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Skywatching

Successful move pushing asteroid off course

Nudging an asteroid

NASA has just achieved a major feat in astronavigation.

A spacecraft has successfully impacted an asteroid that is orbiting another asteroid. This is not just a test of space navigation skill. It has a definite and important purpose— to see what is involved in protecting us from being hit by an asteroid, as has happened many times in our history.

Today, on our resource-stretched world, even a tiny asteroid impact could be devastating.

The only technically feasible method currently available for us is to apply a small nudge that would change an asteroid's orbit enough to turn a hit into a miss.

Because even a 100-metre diameter asteroid could weigh in at almost a million tonnes, changing its orbit is no trivial issue. If the asteroid is close to us and an impact is imminent, the shove required to change its course enough for it to miss us would be enormous, and might even shatter the asteroid.

The result would be that we get hit over wide areas of the Earth by fragments, which would not be an improvement. Fortunately, thanks to the workings of orbital mechanics, a small shove applied months or years in advance, when the asteroid is far from the Earth, could be enough to avoid the impact.

However, to be sure about this approach, it is a good idea to do some sort of test.

An asteroid in a potentially threatening orbit will take one to several years—or longer—to orbit the Sun. The best time to change the orbit is to do so when it is close to the outermost part of its orbit, tens of millions of kilometres from us. It would take months or even years to collect enough information to understand its orbit well enough to see whether we have changed it successfully, and probably months or longer to confirm that. There is a solution; to try changing the orbit of an asteroid orbiting an asteroid, where the orbits have short enough periods and any change we achieved would be quickly visible.

The chosen asteroid, named Dimorphos, is a 160-m diameter rock orbiting a 780-m diameter rock called Didymos. Before the impact, each orbit took 11 hours and 55 minutes. This means that any changes in its orbit due to our test would be detectable soon afterwards.

The Double Asteroid Redirection Test (DART) space vehicle rendezvoused with the pair of asteroids 11 million kilometres from Earth. The half-tonne spacecraft slammed into Dimorphos at about 22,000 km/h.

Our telescopes observed the flash of impact, and soon afterward, saw Dimorphos leaving a trail of debris ejected by the explosion. NASA decided for the test that a change in Dimorphos' orbital period around Didymos of 73 seconds or more would indicate the test was successful. In fact the change was a reduction in the orbital period by 32 minutes.

Hitting an asteroid orbiting the Sun to make sure it will not hit the Earth is challenging, thanks to gravitational tugging by the other planets, especially Jupiter. This makes it hard to predict precisely, years in advance where exactly a potentially threatening asteroid will be.

Given these uncertainties, that small push might be enough to push an asteroid that was going to miss us onto an impact course. The solution is to nudge the asteroid hard enough to change the orbit by more than our prediction errors. The DART experiment suggests this might be easier than we thought.

This still leaves us with some tough problems. Firstly we have to identify potential impact threats early enough for us to deal with them. Then we have to get our spacecraft to the asteroid to deliver the impact kick. The trip time could be months or longer. There is much work still to be done.

•••

• Mercury, the closest planet to the Sun, lies low in the east before dawn.

• In the early evening Jupiter lies in the east and Saturn in the southeast.

• The Moon will be new on Oct. 23.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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