A 300-kilometre space rock has vanished since we saw it in 1995

Kitt Peak National Observatory

On the lookout for lost objects

David Nunuk/All Canada/Alamy

If lost space rocks warranted missing persons’ reports, the entry for 1995 SN55 would read something like this: “First seen 20 September 1995, gliding across the stars. Last seen a few weeks later. Never recovered.”

It was just… lost. We know, based on how it moved over a tiny fraction of its orbit, that it probably follows a path taking it closer than Saturn to the sun, then swings out as far as Pluto. We know this orbit lasts about a hundred years.

Based on its brightness, we know that it’s big – maybe 300 kilometres across. That would make 1995 SN55 among the largest, or maybe even the largest, of a class of objects called centaurs. These are intriguing bodies that orbit in an unstable zone of the solar system, where they are tugged on by the monstrous gravity of the gas giant planets.

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But we don’t really know anything more about this missing world, first spotted by Nichole Danzl and Arianna Gleason through the Spacewatch telescope on Arizona’s Kitt Peak. This is because the Spacewatch team lost track of it in their survey images after the end of October 1995 – and nobody has found it since.

Given the rigour of modern astronomy, this might seem shocking. An object that could be the biggest of its kind, and we can’t find it? But the truth is that solar system objects are lost all the time.

Lost and found

Let’s rewind a few hundred years. In 1612 and 1613, Galileo saw what he thought was a star. It wasn’t – it was Neptune, which was formally discovered on 24 September 1846, after flickering in and out of human attention for more than two centuries.

Or consider the case of Comet 41P/Tuttle-Giacobini-Kresak, which swings around the sun every 5.4 years. It was first discovered in 1858, then lost but found again in 1907, and then lost again and found a final time in 1951. Today, its name credits each of its three “discoverers”.

The issue is still widespread now, says Spacewatch team member Melissa Brucker at the University of Arizona in Tucson. In modern terms, “lost” means that astronomers have a better chance of stumbling across these objects in an unrelated survey than of tracking them down from what we know of their orbital paths, she says.

Right now, there are hundreds of lost objects in the Kuiper belt – the frozen wastes of the outer solar system.

More alarmingly, at this very moment there are 135 lost asteroids hanging out near Earth that also fall into the category of “virtual impactors”. This means that based on what we do know of their orbits, there is some – extremely small but not zero – chance that they will hit Earth.

Closing the case file

As for 1995 SN55, one of two things probably happened, says Alex Parker at the Southwest Research Institute in San Antonio, Texas. It could be that noise in some of the Spacewatch images gave a misleading idea of how it was moving, leading future searchers to look in the wrong place.

Or this huge cosmic object might never have existed at all. Something smaller might have brightened temporarily because it erupted or fell apart; or two smaller things could have collided, flashing bright before dimming – and tricking planetary scientists into chasing a ghost.

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