Asteroid probe: "We need to develop faster defence missions after DART".
Alan Harris from the Institute of Planetary Research at DLR has been researching asteroids for 25 years. In this interview he talks about DART.
DART-Mission
(Bild: NASA)
(Hier finden Sie die deutsche Version des Beitrags)
Following the successful impact of the DART probe on the asteroid Dimorphos on the night of 27 September, Prof. Alan Harris comments on the expected results and whether DART brings us closer to a "planetary defence".
Technology Review: Prof Harris, DART has hit Dimorphos head on. What will we learn now?
Alan Harris: We didn't know anything about the nature of the surface until now. Depending on how soft or hard it is, the impact can have different effects.
Large telescopes on Earth, as well as the Hubble and James Webb space telescopes, have observed DART's impact.
Many observatories will now study the effect on Dimorphos' orbit around its larger parent asteroid Didymos over the coming weeks and months. And in 2024, the European probe Hera will be launched to measure the two asteroids on the ground in 2026.
Why exactly a binary system? Wouldn't it have been better to influence a single asteroid in its orbit around the sun?
That would have been more complex. Depending on the asteroid, the probe would have had to be larger and the subsequent observation of the orbit would have been more protracted. Because: Dimorphos orbits Didymos in such a way that we can see directly with telescopes from Earth how the small one moves around and over the large asteroid. The intensity of the reflected sunlight changes periodically during the orbit. By the slight but measurably increased frequency of the changes, we can see the effect left by DART. We can apply these findings to the orbit of asteroids around the Sun, i.e. the path that can bring an asteroid dangerously close to Earth.
What would an asteroid like Dimorphos do if it hit the Earth?
Dimorphos cannot come close to Earth in its orbit. However, an asteroid of its size, about 160 metres in diameter, would cause devastating damage if it hit the Earth. A crater several kilometres in size could wipe out a major city and devastate an entire region. The problem is: there are a great many asteroids as big as Dimorphos or slightly smaller or bigger, and we don't know them all yet.
If so many asteroids are still undiscovered, isn't the risk of a disaster incalculable?
Among the really big asteroids, several kilometres in diameter that would wipe out a continent and cause a global catastrophe, we thankfully find almost all of them. Organisations like the Minor Planet Center and NASA's search programmes make sure of that. Numerous telescopes keep a constant eye on them. Statistically, an impact of such an asteroid on Earth happens once in hundreds of thousands of years. The smaller ones are much more frequent. Of those from Dimorphos size upwards, we have discovered about 10,000 and 17,000 not yet. And of the even smaller ones, from about 50 metres in diameter upwards, which would not burn up on entering the Earth's atmosphere, the ratio of discovered to undiscovered is even worse and the number of undiscovered is even greater.
Do we even have a chance to react and, like DART, deflect an asteroid?
At the moment, we need five to ten years to plan, build and carry out a defence mission. If we know an asteroid and notice that it is changing its orbit in an unfavourable direction, for example due to effects such as the radiation pressure of the sun or the gravity of neighbouring planets, we would have that much time. We could then perhaps use a gentler deflection method than DART: the gravity tractor. This would involve a probe approaching the asteroid until the gravity begins to pull the probe in. The probe would then direct its engine against the asteroid's gravitational pull. With only enough thrust to balance the attraction and maintain the position.
In this way, the probe takes the asteroid in tow and changes its orbit around the sun sufficiently. This could also be achieved with a long-lasting laser or ion beam. However, the gravity tractor alone - not counting planning, construction and flight - would take years. A kinetic impactor like DART achieves the effect immediately.
In the event that the impact is unsuccessful or the effect is too small, a second impact mission would have to be kept in reserve. Ideally, you would send a reconnaissance mission to the asteroid beforehand. In this way, you can determine the asteroid's composition and mass - something we didn't do with DART and Dimorphos - and thus size the impact mission specifically. But that takes extra time again.
With a newly discovered asteroid, we don't have that time ...
Probably not. The smaller the asteroid, the less. The celestial body that exploded in February 2013 not far from Chelyabinsk, Russia, at an altitude of about 25 km, damaged thousands of buildings and injured about 1500 people. The asteroid was probably only 20 metres in size, came out of the daytime sky and was not detected in advance. An asteroid of about 50 metres could already devastate a larger region and cause many deaths.
We should detect it at least a few weeks or months beforehand. Too late for a defence mission, but sufficient for an evacuation of the endangered region. Above 100 metres in diameter, the area at risk is too large for evacuation, but in this size class detection becomes easier and the lead time longer. So we need to keep testing, improving and speeding up defence missions like DART.
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If necessary, a nuclear bomb will help?
No scientist, and hopefully no one, wants such a deployment. But if other defences have not worked and we are running out of time, this would be the last option. The problem is that such a mission cannot be tested. In the absence of an acute threat, the use of nuclear weapons in the atmosphere and in space is internationally prohibited and taboo for most states. Various studies have shown, however, that a nuclear explosion near a large asteroid could change its orbit very effectively. In the case of a small object, smaller than 100 metres, the asteroid could be disintegrated shortly before arriving at Earth in such a way that the fragments fly past Earth or largely burn up in the atmosphere.
Would you go along with Stephen Hawking and Elon Musk that asteroids and comets are probably the biggest threat to us humans?
A very large asteroid could actually wipe out humanity. The consequences would probably be even more serious than a nuclear war or climate catastrophe. Comets can approach the Earth on an extremely elliptical orbit very quickly and with great energy and similar risk. But the statistical probability of an impact is extremely low due to the comparatively small number of comets. And the same applies to large asteroids, which we know and observe. In the next few decades, I don't believe in an event worse than Chelyabinsk. And in 100 years at the latest, humanity should have mastered several defence methods - if we continue after DART. Climate change and nuclear weapons are the more realistic threats today.
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