For hundreds of years human beings assumed that the Solar System was a fairly quiet, benign and predictable place; somewhere where things changed, but only over very long periods of time. But our distant ancestors had a very different view. They knew very well that occasionally there were catastrophic events that changed everything – floo ds, plagues, fire from the sky and so on. But the “age of reason” taught us that this was nonsense, and that our forebears were superstitious and lacked the intellect of “modern” man.

Sir Isaac Newton convinced us that the movements of celestial objects were controlled by strict laws, and that the heavens were essentially a predictable, mechanical system. Sir Charles Lyell persuaded us that geological processes that shaped the world around us were all incredibly slow, and Charles Darwin did the same for biological evolution. The age of catastrophism was dead, replaced by gradualism as the predominant view of the universe.

Then we went into space, and everything changed again. When we got our first close-up views of the other planets in the Solar System it became clear that something was going on; every solid, stable surface that we saw was covered in craters. It is very clear that everywhere in the Solar System we find evidence of massive bombardment from space – everywhere, that is, except here on the Earth. The surface of the Earth seemed to be “crater free” (any craters that could be seen were attributed to volcanic activity), and people assumed that, for some mysterious reason, that the Earth had got away with it.

When we started looking at the Earth from the outside, using satellites and other spacecraft, everything changed again. Craters of all sizes began to appear, often badly eroded and almost invisible, especially from the ground. Many are completely buried, and can only be detected using sophisticated geological tools; many more are hidden by water in the world’s oceans. But it is obvious now that the Earth didn’t get away with it at all – we have been battered by just as much stuff from space as all of the other planets and moons in the Solar System. It is also very clear that this bombardment didn’t finish millions of years ago. It is a problem that we still face.

NEO SEARCH

Near Earth Objects are asteroids or comets that have orbits around the Sun that bring them close to the Earth. The actual definition of an NEO is a comet or asteroid whose orbit brings it close to Earth's orbit. The criterion is a perihelion distance < 1.3 AU.

Since 1998 , NASA has part-funded the “Spaceguard System” consisting of 5 asteroid detection programmes, and there have been some other search programmes operating around the world such as the Japanese Spaceguard Project at Bisei, the Asiago - DLR Asteroid Survey in northern Italy and the SCAP Programme in China.

So how are they doing? The number of known near-Earth asteroids (NEAs) larger than 1 km in diameter (or, more precisely, brighter than absolute magnitude 18) has passed the 900 mark. If the population of NEAs larger than 1 km is about 1000 (the consensus figure from several studies), 900 represent ~90% completeness.

At the end of 2005 , NASA was directed to set up a follow-on project to find 90% of NEOs with diameters greater than 140 metres by the end of 2020. To do this a new generation of telescopes will be required, but have not yet been funded. There are a number of instruments that could be used for NEO detection including the enormous LSST ( The Large Synoptic Survey Telescope), the DCT (The Discovery Channel Telescope) and Pan-STARRS (The Panoramic Survey Telescope & Rapid Response System). There is also a proposal for a space based telescope (NEOCam) to cover the space between the Earth and the Sun.

FOLLOW-UP

Orbit determination

It is impossible to calculate the orbit of an object from a single observation. With a single observation the real distance cannot be determined; all we know is that the asteroid lies within a cone with an angular dimension given by the error of the measured position. All the points in the cone are known as “virtual asteroids”. As time passes , the Earth and the virtual asteroids move, following different orbits. Since each virtual asteroid is at a different distance from the Sun they will have different speeds (Kepler’s 3 ^rd Law - The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axes [the "half-length" of the ellipse] of their orbits). This means not only that larger orbits have longer periods, but also that planets further from the Sun travel slower than ones closer in. After some time the Earth and the Virtual Asteroids have moved to different positions - the region of uncertainty has moved and changed shape. If we make another observation of the same target a second cone will appear, and the real position of the asteroid will be where they cross. This will exclude may VAs and the region of uncertainty will shrink. If the period is longer (1 year) the uncertainty region is much more elongated and the asteroid’s position can be determined much more accurately.

Physical Properties

NEOs are often very near to Earth. They can be studied not only with classical astronomical instruments and instruments that aren't often thought to be used in astronomy, such as radars.

Photometry is often used to study the brightness variations of asteroids over a period of time, obtaining curves where these variations of brightness are represented: the lightcurves . Asteroids have irregular shapes and usually rotate. When an irregularly shaped object rotates, it will reflect different amounts of light as time goes on, so the brightness of the point of light observable will change with time, depending on the observable area. Time series measurements of the asteroid's brightness variations produce light-curves. The time it takes for a lightcurve to start repeating is the length of the asteroid's day, called its rotation period. The lightcurve amplitude (how much the curve goes up and down) gives us some information about the asteroid shape.

Chemical composition can be determined by spectroscopic observations - by collecting and analyzing their spectra .

Radars are powerful sources of information about asteroids' orbits and physical properties, but the best way to find out about asteroids and comets is to go to them and look close - up. Over the past decade a number of US and European spacecraft have visited asteroids and comets, and have provided invaluable “in situ” data.

MITIGATION

When we find an asteroid or comet that has a high probability of hitting the Earth we have a number of options open.

Evacuate/Ride the Storm

Given enough warning, and an accurate ground-zero prediction (both entirely feasible), it might be possible to evacuate the point of impact and areas in danger such as low-lying coastal regions. This might be an adequate course of action for small impacts (with local or regional effects), but for larger, globally threatening events, long term protection and supply will be necessary for any surviving population - it might be a case of “out of the frying pan, into the fire”.

Destruction

The possibility of destroying potential impactors, probably with high yield nuclear weapons, has been studied in some detail. With the current lack of detailed knowledge of the exact composition of particular objects, and their structural strength, there is an element of doubt as to the effectiveness of this course of action. The fear would be that incomplete disruption of the object would subject the Earth to multiple impacts from pieces of the original body. The effects of transforming a cannon ball into a cluster bomb could be more far-reaching than the original threat.

Deflection / Acceleration / Deceleration

Assuming that a potential impactor can be identified early enough, its orbit could be modified sufficiently to ensure that an impact would not occur. Methods considered include the use of gravity, by parking a spacecraft close to the body and then moving it away, relying on the mutual gravity to change its orbit, or the use of propulsion units or mass drivers (using the material of the object itself as fuel) to physically drive it from its path. Only very small adjustments well in advance would be required to ensure a miss rather than a hit.

SUMMARY

It is obvious that, over long periods of time, NEOs pose a substantial risk to the Earth’s ecosphere, and to other bodies in the Solar System. However, for the first time in history there is a species that has the technical ability to protect not only itself, but all life from the impact hazard. But so far we have not done enough to ensure we can protect humankind from these impacts. The US conducted the Spaceguard programme to detect large (>1km) potentially hazardous asteroids, and have detected an estimated 96% of that population. They are now searching for smaller (>140m diameter) objects. Europe is conducting research into mitigation methods (NEOShield 2). There has been sporadic work in Japan, China, Russia and Europe, but, as detailed in the NASA Authorization Act 2008, the level of NEO related activity worldwide falls well below that required. In the UK, in 2000, the report of the government’s Task Force on Potentially Hazardous Near Earth Objects detailed the dangers and described methods of avoiding them. This document was well received around the world, but since then the UK has made no substantial commitment of resources to address the practical details of finding and dealing with NEOs.

We need to develop a properly funded, international programme to detect and characterise potentially hazardous objects, and prepare to defend ourselves.

Dinosaurs are extinct because they couldn’t do anything about NEOs. What’s our excuse?