Sorry guys, I meant to post it yesterday when I got home from work but I forgot... :unsure: This was written two years ago and I haven't looked at it recently so be aware some of these facts and figures have since changed. However, it was accurate at the time because the guys listed at the end of the piece checked it for me (particularly Brian Marsden and William Bottke, who were great sources of information)

Anyway, here it is...

On October 28 1937, a small asteroid was discovered by the German astronomer Karl Reinmuth and named Hermes. Two days later it came within 750,000 km of the Earth, the closest known approach by an asteroid at that time. It was then lost and has not been seen since. Its fate remains unknown.

Twenty-eight years later the asteroid 1999 VP11 came within 400,000 km of the Earth – just a little further than our own Moon. Unlike Hermes, no one knew about this close encounter until 34 years later and yet both asteroids could have caused a global catastrophe. 1999 VP11 is now classed as Near Earth Object. (NEO).

NEO’s are asteroids that come within 45 million kilometers of the Earth’s orbit, three-tenths the distance from the Earth to the Sun. At the time of writing, there are 1,413 known NEO’s, 489 of which are wider than a kilometer.

310 of the known NEO’s are larger than 200 meters across and come within 7,500,000 km of the Earth. These are classed as Potentially Hazardous Asteroids (PHA) – 1999 VP11 is one such example.

Four and a half billion years ago, the unwanted building blocks of the solar system formed a belt of rocks between the orbits of Mars and Jupiter. Many of the smaller rocks coalesced to become single, larger rocks bound together only by the mutual attraction of gravity. In time, these vagabonds would be known as the “rubble pile” asteroids.

Such is the case with asteroid 1997 XB26, a kilometer wide rubble pile asteroid that exists only within the infinite space of this author’s mind. Although this asteroid is fictitious, the following scenario could happen.

For much of 1997 XB26’s life, it roamed the asteroid belt before falling under the influence of Jupiter for roughly a million years. With the asteroid orbiting the sun every four years and Jupiter taking 12, the pair fell into a resonance which caused them to be regularly aligned at the same place in their orbits. Despite at least a hundred million kilometres between them, Jupiter was able give the asteroid the boost it needed to leave its peers behind and join the inner solar system.

For countless years it circled its parent star once every 671 days. Despite missing Mercury by four hours in February 1974, it remained unnoticed until one cold winter night, twenty-three years later. The date was December 5 1997 and amateur astronomer Tomi Andrews had spent the night photographing faint galaxies in Leo. At 5:30 a.m. she packed away her equipment and retreated to the warm comfort of her bed.

The resulting images showed a streak by her target galaxy, NGC 3611. Given the streak’s magnitude and with no faint halo surrounding it, Tomi surmised that this was no comet but rather an asteroid instead. The exposure time for the image had been 45 minutes. With the asteroid moving nearly three arc minutes across the sky during that time, she knew her rock was close.

An online search for a predicted asteroid close approach proved fruitless so she sent an e-mail to the Minor Planet Center to verify the discovery.

After comparing the data against all known asteroids, the Minor Planet Center in turn analysed the observations from Project LINEAR (Lincoln Near Earth Asteroid Research), the team responsible for 75% of all potential NEO observations. With a match found, the estimated orbital elements of the asteroid could be calculated. 1997 XB26 was now 90 million kilometers away and had passed into the constellation of Virgo. In the four days since its discovery it had moved 10 million kilometers closer to the Earth.

With the resulting ephemerides being published online, astronomers worldwide began searching for the wayward asteroid. However, despite their efforts, time was running out and by the end of the month, the asteroid had passed through Libra and into Scorpius, moving too close to the Sun for further observation.

For the Minor Planet Center, 1997 XB26 was cause for concern. At its closest approach, on Christmas Eve 1997, the asteroid had come within 68 million kilometers of the Earth and calculations hinted at a much closer encounter in 2001. Nearly two years would pass before the asteroid would be found again, appearing as a faint, 19th magnitude star moving slowly through Cancer.

Further observations refined the asteroid’s orbit and a team of experts convened to verify the computations. Within 72 hours, a meeting of the United Nations was arranged through the Office of Outer Space Affairs in Vienna. It was confirmed. - on October 31 2001 at 15:31 UT, asteroid 1997 XB26 would hit the Pacific Ocean at 20 km per second. Hawaiian observers would have a ringside seat to the last show on Earth. As they watched the rock rise at 5:00 a.m. local time, it would pass by a close conjunction of Mercury and Venus before falling below the horizon less than 30 minutes later. In that time, it would brighten from a 4th magnitude star to magnitude –0.6, a rival to Mercury in the sky.

From that point on, there would be nothing to do but wait. Just a few minutes later, the asteroid would hit the ocean, releasing half a million megatons of energy and causing hundred-meter tsunamis to hit the coastlines of every nation bordering the Pacific. Millions would die by water alone with many more being killed by the tons of burning rock raining down from the skies above. The shockwave of the impact would cause earthquakes around the world and volcanoes to erupt, throwing more dust into the air and darkening the skies for years to come.

The press called it Asteroid Andrews and like its discoverer, 1997 XB26 became a celebrity.

But the Earth is not without hope. Although years are needed to plan a mission, it would still be possible to deflect the asteroid with a nuclear detonation. A change in velocity of only a few meters per second is all that’s needed. However, with a rubble pile asteroid such as our theoretical example, care must be taken not to destroy it completely and therefore create a multitude of targets to contend with. The Earth’s atmosphere can only deal with rocks smaller than 50 meters.

Fortunately, LINEAR is not the only project searching for NEO’s. In 1998, NASA was assigned the task of finding 90% of the asteroids larger than a kilometer by 2008. Other organizations include NEAT (Near Earth Asteroid Tracking), Spacewatch, Spaceguard and LONEOS (Lowell Observatory Near Earth Object Search). In the past 20 years, these programs have increased the number of known NEO’s tenfold. There are also many amateur astronomers around the world actively observing potential NEO’s on behalf of the Minor Planet Center.

Out of the world’s estimated $20 trillion annual gross product, only $1.5 million is spent annually searching for NEO’s. The next time an asteroid shows up on a deep sky photo, it’s worth remembering that if every reader of Astronomy was to contribute $5, our global insurance policy could be renewed for another year.

The author would like to thank the following for their help in writing this article: William Bottke, Andrea Carusi, Jenifer B. Evans, Brian Marsden and Robert McMillan.

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"The stars are my home"
"I've seen things you people wouldn't believe... Attack ships on fire off the shoulder of Orion... I've watched c-beams glitter in the dark, near the Tanhauser Gate... all those moments will be lost, in time... like tears in the rain..."