What Are Asteroids? Leftovers of the Solar System and the Odds of a Hit

Asteroids are sorted by what their surfaces reflect. About three quarters are carbonaceous, dark as coal, rich in carbon compounds and water bound into their minerals, and unchanged since the solar system's birth; these are the most primitive material anywhere within reach, and the samples brought back from two of them by Japanese and American spacecraft since 2010 contain amino acids and the raw ingredients of life. Most of the rest are stony, made of silicate rock like the Earth's mantle, and a few percent are metallic, nearly solid iron and nickel, the cores of small bodies that grew large enough to melt and separate and were then shattered. The largest, Ceres, is 940 kilometres across, round, and counted as a dwarf planet; the next three, Vesta, Pallas and Hygiea, are around 500. Everything else is smaller and irregular, and the whole belt together weighs about four percent of the Moon.

The planets grew by accretion, small bodies colliding gently and sticking until they were large enough to pull in more. In the region beyond Mars, Jupiter formed first and fast, and its gravity swept through the belt in resonances that pumped up the speeds of the bodies there, so that collisions broke them rather than joined them. The belt also lost most of its mass early, thrown into the Sun or out of the solar system by the same disturbance. What survives is a thin ring, and the asteroids in it are so far apart, on average a few million kilometres from their nearest neighbour, that a spacecraft crossing it has never had to steer around one; the dense field of tumbling rocks in films is a fiction.

Some asteroids are nudged out of the belt by Jupiter's resonances or by collisions and into orbits that cross the Earth's, and those are the near-Earth objects that matter:

• •About 35,000 are known, and the count grows by about 3,000 a year as surveys sweep the sky
• •Nearly all of the roughly 1,000 larger than a kilometre, the size that would cause global damage, have been found, and none is on a collision course this century
• •Of the estimated 25,000 larger than 140 metres, the size that would devastate a region, about 40 percent have been found; a space telescope to find the rest is due to launch in 2027
• •Objects the size of the one that exploded over Chelyabinsk in Russia in 2013, about 20 metres, arrive roughly once a century and are mostly undetected until they hit the atmosphere
• •Sand-grain to pebble-sized debris hits the Earth constantly, some 100 tonnes a day, and burns up as meteors

Impact energy scales with mass and with the square of speed, and asteroids arrive at 15 to 30 kilometres a second, so even a modest one delivers the energy of a nuclear weapon. The 20-metre Chelyabinsk object released about 30 times the energy of the Hiroshima bomb high in the air and broke windows across a city, injuring 1,500 people with flying glass; the 50-metre Tunguska object of 1908 flattened 2,000 square kilometres of Siberian forest. Meteor Crater in Arizona, 1.2 kilometres wide, was dug by a 50-metre iron body 50,000 years ago. The 10-kilometre asteroid that struck Mexico 66 million years ago threw enough rock and soot into the sky to darken the planet for years and ended three quarters of all species, and an impact of that size happens roughly once every hundred million years.

In September 2022 a spacecraft the size of a vending machine hit the 160-metre asteroid Dimorphos at six kilometres a second and shortened its orbit around its larger companion by 32 minutes, far more than predicted, because the debris thrown off acted as extra thrust. It was the first test of moving an asteroid on purpose, and it showed that a body found years in advance could be deflected by a small push. That is why the surveys matter: the warning time is the whole defence, and an object found a decade out needs only a nudge, while one found a month out can only be evacuated from under. For the first time in the history of life on Earth, a species has the means to look for the next one, and a reasonable chance of doing something when it finds it.

News stories sometimes report that the chance of an asteroid hitting Earth has gone up, and then, weeks later, that it has dropped to zero. This pattern is normal and does not mean the scientists were wrong. When an asteroid is first spotted, only a short stretch of its path has been observed, so its future position is known only as a long, thin zone of possibilities. If Earth lies inside that zone, there is a small calculated chance of impact. As more observations arrive, the zone shrinks, and while Earth is still inside it, the fraction it occupies grows, so the odds rise.

Eventually the zone shrinks enough to slide off the Earth entirely and the chance falls to zero. The asteroid Apophis followed this path after its discovery in 2004: its estimated chance of hitting Earth in 2029 briefly rose to a few percent, the highest yet seen for an object of its size, before further tracking ruled out that impact and, later, any impact for at least the next century. It will still pass closer than many communication satellites in 2029, close enough to be seen with the naked eye from parts of the world. Old images that happen to show a newly found asteroid can settle its orbit quickly.

Most meteorites in museum drawers are fragments of asteroids, and matching them to their parent bodies is how much of the composition story was worked out. Astronomers split the sunlight reflected by an asteroid into its colours and compare the pattern of absorbed wavelengths with that of meteorites measured in the laboratory. One family of meteorites, the HED group, matched the spectrum of Vesta so well that it was proposed as their source decades before any visit. When NASA's Dawn spacecraft orbited Vesta from 2011, it found a giant impact basin near the south pole that could have blasted out exactly this kind of rock.

The most direct link came in 2008, when a small asteroid named 2008 TC3, a few metres across, was discovered less than a day before it struck the Earth, the first time an impact had been predicted in advance. It exploded high over the Nubian Desert in Sudan, and a team of researchers and students searched the area and recovered hundreds of fragments, now called the Almahata Sitta meteorites. For the first time scientists held rock from an object they had also watched and measured in space, and it proved to be an unusually varied mixture, a reminder that a single asteroid can carry material from several sources.

A newly found asteroid first receives a provisional code based on the date of its discovery. Once its orbit is known well enough for it to be reliably found again, it is given a permanent number, and its discoverers may then propose a name, which must be approved by a committee of the International Astronomical Union. Names cannot be offensive, too similar to existing ones or honour political figures until long after their deaths.

The results are wonderfully varied. Asteroids have been named after scientists, writers, places, mythological figures and musicians, including all four Beatles and Freddie Mercury. Some names honour amateur astronomers or schoolchildren who helped with discoveries. With more than a million asteroids known, only a small fraction have names; most are identified simply by their number and code.

Asteroids are the rock, carbon and metal fragments of a planet that Jupiter's gravity prevented from forming, most of them in the belt between Mars and Jupiter and none large enough together to make a body a twentieth the mass of the Moon. A few tens of thousands cross the Earth's orbit, the kilometre-sized ones are found and safe for now, the smaller ones are being counted, and the 2022 impact test showed that a deflection works given enough warning.