When I was a kid growing up, I loved watching science documentaries of all kinds. A rather popular topic was any variation on DOOM FROM THE SKIES – how we’re going to be destroyed by xyz astronomical event. It’s pretty easy to write a show around how the dinosaurs were killed by an asteroid impact and we might be next. But now we have NASA. And they’re on the job.
NASA and other organizations keep tabs on pretty much all large asteroids. Asteroids whose orbits carry them near the Earth are called, fittingly, “Near Earth Objects”, or NEOs for short. Ones which come close enough and are large enough to threaten us are designated as “Potentially Hazardous Asteroids” (PHAs). We know where all of the Earth killers are, and you can rest assured that none of them are likely to cause problems any time soon. But we still keep an eye on them just in case. And we want to be able to do something if one of them poses a serious threat.
This Monday, NASA’s planetary defense efforts made a huge leap forward with the successful impact of the Double Asteroid Redirect Test (DART) spacecraft onto the asteroid Dimorphos. The primary objective of this mission is to see what kind of change results in Dimorphos’s orbit as a result of this collision. This information could prove invaluable in the future if we need to do something about a rock headed our way. You can watch the final moments of the DART spacecraft here.
Dimorphos was specially selected for this mission, as it orbits around another asteroid named Didymos (the Greek word for “twin”). This pair of asteroids forms what is known as an “eclipsing binary”. Dimorphos’s orbit is almost edge on from our point of view, so Dimorphos periodically passes in front of and behind Didymos. By monitoring the regular variations in brightness of the system, ground-based telescopes will be able to very precisely determine how much Dimorphos’s orbit has changed. The kinetic energy of DART’s impact with Dimorphos is expected to shorten its orbit around Didymos by just a few minutes.
Neither Didymos nor Dimorphos will come anywhere close to Earth, so they do not pose a threat to us. However, in order to be able to respond to any potential threats, it is important for us to understand how asteroids like these respond to kinetic impacts. The easiest way to deflect an asteroid on course for Earth is to give it a little push long before it gets to us. That small perturbation will, over time, lead to a large deflection in its path of travel. (By the way, you would not want to blow up the asteroid, à la Armageddon, as then you’d just end up with a bunch of smaller asteroids headed your way – a shotgun blast).
But asteroids like Dimorphos are not solid bodies. They are more like loose concretions, held together through some cementation and gravity. As a result, they may not react to an impact in the same way that a solid rock would. This is one of the reasons the DART mission is so crucial. Didymos and Dimorphos present a perfect laboratory to test exactly what will happen if a loosely concreted asteroid is impacted.
For the next few years, astronomers around the world will be studying this system. The Hubble Space Telescope and James Webb Space Telescope have already captured images of the impact. And in 2024, the European Space Agency (ESA) will be launching the Hera space probe to study Didymos and Dimorphos, and to examine the crater left by DART. It will be at least a few months before we can determine how much Dimorphos’s orbit has changed.
P.S. – if you search “dart mission” on Google, you’ll get a fun little animation.
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