Astronomers from St Petersburg University calculate how a ‘gravity tractor’ can help protect the Earth from an asteroid

The research findings are published in the journal Astronomy Reports.

The ideas to save the Earth from pestilent asteroids captivate not only Hollywood filmmakers but also scientists around the world. Their fears are understandable. Provided that the celestial body is relatively small, about 20 metres in diameter, a collision with it might lead to a repetition of the scenario with the Chelyabinsk superbolide. But if the diameter is like that of the Tunguska space body, about 100 metres, the consequences will be equivalent to the explosion of several atomic bombs. If such an asteroid falls into the desert, nothing bad will happen. However, if it hits a city, it will turn it into ruins.

One of the ways to deflect asteroids is to hit a dangerous celestial body with a rocket.

This should result in a large momentum that will force the asteroid to change its orbit. This method is suitable for bodies with a diameter less than 100 metres.

Konstantin Kholshevnikov, the main author of the article, Head of the Department of Celestial Mechanics at St Petersburg University

The most radical method is the explosion of an asteroid with a nuclear bomb. If the diameter of the body is less than 100 metres, it can be destroyed. If the dimensions are larger, the explosion will not destroy the entire asteroid, but the generated reactive force will help deflect the remaining part and secure the planet. However, if the body is already on its way to Earth, this method will not work. The fact is that most of the asteroids approach the planet several times before colliding with the planet. Thus, after yet another approach, several years may pass before the body will fly directly towards the Earth. It is at this time when the body must be blown up. The expert notes that this is the only way that makes it possible to ensure that the debris will scatter in space and will not threaten humankind.

However, the probability that the asteroid will fly to Earth ‘without warning’ still exists.

The research team from St Petersburg University calculated the capability of using a peaceful method of counteracting asteroids, which excludes explosions and collisions with rockets. The astronomers believe that it is possible to deflect a cosmic body from a collision orbit with the Earth using a low-thrust engine.

The research was aimed at establishing the fundamental capability of using this method. According to our calculations, asteroids up to 55 metres in diameter can be deflected in a year with an engine thrust of 1 newton. With a thrust of 20 newtons, asteroids up to 50 metres in diameter can be deflected in a month, and asteroids with a diameter of up to 150 metres, in a year.

Konstantin Kholshevnikov, the main author of the article, Head of the Department of Celestial Mechanics at St Petersburg University

The scientists have created a model problem, according to which the engine provides constant tangential acceleration. In this case, the impulse is directed tangentially to the trajectory of the asteroid and, if necessary, increases or decreases the speed of the body. As a result, the asteroid avoids the collision with the Earth. Diverting larger asteroids requires more powerful engines. However, according to Konstantin Kholshevnikov, a low-thrust engine is the most profitable option considering fuel consumption.

Another challenge is how to mount the engine on a space body. The matter is further complicated by the fact that the asteroid is constantly rotating. This task requires a targeted solution and new calculations. However, it can be circumvented with a ‘gravity tractor’.

The research is supported by a grant from theRussia Science Foundation (project No 18-12-00050).

‘Using a “gravity tractor” is like pulling yourself by the hair. Suppose there is a relatively large asteroid, say 100 metres in diameter. A spacecraft flies by next to it and stops against this body. As a result of gravitational attraction, it begins to fall onto this asteroid. At this very moment, it is necessary to turn on the engine so that the spacecraft diverts. As a result, it pulls this large asteroid with it. It turns out the same thing – low thrust. The ‘tractor’ is also fit for this purpose because the rotation of the asteroid does not matter. If the engine is mounted directly on the asteroid, it will rotate with it. However, the tractor will not rotate,’ explains Konstantin Kholshevnikov.

However, both spacecraft – the ‘gravity tractor’ and the low-thrust engine, which is supposed to be mounted on the asteroid – exist today only in theories and models.

‘Humankind might suddenly face unforeseen danger. Could anyone have predicted the coronavirus pandemic? Or the fall of the Chebarkul meteorite? It is impossible to know in advance what resources will be required to save humankind next time. The advancement of science can help to cope with such threats. Moreover, it is of paramount importance to develop namely fundamental science,’ says Vladimir Titov, a co-author of the article, Associate Professor in the Department of Celestial Mechanics at St Petersburg University.

Further research will require more serious calculations. However, the scientists are convinced that humankind still has time for this.

Together with Konstantin Kholshevnikov and Vladimir Titov, doctoral students at the Department of Celestial Mechanics at St Petersburg University Danila Milanov and Khristina Oskina worked on the article.