Nov 23 2021

DART Asteroid Deflection Mission Ready for Launch

Published by under Astronomy
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Why is NASA planning on deliberately crashing a spacecraft into a small asteroid that poses no threat to the Earth? It’s a test of an asteroid deflection system – DART (Double Asteroid Redirection Test). Why the “double”? Most articles on the topic don’t say, and I had two hypotheses. The first is that the mission is targeting two asteroids, or actually a binary asteroid, Didymos (Greek for “twin”). Didymos has a primary asteroid that’s 780 meters across, and a smaller secondary asteroid 160 meters across that actually orbits the primary asteroid, and is therefore called a “moonlet”. However, the mission was originally supposed to be part of a pair of missions, with the second one by the ESA who were going to send their AIM probe to orbit and monitor Didymos during the DART mission. The ESA cancelled this mission, however, and now Didymos will be monitored by ground telescopes. But it turns out the “double” refers to the twin asteroids.

In any case, the purpose of the mission is to test out an asteroid defense system known as a kinetic impactor. The course of an asteroid can be altered by ramming something into it very fast. At first this seems like a crude method, but sometimes simple is best. The mission is part of NASA’s Planetary Defense Coordination Office. The European Space Agency (ESA) is also engaged in planetary defense, although their cancelling of AIM was disappointing. There are also international meetings on planetary defense, with calls for the USA, Russia and China to work together on this project. Russia, for their part, has proposed repurposing old ICBMs as asteroid busters. This would not be a kinetic impactor, but actually use nukes to blow up asteroids.

The DART mission is the first real test of an asteroid defense system. The spacecraft uses electric motors powered by solar panels, and will be going 6.6 km/s when it impacts the smaller Didymos asteroid. This impact will only divert the orbit of the asteroid by less than a percent, but that will be enough to change its orbit around the larger asteroid by several minutes, which can be observed from Earth. The craft is scheduled to launch tonight, November 23rd, at 10:21 pm PST aboard a SpaceX Falcon 9 rocket. It will intercept Didymos in late September 2022.

The kinetic impactor method is simple and relatively inexpensive compared to some other proposed methods, but has one drawback – we need a lot of lead time to use this method. The incoming asteroid will need it’s orbit accurately measured, and there needs to be time to design and execute the mission. Travel time to the asteroid can also be a year or longer. But most of all, we need years of warning in order for the tiny changes in the orbit of the asteroid to be enough to deflect it away from impacting the Earth. Each impact is a tiny nudge, but over the course of several years this small deflection can add up. For larger asteroids multiple impacts may be necessary.

The impactor method may also, if necessary, be combined with other methods of diverting an asteroid. We could, for example, land rockets on an asteroid an push it. This can be a very complex maneuver, however, especially if the asteroid is spinning. There is also the gravity tug method, parking a heavy spacecraft near an asteroid and using rocket to keep the ship close to the asteroid but keeping its distance so that the gravity of the asteroid does not pull it in. Instead, the gravity of the spacecraft give a tiny tug on the asteroid, but over years this can alter the orbit of the asteroid.

All these methods require years of warning. This is the reason for the UNs Office of Outer Space Affairs program to catalogue all near-earth asteroids. There are an estimated 26,000 near Earth asteroids over 140 meters that could potentially threaten the Earth, but only about 2,600 have been identified. There are potentially a million asteroids that could destroy a city as large as New York. There are no known asteroids that threaten the Earth for the next 100 years (again – known), but a large impact is inevitable. It may hopefully be thousands of years before a large impact, but we can’t build the fire house after the first starts so we need to be ready. This seems like a worthy investment, and one that we hope we will never need.

What if, however, we detect an incoming asteroid only weeks or days away? Then all these systems will be useless. That is where Russia’s proposal comes in – ICBMs have solid rocket fuel which means they can be in the silo ready to launch at a moment’s notice. The goal here is to literally blow up the asteroid with nukes. This would work of the asteroid is turned to mostly dust. But if larger chunks remains this could potentially do even more damage, with the Earth being hit by multiple asteroids rather than just one. It would be like a shotgun instead of a cannon ball – yes, both are bad, but the shotgun is not a good option. Still, it might be worth a try if the only other option is certain extinction. Perhaps dozens or even hundreds of nukes can be used to blow up as much of the asteroid as possible.

The best option, of course, is not to be in this situation. That is why asteroid identification and tracking is critical. We need as much advance warning as possible. Then methods like the DART impactor method will be enough. Here’s hoping the test goes well.

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