The Near Earth Asteriod Threat

  Professor N C Wickramasinghe

Our speaker was Jay Tate who heads the organisation Spaceguard UK from a base in mid-Wales. It is the only body in the UK addressing the hazard of Near Earth Objects (NEOs) and receives no government funding. The organisation was set up in 1996 and the Spaceguard Centre opened in 2001 where for a small fee the general public can take a guided tour.

Jay explained that it can be quite difficult to bring home the consequences of an asteroid impact as they happen relatively infrequently on human timescales. However, when they do occur the effects may be devastating to our culture which is so highly reliant on its technological infrastructure.

There are more than 250 known impact craters on Earth, many of which are visible from space such as the Aorounga Crater in the Sahara Desert in northern Chad. This is 17km across and is thought to have formed several hundred million years ago. As our planet is geologically active we do not see many more of the craters that peppered the surface in the past which can lead to a false sense of security. A more accurate record of the cratering that has taken place in our Solar System is on the Moon where the lack of an atmosphere and the absence of plate tectonics allow the signs of impacts to accumulate.

Scientists do know that there was an intense burst of crater-forming events about 3.8 to 4.1 billion years ago. They came to this conclusion by the radiometric dating of lunar rock samples brought back by the Apollo astronauts. Impacts are still occurring today though, as shown by the collision of a comet named Shoemaker-Levy 9 with Jupiter in 1994. This comet actually broke up into pieces up to 2km in diameter before impacting with the Jovian atmosphere at 60km/sec. At the impact sites temperatures of 24,000 Kelvin were observed and seismic waves radiated outwards for over 2 hours at speeds of 450 km/sec.

Earth´s atmosphere protects us from most NEOs smaller than 40m in diameter. However, from this size upwards to objects 1km across an impacting NEO can wreak devastation over a local area. Impacts of objects around 2km in diameter can produce severe environmental damage across the globe resulting in an overall cooling of the Earth´s climate, loss of harvests and subsequent starvation and disease outbreaks.

Astronomers estimate that there are approximately 1,100 Near Earth Asteroids (NEAs) larger than 1 km in diameter, and more than a million larger than 40 m. What is needed is an international programme to identify, track and then plan how to deflect them. Sadly, many governments are not taking much of an interest in protecting their citizens from impact hazards. Most of the surveys are being carried out by US-funded projects and they successfully tracked an asteroid (2008 TC3) last October which exploded in the sky above northern Sudan last October.