The threat to Earth by celestial ‘vermin’

On Jan. 26 a tiny asteroid passed Earth, a mere 3600 km away, at a speed of around 9.3 kilometres a second.

Known as 2023 BU, it was discovered by Crimean amateur astronomer Gennadiy Borisov about five days earlier. Fortunately, the object was too small to constitute a real threat to us. However, this event showed that even with special telescopes designed to spot asteroids, some of them slip through, only to be discovered by dedicated amateurs, or possibly missed altogether.

Until the end of the 18th Century, the Solar System presented a puzzle. There is a big gap between the orbits of Mars and Jupiter, which should not exist. There should be a planet there. In 1801, Giuseppi Piazzi, an Italian priest and astronomer, found something. He called the "new planet" Ceres. It was a bit small, with a diameter of about 1,000 kilometres, but at least there was planet where one was expected to be.

However, soon after, three more "new planets" were found orbiting in that gap. They were named Pallas, Vesta and Juno, all in similar orbits. This was entirely unexpected. It was agreed the bodies were not planets, so they became known as "asteroids", because through the telescopes of the time, they just looked like stars. They are better referred to as minor planets.

Now we know there are millions of these objects ranging in size from Ceres down to rubble. We now believe many of the asteroids are bodies that would have joined together to form that expected planet. However, Jupiter's strong gravitational pull stopped that happening. Others are wandering lumps of material that somehow never got captured by a growing planet. Although most of them spend their time between Mars and Jupiter, others range around the Solar System, with a good number venturing in among the inner planets, flying past us, and in a few instances in our history, have hit us.

When astronomers started using cameras on telescopes, which involved carefully controlling the telescopes' tracking during the hours-long photographic exposures that were needed to image the most distant galaxies and nebulae, the astronomers were not happy to see tracks across their laboriously obtained images. These were due to asteroids drifting across their field of view during the exposure. That led to asteroids being referred to as the "Vermin of the Skies".

With people living over most of the surface of the Earth, and its resources stretched to meet their needs, even a relatively small asteroid impact could be a disaster. That led to a number of sky monitoring programmes to detect threatening ‘vermin.’ A recent experiment has shown it is possible to change the orbit of asteroids on potential collision courses if they are detected early enough—years or many months in advance.

Most of the asteroids were formed from dust and ice. Many of the smaller ones are basically orbiting rubble piles just about holding themselves together by gravity. One of those approaching Earth would be torn apart by the Earth's gravity, due to it pulling at the nearest parts on the asteroid more strongly than the most distant parts. Much of the resulting rubble stream, moving at many kilometres per second, would burn up in the atmosphere.

If 2023 BU had come closer to us, that would probably have been its fate. Asteroids a few kilometres across or more, like the one that sealed the fate of the dinosaurs some 64 million years ago, are much more solid. There are solid, rocky ones out there, and some that are lumps of nickel iron.

That is why we take the celestial ‘vermin’ threat so very seriously.

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• Venus shines brightly, low in the sky after sunset. It looks like an aircraft landing light.

• Jupiter, almost as bright and yellower, lies in the southwest, with Mars, redder and fainter, high in the south.

• The Moon will reach last quarter on Feb. 16.

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