An asteroid was streaking toward Earth. A collision seemed all too possible

Saturday, 1 July 2017

On December 6, 1997, Jim Scotti of the Spacewatch programme at the University of Arizona, in the US, spotted an asteroid.

This wasn't entirely unusual. The problem: It appeared the asteroid was on a possible collision course with Earth.

Asteroid 1997 XF11, as it was later called, was a big one - 1.6km in diameter. The International Astronomical Union's Minor Planet Centre was alerted. Other astronomers weighed in. After a few months of study, Brian Marsden of the Harvard-Smithsonian Centre for Astrophysics announced the asteroid discovery and warned there was a small but 'not entirely out of the question' chance it would hit on October 26, 2028.

On June 30, 2017 - Asteroid Day - it's worth remembering that the prediction turned out to be wrong.

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But for a while, Marsden's calculations set off a panic. 'Asteroid Is Expected to Make A Pass Close to Earth in 2028,' read The New York Times front page headline on March 12, 1998. The Washington Post's story, running on an inside page, said the asteroid was 'virtually certain' to come closer to Earth than the distance to the moon.

This was not good. People seemed to recall learning at school that an asteroid had once wiped out dinosaurs.

'With regard to the asteroid,' wrote the Post's editorial board, 'we'd like a bit more reassurance.'

But was Marsden right? Would the asteroid come that close?

'Sure, there was some uncertainty associated with the actual miss distance,' he later wrote, 'but the tests that I made strongly suggested that the object would come closer than the moon.'

Astronomers continued investigating.

Eleanor Helin and Kenneth Lawrence, of the Jet Propulsion Laboratory in Pasadena, California, looked for and found previous images of 1997 XF11 taken accidentally in 1990. These images, upon being analysed by Paul Chodas and Donald Yeomans - also of JPL - ruled out the chance of collision in 2028.

'Zero chance,' was the JPL's ruling.

Marsden and his team later acknowledged they were wrong. As Dan W.E. Green of the Harvard-Smithsonian Centre for Astrophysics then told The Washington Post: 'There was no debate, of course not… We never disagreed. We quickly threw into our own saw that the closest approach moved out to 600,000 miles.'

The danger was over.

However, the debate over the XF11 affair continued. In 1998, Nasa created the Near Earth Object Program Office, now the Centre for Near Earth Object Studies, to find the 90 per cent of so-called near Earth objects larger than 1km in diameter. Congress also held hearings about the impact hazard of near earth objects.

Marsden maintained he was right to announce the presence of asteroid 1997 XF11 quickly. He wanted to make sure future observations of the asteroid wouldn't be missed for future study.

Underestimating the miss distance, he wrote in 2007, 'was the one and only shortcoming to my calculations'.

Marsden died on November 18, 2010.

The astronomy community had learned much from the XF11 affair. Now it was time to look forward, and to keep looking for asteroids.