alter its trajectory through space just enough to steer it away from its destiny with Earth's surface.
The Double Asteroid Redirection Test (DART) was an attempt to see if this was feasible. The target was carefully chosen: Dimorphos, a moonlet orbiting a larger asteroid called Didymos. Because the orbital period of the two objects has been well characterized, any change in Dimorphos' trajectory would be detectable as a change in its orbital period.
At around 160 meters (525 feet) across, Dimorphos orbits the 780 meter-wide Didymos roughly once every 11.9 hours. The DART impact was expected to alter this orbital period by around 7 minutes.
As described in a paper led by planetary astronomer Cristina Thomas of Northern Arizona University, the change in orbital period was much more dramatic: Dimorphos now orbits Didymos 33 minutes faster than it did prior to the impact. Two separate measurements of the orbit using different methods found the same result.
That larger-than-expected change to the orbital period of the binary asteroid system can't be accounted for by the transfer of momentum from the DART spacecraft alone.
A paper led by astronomer Jian-Yang Li of the Planetary Science Institute makes a detailed study of the ejecta – the material that was ejected from the asteroid as a result of the explosive impact. It wasn't just the immediate kaboom: for nearly two weeks after the impact, Dimorphos continued to spew out tails of dust, like a very dry comet.
A third paper, led by astronomer Ariel Graykowski of the SETI Institute in the US, studied the light reflecting off Dimorphos before, during and after the impact. A little over three weeks after the collision, Dimorphos's brightness returned to its normal, pre-impact levels. The level of the brightness over that period suggested that the asteroid lost 0.3 to 0.5 percent of its total mass.
According to a paper led by astronomer Andrew Cheng of Johns Hopkins University Applied Physics Laboratory, that ejecta was responsible for most of the change in the binary asteroid's orbit. That escaping material transferred more momentum to Dimorphos than was transferred by the DART spacecraft during its moment of impact.
"DART's impact," they write, "demonstrates that the momentum transfer to a target asteroid can significantly exceed the incident momentum of the kinetic impactor, validating the effectiveness of kinetic impact for preventing future asteroid strikes on the Earth."
Finally, a team led by planetary scientist Terik Daly of Johns Hopkins University Applied Physics Laboratory reconstructed the impact event from the collected data, including the timeline leading up to the impact, a detailed characterization of the impact site, and the size and shape of Dimorphos.
Their findings are promising. Humanity can successfully deflect an asteroid from its course with limited knowledge of its composition and surface conditions, without conducting an expensive and lengthy reconnaissance mission first.
An asteroid deflection mission, ideally, would be conducted decades in advance of the projected impact. Fortunately, time is a resource we have plenty of right now: no asteroids that we know of will threaten Earth for at least 100 years. This gives us time for a number of reconnaissance missions to any peripheral threats, which would improve the chances of successful deflection should anything change in the far future.
In light of that, the information we have from DART is invaluable. It will contribute towards modeling and planning future asteroid deflections, if we need them, for better predictions of the outcomes of exploding spaceships into space rocks.
"The successful impact of the DART spacecraft with Dimorphos and the resulting change in Dimorphos's orbit," Daly and his team write, "demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth if necessary."