Avoiding the fate of the dinosaurs

Around 65 million years ago, an asteroid 10 kilometres to 15 kilometres in diameter hit the Earth close to what is now the Yucatan Peninsula.

Already stressed ecosystems were devastated, leading to the extinction of 75% of the species living at the time, including the dinosaurs, ammonites, belemnites and others.

We now live in a world filled with people, in highly stressed ecosystems. We would be very unlikely to survive a similar impact were it to happen now. Therefore, we are working hard to avoid it. There are telescopes dedicated to detecting potential Earth-threatening asteroids, and programs to estimate whether they are an immediate or long-term threat and the probability of an impact.

Of course, detection is only part of the solution. If we find a potentially threatening asteroid, what can we do to remove that threat? Proposed solutions include sending a missile to the object and blowing it up or finding a way to nudge it onto a different and safer path.

A lot depends upon what asteroids are like. Are they rigid lumps of rock or are they rubble piles held together by their weak gravity. That has led to space missions to asteroids to see what they are like and, recently, an experiment to see if we could change its orbit enough to avert a hit.

A few close encounters with asteroids by spacecraft suggest most of them are basically loosely consolidated rubble piles. Whether one of these could be diverted into a new orbit using available space technology led to a NASA mission to Dimorphos, an asteroid with a diameter of about 180 metres (small, but large enough to do a lot of damage). This asteroid happened to be orbiting a somewhat larger asteroid, Didymos, with a diameter of around 780 metres.

The spacecraft, named DART, was intended to smash into Dimorphos and see whether its orbital path was changed by a useful amount. A double asteroid was chosen because the change in their orbits around one another would be far easier to detect, over less time.

The spacecraft was launched in November 2021 and hit it head-on in September 2022. The mission was a success in that the orbit was changed by more than expected.

However, something else happened. The shape of the asteroid was changed. Dimorphos was a rubble pile and it is likely that, if it were hit harder, it would have come apart. If it was an Earth-threatening asteroid, that could be a disaster.

Being shot with a rifle is bad. Being shot by a shotgun is far worse. If a larger rubble pile were heading in our direction, how hard could we hit it in order to change its path without smashing it? Another important consideration is the spacecraft took close to a year to get to the asteroid. We have to know at least a year in advance in order to act usefully.

The need to apply a push that changes the path enough without smashing the asteroid, together with having enough time to reach it, means we need to identify the threatening asteroids well in advance. That way we can apply a gentler push, or over a longer time.

We certainly know how to determine orbits with precision. However, predicting where an asteroid will be in a few years' time is made harder by the constantly changing gravitational influences of the other planets, especially Jupiter, the largest planet in the Solar System. Small perturbations build up rapidly over time, changing asteroid orbits significantly.

On the other hand, we are getting better at it, and spacecraft can be made that make small navigational changes en route. It is a challenge spotting small, dark objects against a black sky early enough to identify threats and act on them. However the incentive is certainly there.

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• Venus and Mars are getting lost in the dawn glow.

• Jupiter shines high in the southwest after sunset.

• The Moon will be new on March 10.

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