How serious a threat do near Earth objects pose, and what is being done to prevent the devastation that a collision would inevitably cause?
On 18 March a mysterious object raced across the evening sky over the South Atlantic. Large enough to destroy a city, a 30m (100ft)-plus diameter rock with an energy equivalent to a 0.5 megaton bomb brushed past the Earth and hurtled onwards through space. Only two months earlier, the NASA-funded Spaceguard telescope spotted a massive asteroid on an apparent collision course with the planet. With around 24h to go before the estimated impact in the northern hemisphere, astronomers recalculated the near Earth object's (NEO) trajectory and discovered that it would clearly miss the planet.
Although this all sounds familiar as the fodder for science fiction writers and "doomsday" movie scripts, the threat to Earth from NEOs is real and immediate, says an international group of scientists and engineers. They hope that news of these two recent incidents, plus revelations of the frequency of other near misses, will be a wake-up call to the international community. "The public has to learn we are living in a shooting gallery," says a delegate at the first American Institute of Aeronautics and Astronautics planetary defence conference held recently in California. With input from the conference, the group is drawing up a "white paper" to help guide what it hopes will be a blueprint for the first organised global defence against asteroids and comets.
How real is the threat? Some NEOs - like the 10km (6.2 miles)-diameter Cretaceous-Tertiary asteroid judged to have possibly led to the extinction of the dinosaurs - are large enough to destroy the human species, while many smaller ones have the potential to wreak nuclear weapon-like havoc.
Previous collisions
Large NEOs are known to have hit the planet more than 139 times and evidence exists of more than 93 craters between 5km and 200km in diameter. In 2002 three NEOs came close to the Earth, two of which were not discovered until after they had passed. As recently as 13 January a large NEO, now estimated to be around 500m in diameter, was discovered approaching the planet.
Then there are the smaller NEOs, such as the one that grazed the atmosphere on 18 March, which have the potential to cause disaster if they are misinterpreted by a tense nation as a nuclear attack. Outgoing US Air Force Space Command development and transformation director Brig Gen Pete Worden describes one such incident on 6 June 2002 when a "small NEO impact" of around 10 megaton equivalent size lit up the night sky as it exploded in the atmosphere over the Mediterranean. "We were in the middle of a crisis situation as Pakistan and India were at loggerheads with each other. What would have happened if that had gone off over New Delhi or Islamabad? Neither country has the ability to clearly define what has happened. This is the kind of NEO issue we need to deal with - it's not just about dinosaur bones."
A recommendation of the conference will call for the funding of a next-generation Spaceguard survey effort (originally set up with NASA funding in 1998) to detect and catalogue potentially hazardous NEOs larger than 140m, a move embraced within a bill introduced in the US Congress on 11 February. Similar actions are under way in Europe with plans to support NEO surveys from advanced 3m-class telescopes in the Canary Islands. The US Department of Defense will be asked to speed up the release of data on NEOs and meteorites, particularly over areas where unexpected high-altitude detonations could be misinterpreted.
Tracking the threat
Thanks largely to NASA's Spaceguard survey conducted through the MIT-Lincoln Lab military telescope in New Mexico, and mostly amateur astronomers, more than 680 NEOs with diameters greater than 1km have been discovered and tracked. Between 300 and 500 NEOs in this class are estimated to exist, but remain undetected, says NASA. An estimated 200,000 of the smaller NEOs in the 100m diameter range also await discovery and tracking.
One of the expected recommendations from the conference is the need to establish a globally recognised authority or "home" for the planetary defence initiative, as well as the setting up of a chain-of-command structure to handle detection, threat verification, countermeasures and alerting. The paper is also expected to recommend the launch of low-cost mini/microsatellites to rendezvous with NEOs. The missions would help characterise the nature of different NEO types and could also be used to "tag" them with transponders.
One concept under study is SIMONE (smallsat intercept missions to objects near Earth), a project being led by the UK's Qinetiq in partnership with the Planetary and Space Sciences Research Institute. The concept covers the deployment of five identical 120kg (265lb) spacecraft designed for low-cost piggyback launch on the Ariane 5. Each will use a high specific-impulse ion engine to rendezvous with a different NEO target. The microsatellites will provide close-up reconnaissance of the NEOs to report back on the critical physical and compositional information needed for assessing the best method of either deflecting or destroying the object should it become a threat.
To help promote further study and to aid the planning of countermeasures for the "unthinkable" threat, the US-based Aerospace Corporation has funded development of four fictitious "defined threat" scenarios in which three asteroids and one comet would strike Earth.
The fictitious scenarios include Porthos, a comet scripted to be detected in February 2013, which, if not deflected, would hit the Mississippi valley in the USA in October 2015; D'Artagnan, a 120m diameter asteroid detected in 2004 that would probably hit Europe in 2009; Athos, a small asteroid headed towards a spot in the Pacific Ocean off the California coast in February 2016; and Aramis, a large asteroid almost 2km long, aiming to make an impact on the Indian Ocean in May 2033.
While planning for the "big one" is slowly taking place using these fictional threats as a guide, the group acknowledges its biggest problem is convincing the world that the threat is real. Author Larry Niven suggests that statistics are not useful for a threat that is "locally improbable, but globally certain". His suggestion: make another movie about an Earth-threatening asteroid - only this time with no "happy" ending. With the apparent increase in the detection of NEOs, and the number of recent close shaves, this may be the sort of blockbuster no-one will want to ignore.
GUY NORRIS / LOS ANGELES
Source: Flight International
