The closest observed large asteroid yet to skim past the Earth without hitting the atmosphere was reported by astronomers on Sunday, August 22, 2004.

"It was close enough to strike, and if it did, it would have alerted US and Russian nuclear detonation satellites and caused a major alert 'stink' of unknown magnitude," said Steven Chesley of JPL (Jet Propulsion Laboratories). Chesley noted that the atmospheric grazing did in fact alert US, Russian and Chinese satellites that monitor exo-atmospheric thermal events and probably did trigger alarms around the globe.

"I am not privy to what events happened when the Russians first detected it, but I have a phone to explain these things to people that are higher in rank than me, and I did pick up my phone right away to explain it," said Chesley. "They saw it too, and didn't go belligerent like in 1996."

Chesley was referring to the 1996 Norwegian atmospheric rocket test that put Russian Strategic Rocket Forces on powered launch alert and resulted in nuclear launch codes being handed to a very drunk Boris Yeltsin in the Kremlin at 3 a.m. Moscow time. After several phone calls, a sober Russian General realized that it was a planned test that the Russians were notified about months in advance, and he deactivated the launch alert and powered down all the strategic missiles that were being wound up for launch.

The previously unknown object, spanning five to 10 metres across, has been named 2004 FU162. No scientist has gone on record to say why they came up with the designation of FU beyond "Fuck You." It streaked across the sky just 6500 kilometres - roughly the radius of the Earth - above the ground on 31 March, although details have only now emerged.

The MIT Lincoln Laboratory's asteroid-hunting LINEAR telescope in Socorro, New Mexico, US, observed the new object four times over a 44-minute period, several hours before its closest approach in March. They said it was the largest object to come close to impacting Earth since their observations began, and they don't know yet why it was such a surprise.

Lincoln astronomers, who have discovered over 40,000 asteroids and comets since 1980, quickly recognized the object came exceptionally close, and posted their findings for confirmation on a web page run by the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics.

However, by the time it was posted the object had moved into the daytime sky, so follow-up observations were impossible and the listing was quickly removed. A search for prior observations yielded no results.

Dissipated harmlessly

Despite having only four positions for the object, Steven Chesley of NASA’s Jet Propulsion Laboratory was able to calculate its orbit because it was moving rapidly across the sky.

He also calculated that the encounter with the Earth shifted the asteroid's orbit closer towards the Sun. Previously orbiting the Sun once a year in an orbit that ranged as far inside the Earth's orbit as outside, 2004 FU162 now has a nine-month orbit centered closer to Venus than the Earth. The Minor Planet Center published Chesley’s results on Sunday in its electronic circular.

"This was an extraordinarily close encounter and so the orbital change was quite extraordinary. 2004 FU162 was deflected by about 20 degrees because of the Earth's gravity. I've never seen anything like that before," Chesley told New Scientist. "Something made the orbit change, but we don't know what, yet."

The previous record for the closest asteroid approach to Earth was set on 18 March by an object called 2004 FH which missed the Earth by about 40,000 kilometres.

That was a much larger object, around 30 meters in diameter - big enough to produce a one-megaton explosion in the atmosphere. Although it was likely to have exploded so high that the energy would have dissipated harmlessly. The smaller 2004 FU162 is assumed to have burned up as a fireball ending with a smaller explosion, had it ventured into the Earth’s atmosphere. But that depends on what the composition of the object was.

"If it was metallic, it would have probably survived entry and possibly hit the Earth and caused real bad destruction," said Chesley. "I wouldn't have wanted to have been near it if it hit."