Preventing an Impact
Attention is beginning to focus on the destructive forces that lie beyond planet Earth. Such an important task of tracking earth crossing asteroids and comets should not have to be carried out on a shoestring budget as has been the way up to recent times. As we discover more about the make up and workings of the solar system we now realise that the Earth is under a constant threat from unseen hazards hurtling through space.
And in the 1990s public awareness was raised of the probability of an impact in the near future rather than the possibility. Especially after the 'end of the world' was announced by news media around the world in 1998. Brian Marsden, an astronomer in the Minor Planet Centre in Harvard, announced that he could not rule out a collision of asteroid 1997 XF11 and Earth in 2028. An impact of an asteroid this size, 1-2Kms, would have global effects. However, after subsequent observations and analysis of earlier pre-discovery sightings, its trajectory was refined and it is no longer seen as a threat. In fact it will miss Earth by 2-3 Earth-Moon distances. But at least this prompted governments around the world to take the threat more seriously. In the US, NASA was provided with funding to start a programme to identify, by 2008, 90% of one Kilometer sized asteroids that could collide with Earth.
Predicting the precise orbit of asteroids can be very difficult. Highly accurate data is required to be collected for attritubes such as the mass, shape, size, spin, light reflectivity and heat radiation. Even the gravational effects of unknown neighbouring asteroids can influence the orbit. Small unknown values of these characteristics can lead to large diferences in orbital trajectories and for the Earth can mean the difference between a hit or miss.
After years of searching astronmers found one of these potential killers. A kilometer wide asteroid, with our name on it - 1950 DA (originally discoverd in 1950, but lost from view and rediscoverd in December 2000). It will collide with us in 2880. It is the most dangerous known asteroid at present. Its impact could kill hundreds of millions of people. There is a good probabililty that this could be a date with Armageddon. But there are probably hundreds more undiscoved asteroids of this size near earth that could collide with us a lot sooner than 2880. An awesome task is upon us to identify these killer mountains of space debris. How do we save the Earth if it becomes necessary, or should we say when it becomes necessary. Years of warning to allow adequate preparation to defend our planet would be required.
So is it all impending doom? Maybe not. Surely one of NASA's great achievements was the soft-landing of the spacecraft NEAR-Shoemaker on Eros, a 30km wide asteroid, in 2001. The Near Earth Asteroid Rendezvous (NEAR) mission was a huge success. Not only was it the first spacecraft to orbit an asteroid, but it managed a controlled descent with a touch-down speed of about four miles per hour within 650 feet from the projected landing site. NEAR managed to send home detailed images of the asteroids's surface features as well as carrying out analysis of the composition of the asteroid.
So perhaps given enough warning time it would be possible save the planet. We could launch a spacecraft to rendezvous with an approaching asteroid and detonate a nuclear weapon on an asteroid. But this might not necessarily go as planned. A detonation may just split the asteroid into numerous dangerous pieces, which could rain down and turn one huge killer into numerous killers each equally as lethal.
Another option is to detonate a nuclear weapon some distance away from the asteroid. The resulting shock waves could deflect the asteroid away from an Earth impact.
Or it may be possible to gently push an asteroid off course. By deploying a solar collector close to the asteroid, and focusing solar energy onto the surface of the asteroid for a number of months. It would eventually vapourise the surface of the asteroid and gently nudge its orbit into a safer orbit to avoid a collision with the Earth.
Another method currently being pursued is the so-called asteroid tugboat. The tugboat would approach the asteroid and nudge it continuously for a period of perhaps months using its on board engines until its orbit changed from an Earth impact orbit. The rapid spin of asteroids is just one of the many problems to ovecome with this method.
But the theoretical techniques for diverting asteroids are still in there infancy. Its difficult to predict or control there outcome. So we will need time on our side to tackle such a mammoth task. But will we have time on our side or will an unseen asteroid creep up on us?
The Spaceguard Survey is addressing the impact hazard of Near-Earth Asteroids (NEAs). The project's mission is to discover and characterise the asteroids that pose a global threat, and establish the probability of impacts. To do this as quickly as possible requires much international cooperation and funding.
NASA's stated goal in the mid 1990s was to discover 90% (what about the other 10%?) of all NEAs greater than 1km diamater by 2008. There was thought to be about 1200 NEAs greater than 1Km diamater. This 1km diamater has since been substantially revised to 140 meter diameter (I think), which might be a very good idea. A number of different survey teams are currently working on achieving this goal (NEAT, LINEAR, LONEOS, Catalina). See here for discovery statistics.
At long last it is now recognised that it is essential for us to detect, track and calalogue all NEAs. We must know what's heading our way. And we must also investigate the properties of these asteroids - what they are made of - whether they are rock, metal, porous, non-porous. This can only happen by spectroscopic analysis of the asteroids and/or mounting space rendezvous missions using relatively inexpensive spacecraft. And we must investigate deflection technologies - ways to deflect NEAs if this becomes necessary. And, in the worst case scenario, we must be adequately prepared to minimize the loss of life, property, society breakdown in the event of an impact.