Sunday, December 4

DART has been a success. The question is whether it will really save us when an asteroid heads for Earth

The first part of NASA’s DART mission has been a success. According to the plans, the main probe of the mission crashed at 1:14, peninsular time (CET), against Dimorphos, the smallest of the asteroids in the Didymos system. It is certainly a feat to celebrate, but what are the chances that it will be of any use to us?

Success for now.
Everything went according to plan, and DART sent back its images at a one-second cadence, getting closer and closer to the surface of the asteroid it was meant to hit. In the last ones, the regoliths, stones and sand of the asteroid’s surface could be seen with good definition. The last frame, in red, indicated the loss of the connection. What in another mission would be a failure here implied success.

For now. And it is that the impact of the probe on the asteroid was only the first part. From now on, the DART team will have to carry out measurements on the orbit of Dimorphos to verify the effect of the collision on the orbit that Dimorphos traces over Didymos, the largest of the asteroids in the homonymous system.

Objective: planetary defense.
DART is just a test of a planetary defense project that aims to protect us from a possible impact of an asteroid. The idea is to divert the trajectory of the object that threatens the peaceful existence of humanity by “firing” at it with a spacecraft as has been done with DART and Dimorphos.

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When evaluating whether DART will be useful for something, two variables must be taken into account. First, the probability that an asteroid is headed on a collision course for Earth. The second is the probability that, if this circumstance occurs, the successor DART mission will succeed in diverting the object.

The higher the damage, the lower the probability of impact.
Thanks to decades of research, we have a rough idea of ​​what kinds of objects can threaten our existence. We know, for example, that as with other types of catastrophic events, the probability of an asteroid hitting us is inversely proportional to its destructive potential.

That is, events like the one that caused the extinction of the dinosaurs are much less frequent than events like the Chelyabinsk fireball. How much less? According to NASA’s own estimates, the impact of an asteroid of about 10 kilometers in diameter occurs on average every 100 or 200 million years. There are four objects of this size in our vicinity, so it can be concluded that the risk is small.

If we go down in size the probability increases. A one kilometer asteroid would not cause a mass extinction but its damage could send us back to the stone age. This impact could occur every half a million years.

The risk is in the middle ground.
Dimprphos has a size of about 160 meters. The impacts of these bodies could occur every 20 millennia. For reference, the Homo sapiens arose about 300,000 years ago and anatomically modern humans about 200,000 years ago. These impacts cause serious damage but on a limited scale. If it hit densely populated areas such as cities, the damage would be catastrophic. We estimate about 25,000 NEOs of this size.

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The fireballs that we can expect to see throughout our lives are those smaller than 25 meters. Those of 25 meters reach us once every 100 years on average and can cause severe damage as they pass through cities. There are millions of those around us. The little ones can pass often and almost without leaving a trace at ground level.

See it come.
The size of the asteroid is also very closely tied to our ability to see it coming. Just as we have duly located the largest asteroids, as it decreases, our records weaken.

Thus, we have located more than 95% of the asteroids larger than a kilometer in size, however, we barely have under control 40% of the asteroids with a range of 140 meters, that is, those similar to Dimorphos. If we go even lower, we have controlled less than 0.5% of the asteroids 25 meters or less.

Sun and shadow.
There is a second factor when it comes to seeing one of these rocks coming, and that is its origin. Asteroids do not emit light, so we rely on the light they reflect from the Sun.

Their composition can alter their albedo, but the key here is whether they come to us from the Sun (in which case we won’t be able to see them coming until too late), or from our “night.” In this second case, the asteroid will reflect sunlight and we will be able to see it more easily.

Hence the importance of monitoring the orbits of these objects and thus knowing years from now if they are going to come dangerously close to us.

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A matter of time.
There is still a lot of work ahead for teams at NASA and many other institutions before they can come up with an effective way to deflect an asteroid. We may have to wait decades until we know how to do it. The result won’t necessarily be a vehicle and a mission either, but more likely a series of mathematical models that will allow us to do the diversion calculations when the day comes.

The hypothetical future mission to divert it will depend on many factors, such as the size of the object, its speed and the angle at which it is going to intercept us. This can involve years of work until a satellite can be diverted. Hence the tremendous importance of keeping an eye on as many NEOs as we can and deciphering their orbital trajectories years, if not centuries, ahead.

Only time will tell if the DART mission has been worth anything. In the meantime we can continue to delight you with the accuracy with which the probe has hit a target located a whopping 11 million kilometers away.

Image | NASA/Johns Hopkins APL

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