Near-Earth-Objects (NEOs) are small bodies in the solar system (asteroids and short-period comets) with orbits that regularly bring them close to the Earth and which, therefore, are capable someday of striking our planet. Sometimes the term NEO is also used loosely to include all comets (not just short-period ones) that cross the Earth's orbit. Those NEOs with orbits that actually intersect the Earth's orbit are called Earth-Crossing-Objects (ECOs).
The Earth's atmosphere protects us from most NEOs smaller than a modest office building (50 m diameter, or impact energy of about 5 megatons). From this size up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale. Above an energy of a million megatons (diameter about 2 km), an impact will produce severe environmental damage on a global scale. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65 million years ago (15 km diameter and about 100 million megatons).
There are many more small NEOs than large ones. Astronomers estimate that there are approximately 2000 NEOs larger than 1 km in diameter, and more than a million larger than 50 m in diameter (the threshold for penetration through the Earth's atmosphere). The largest NEOs are less than 25 km in diameter.
Several teams of astronomers worldwide are surveying the sky with electronic cameras to find NEOs, but the total effort involves fewer than 100 people. The most productive NEO surveys in 1997-98 are: the LINEAR search program of the MIT Lincoln Lab, carried out in New Mexico with US Air Force support; the NEAT search program in Hawaii, carried out jointly by the NASA Jet Propulsion Lab and the US Air Force; and the Spacewatch survey at the University of Arizona, funded by NASA and a variety of private grants. Other searches in the US, France, Japan and China also contribute to discovery of NEOs, while additional astronomers follow up the discoveries with supporting observations.
The Earth has been hit throughout its history, and certainly it will be in the future. But none of the known NEOs is on a collision course with Earth. All known NEOs and their predicted future positions are openly available to everyone with access to the Internet. The problem is that astronomers have discovered only about 10% of even the larger NEOs (diameter greater than 1 km). So 90% of them remain unknown, and we have no way of predicting the next impact from an unknown object.
No! There was never a prediction that XF11 (or any other asteroid) will hit the Earth. In March 1998 one astronomer told the press that XF11 would come close to the Earth in 2028 and that a collision could not be ruled out, but fortunately better calculations and additional observations quickly revealed that there is no risk of a collision.
We don't know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1 million megatons energy (roughly 2 km in diameter). On average, one of these collides with the Earth once or twice per million years, producing a global catastrophe that would kill a substantial (but unknown) fraction of the Earth's human population. Reduced to personal terms, this means that you have about one chance in 20,000 of dying as a result of a collision. Such statistics are interesting, but they don't tell you, of course, when the next catastrophic impact will take place -- next year or a million years from now.
With 90% of even the larger NEOs remaining undiscovered, the most likely warning today would be zero -- the first indication of a collision would be the flash of light and the shaking of the ground as it hit. In contrast, if a survey is carried out and NEO orbits are calculated, we would expect many decades of warning. This is the purpose of the proposed Spaceguard Survey. In almost all cases, we will either have a long lead time or none at all.
NEO impacts are the only major natural hazard that we can effectively protect ourselves against, by deflecting (or destroying) the NEO before it hits the Earth. The first step in any program of planetary defense is to find the NEOs; we can't protect against something we don't know exists. We also need a long warning time, at least a decade, to send spacecraft to intercept the object and deflect it. Many defensive schemes have been studied in a preliminary way, but none in detail. In the absence of active defense, warning of the time and place of an impact would at least allow us to store food and supplies and to evacuate regions near ground zero where damage would be the greatest.
The US Congress has held hearings to study the impact hazard (in 1993 and 1998), and both NASA and the US Air Force are supporting surveys to discover NEOs. In 1997 NASA adopted the objective of finding 90% of the NEOs larger than 1 km diameter within the next decade. In 1998 NASA created a NEO Program Office, and it is expected that at least $3 million per year will be spent on NASA-supported NEO searches and orbit calculations. At current (1998) discovery rates, however, the surveys will require a century, not a decade, to achieve 90% completeness. In addition, the US Department of Defense is studying a space mission called Clementine 2 to test the technology for intercepting an NEO. Other governments have expressed concern about the NEO hazard, but none has yet funded any extensive surveys or related defense research. A private Spaceguard Foundation based in Europe also promotes NEO surveys on an international basis.