Huge comets called centaurs deserve greater recognition as potential destroyers of life on Earth, according to a team of astronomers from the Armagh Observatory and the University of Buckingham. Measuring 50 to 100 kilometers (30 to 60 miles) across, these enormous masses of ice and rock have been identified in their hundreds over recent decades in the trans-Neptunian region – the area beyond Neptune, which is the outermost planet in the Solar System.

However, these centaurs can make their way towards the inner planets as their orbits become deflected by the gravitational fields of Neptune, Uranus, and Saturn. Publishing a review of their research in Astronomy and Geophysics, the team estimates that centaurs are likely to cross the Earth’s orbit every 40,000 to 100,000 years.

Depending on the size, composition, and distance of a given comet, its effects on terrestrial ecosystems are likely to be highly variable. Because centaurs are thought to be unstable, the authors expect the majority of these effects to be caused by dust and other small fragments that result from the disintegration of the comets.

They estimate that a single centaur measuring 100 kilometers (60 miles) could contain about 100 times the mass of all the Earth-crossing asteroids detected to date. This translates to an awful lot of dust, leading the researchers to suggest that comets of this size could fill the Earth’s atmosphere with tiny particles, reducing the amount of sunlight that can pass through to roughly the level of moonlight, for up to 100,000 years. This, they say, would put an end to commercial agriculture.

A map of the Solar System, showing the orbits of several planets. In red are the orbits of 22 centaurs. In yellow are the orbits of 17 trans-Neptunian objects. Duncan Steel/Royal Astronomical Society

Additionally, larger fragments – which could extend for several kilometers in length – may generate catastrophic impacts, similar to that which is believed to have led to the extinction of the dinosaurs. According to the report, several such collisions may have occurred in the past, with the authors citing craters in the Gulf of Mexico, Ukraine, Siberia and Chesapeake Bay as likely candidates for centaur impact sites.

Given the probability of a centaur crossing Earth’s orbit at some point in the future, and the sheer volume of debris caused by the disintegration of these comets, the team claims that some level of impact is inevitable, although the nature and extent of the effects are likely to depend on various factors.

At present, attempts to quantify Earth’s risk of being affected by a collision are centered around NASA’s Spaceguard initiative, which seeks to map up to 90 percent of near-Earth objects (NEOs) larger than 1 kilometer (0.6 miles). This program focuses mainly on the asteroid belt that sits between Mars and Jupiter, although Professor Bill Napier, who helped to conduct this research, is now calling for the scope of this search to be extended to the outer reaches of the Solar System.

“Our work suggests we need to look beyond our immediate neighborhood and look out beyond the orbit of Jupiter to find centaurs,” he said in a statement. “If we are right, then these distant comets could be a serious hazard, and it's time to understand them better.”