The time has come. We are going to smash a spacecraft into an asteroid.
The asteroid is Didymos B, the smaller of the two objects in the Didymos binary asteroid system. The spacecraft is NASA's Double Asteroid Redirection Test (DART). The reason: to test whether a spacecraft impact can deflect an asteroid's trajectory, as a means to protect Earth from rogue space rocks.
The Joint Asteroid Impact and Deflection Assessment (AIDA) project by the European Space Agency (ESA) and NASA was announced in 2015, but recent surprise findings from subsequent asteroid missions may have implications for the test.
For example, when JAXA's Hayabusa2 bombed the Ryugu asteroid in April of this year, it made a much bigger crater than expected. In addition, the material on the asteroid's surface behaves a lot like sand;
"The impact with Hayabusa2 showed that there was no cohesion on the surface and the regolith behaved like a pure sand. Gravity was dominating the process, rather than the intrinsic strength of the material from which asteroid is made, "explained planetary scientist Patrick Michel of CNRS.
" If gravity is also dominant at Didymos B, even though it is much smaller, we could end up with a much larger crater than our models and lab- Based on experiments to date have shown. Ultimately, very little is known about the behavior of these small bodies during impacts and this could have major consequences for planetary defense. "
Following an AIDA workshop last week in Rome, scientists have met at the EPS-DPS Joint Meeting 201
"Today, we are the first humans in history to have the technology to potentially deflect an asteroid from impacting the Earth, "astronomer Ian Carnelli of the ESA told Technology Review.
" The key question that remains to be answered is, are the technologies and models that we have good enough to actually work? Before you drive a car, you need to have an insurance policy. Well, AIDA is the insurance policy for planet Earth. "
The Didymos system is a perfect testbed, too. It's a near-Earth object – so, not too far away – that it's not on a collision course with Earth,
"DART's target, Didymos, is an ideal candidate for humankind's first planetary defense experiment," said planetary scientist Nancy Chabot of Johns Hopkins University Applied Physics Laboratory.
"It's not on a path to collide with Earth, and therefore poses no current threat to the planet. However, its binary nature enables DART to trial and evaluate the effects of a kinetic impactor. "
In this asteroid binay, the larger object, Didymos A, is about 780 meters across; the smaller, Didymos B, is 160 meters, and is sometimes called "Didymoon." It orbits the larger asteroid every 11.92 hours.
When DART rams into Didymos B at a speed of 23,760 kilometers per hour (14,760 miles per hour), it will only change the asteroid's speed very slightly – just a centimeter per second or so.
In a single asteroid , we might not be able to detect this at all; but in the Didymos system, the impact is expected to slightly change the orbital period. Rather than 11.92 hours, Didymos B may take a few minutes to go around Didymos B.  That doesn't sound like much, but if we can intercept an Earth-bound asteroid early enough, that small ve
DART is scheduled to launch in July 2021, for an impact in September 2022. A small cubesat called LICIAcube will detach from the spacecraft just prior to taking photos of the impact to the beam back to Earth. And Earth-based telescopes will observe Didymos to see if transit times change based on regular dips in the system's light curve.
The second part of the mission is ESA's Hera. This is a small observation spacecraft that will launch in 2023, and arrive at such observations of Didymos B in 2027. Since we cannot really see the asteroid system clearly from Earth, Hera will be able to tell us all the finer details – such as, for instance, if the DART impact makes Didymos B wobble longitudinally.
Hera has passed its system requirements review, and is now proceeding into the development stage.
"In addition to just technology and science, AIDA is also a really good experiment in terms of collaboration between scientists and agencies around the world. It's the sort of thing that would be needed to be an asteroid on a collision course for Earth. "