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Saturday, 19 January 2013

Death from the skies

Starting 2013 with a bang, we continue our armageddon series about the science behind the end of the world.

On 8 November 2007, the Minor Planet Center issued a routine "Minor Planet Electronic Circular", reporting on observations of a newly discovered object, designated 2007 VN84 - some 4 million km from the Earth. It was heading towards us at a speed of 45,000 km/h - over 12 km every second. It was set to pass within 5,300km of Earth just five days later.

Diagram of the orbits of Rosetta, Mercury, Venus, Earth and Mars between the Mars and Earth swingby events in 2007
The gravitational pull of planets can change the orbits of smaller bodies. This "slingshot effect" is sometimes used by interplanetary spacecraft. Image courtesy DLR
Obviously, 2007 VN84 didn't hit the Earth, and nobody was hurt. But how bad would it have been if it had? Well, it's unlikely anybody would have been hurt, but it certainly would have been embarrassing. This was no space rock, but a spacecraft, carefully being flown past the Earth by the European Space Agency to change its orbit - in exactly the same way they'd done at Mars, several months earlier (and at the Earth, before that). 2007 VN84 was more commonly known as Rosetta.

This mix-up was embarrassing, but also quite alarming for some. After retiring the designation 2007 VN84, the Minor Planet Center stated: “This incident highlights the deplorable state of availability of positional information on distant artificial objects.

But what if this had been a real asteroid, heading towards us? Five days is not a lot of notice, though Rosetta is only quite small. A larger asteroid would (hopefully) be spotted sooner. Most people have heard the theory that dinosaurs were wiped out by an asteroid, but is the Earth endangered by a similar collision?

Essentially, we need to ask three questions: "How bad is being hit by an asteroid?", "Is it likely to happen?" and "How much warning would we have?". As it turns out, these questions are very closely connected.


You may have noticed the occasional news report about asteroids passing close by the Earth. If you have a particularly good memory, you may recall news articles about an asteroid striking Sudan on 7 Oct 2008. This asteroid, named 2008 TC3, has the distinction of being the only asteroid to hit the Earth which we were able to observe and track in advance, though much like Rosetta the warning period was very short - in this case, just 20 hours.

2008 TC3 weighed an estimated 80 tonnes, and was about the size of a car. It came down over Sudan, where it scattered some 600 pieces of rock over 29km of desert that we know of. Nobody was hurt, but clearly such an impact occurring over a densely populated area would be concerning, and with just 20 hours notice the possibility to evacuate people in a calm and controlled manner is somewhat limited.

It may seem surprising that we don't know how many asteroids like this are out there, crossing paths with the Earth, but we don't. In 1998, NASA launched their Near Earth Object Program as part of the international "spaceguard" project, with the express aim of discovering 90% of the asteroids 1km or larger. The spaceguard project continues around the world to this day, and are discovering asteroids of this size every 2-3 weeks on average. Significantly larger numbers of smaller asteroids are also being found.

When an asteroid is found which has an orbit that comes close to the Earth, such as asteroid Apophis which will pass by in 2029, it is assessed according to the Torino Impact Hazard Scale. But when it comes to making an impact size matters, and asteroids come in all manner of sizes, ranging from tiny apples to the Big Apple. The Earth is actually being hit constantly by small things, which burn up harmlessly in our atmosphere as meteors. About 500 meteoroids reach the ground each year, but they're small and the Earth is large - there is no record of anybody being killed by one, though at least one person has been struck.

Simulated fireball due to asteroid impact at Sandia Laboratory
Simulation of a "fireball" resulting from an asteroid
 exploding in our atmosphere.
Image courtesy Sandia Laboratory
(Photo by Randy Montoya)
However, in 2007 scientists at Sandia Laboratory issued a press release stating:

"Smaller asteroids may pose greater danger than previously believed"

The scientists at Sandia were using supercomputers to simulate and study the effects of the nightmare scenario; a direct hit by a large asteroid. Specifically, they were attempting to model the infamous 1908 Tunguska event in Siberia, which destroyed an area of forest covering 2,150 square kilometers. This particular impact didn't reach the ground as a solid meteoroid, but a large fireball comparable to a nuclear weapon did result from the asteroid exploding in the Earth's atmosphere. The exact size and nature of the Tunguska asteroid remains a matter of debate, and short of inventing a time machine we're unlikely to ever have a definitive answer. Just how large an asteroid would have to be in order to be dangerous remains an open question, and simulations like this enable us to seek the answer without waiting for one to occur. Some fascinating movie clips of the simulations they produced are available with the press release above.

Ironically, when coupled with human psychology (and international politics), one of the essentially harmless smaller impacts may also have come closer to wiping out humanity than any giant "planet killer" asteroid. Small asteroids tend to burn up, or explode, high in the atmosphere. This causes no problems on the ground, aside from the occasional UFO report, and may often go unnoticed. But in early June 2002 Pakistan and India were at full alert and poised for a large-scale war - which both sides appeared ready to escalate into nuclear war... U.S. early warning satellites detected a flash that indicated an energy release comparable to the Hiroshima burst... caused by the impact of a small asteroid ... Imagine that the bright flash accompanied by a damaging shock wave had occurred over Delhi, India or Islamabad, Pakistan? ... The resulting panic... could have been the spark that would have ignited the nuclear horror we'd avoided for over a half-century.” - Brigadier General Simon P. Worden, Deputy Director for Operations, United States Space Command, July 10, 2002 (the full speech is available here).

Today we know that asteroids impacts do happen, and although it is extremely unlikely that a particularly large asteroid will hit the Earth and wipe out all of mankind, smaller asteroids may still pose a serious threat. In 2010, a National Research Council report called for greater U.S. effort to identify and possibly mitigate the threat of near-Earth objects striking the Earth, and scientists around the world are investigating possible ways of doing just that.