Near-Earth-Objects (NEOs) are small objects 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). From the perspective of impacts, we are primarily interested in the Near Earth Asteroids (NEAs). It is estimated that about 99 percent of the risk is associated with NEAs, which are much more common than comets.
The Earth's atmosphere protects us from most NEOs smaller than a modest office building (40 m diameter, or impact energy of about 3 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. The probable consequence would be an "impact winter" with loss of crops worldwide and subsequent starvation and disease. 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 one thousand Near Earth Asteroids (NEAs) larger than 1 km in diameter, and more than a million larger than 40 meter in diameter (the approximate threshold for penetration through the Earth's atmosphere). The largest NEAs are less than 25 km in diameter. There are probably many more comets than NEAs, but they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 1% of the impact risk.
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 are the Catalina Survey at the University of Arizona, supported by NASA, and the LINEAR search program of the MIT Lincoln Lab, carried out in New Mexico with US Air Force and NASA support. Several other ground-based search efforts are also finding NEOs. These were supplemented recently by the JPL NEOWISE infrared telescope operating for nearly a year in Earth orbit. Other astronomers (many of them amateurs) follow up the discoveries with supporting observations.
By 2013, astronomers had discovered more that ten thousand NEAs, including more than 90 percent of the NEAs larger than 1 km diameter. None of the known asteroids is a threat, but we have no way of predicting the next impact from an unknown object. The count of known NEAs and predictions of close passes by Earth can be obtained daily from the NASA NEO Program Office website at <http://neo.jpl.nasa.gov>.
We don't know when the next 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 roughly once 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 40,000 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. The purpose of the Spaceguard Survey is not to improve these statistical estimates, but to find any individual rock that may be on a collision course. By finding more than 90 percent of the NEAs larger than 1 km, Spaceguard has effectively retired most of the risk from impacts that are capable of global damage, and today there is increasing focus on the smaller but more frequent impacts.
With so many potential impactors 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. We estimate that there are about a million NEAs that are capable of local damage if they hit. In contrast, if the surveys actually discover a NEA on a collision course, we would expect years to decades of warning. Any NEA that is going to collide with the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard. In almost all cases, we will either have a long warning time or practically none at all.
Some press reports express concern that an asteroid could hit the Earth coming out of a "blind spot", such as the daylight sky or high southern latitudes where no Spaceguard telescopes are looking. Some worry that if an asteroid is found after its closest approach to Earth, this is an indication that the system is not working. These concerns seem to be based on the misconception that we are trying to detect asteroids as they approach the Earth on their final plunge toward impact. In fact, any such last-minute warning system is impractical as well as unproductive. In this survey, it makes no difference if a NEA is discovered on approach or departure from the vicinity of the Earth. The important thing is that it is discovered and its orbit determined. The only effect of blind spots, whether they be due to sunlight or moonlight or bad weather or lack of a southern hemisphere survey telescope, is to slow down the completion of the NEA catalog. Objects in blind spots will be picked up later, usually within a few years, in a more favorable geometry.
Cosmic 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 evacuate regions near ground zero where damage would be the greatest.
Both NASA and the US Air Force are supporting surveys to discover NEOs. In 1998 NASA formally initiated the Spaceguard Survey with the objective of finding 90 percent of the NEAs larger than 1 km diameter. In 1998 NASA also created a NEO Program Office, and today more than $10 million per year is being spent on NASA-supported NEO searches and orbit calculations. NASA has also sent the NEAR-Shoemaker mission to orbit and land on NEA Eros, and it is currently developing the OSIRIS-REx sample-return mission to NEA Bennu. Other governments have expressed concern about the NEO hazard, but none has yet funded any extensive surveys or related defense research. Japan, however, sent the successful mission Hayabusa to return samples from NEA Itokawa, and they plan further space missions to study NEAs.
The Spaceguard Survey and most associated search and tracking programs are concentrating on NEAs larger than 1 km in diameter -- large enough to risk a global ecological catastrophe if one of them hit the Earth. But there are many more smaller undiscovered NEAs, and we are likely to be hit somewhere on Earth by one of these, with an energy equivalent to a large nuclear bomb, sometime in the next century or two. The most recent such event took place on February 15, 2013, when a 20 meter NEA exploded over the city of Chelyabinsk, Russia with a energy of 0.5 megatons. While more than a thousand people were injured by broken glass, no major damage was done. A larger impact took place in 1908 in Tunguska (Siberia), with an estimated explosive energy of about 5 megatons, sufficient to destroy a city. The actual risk to each of us from Tunguska-like impacts is very small, much less than the risk from many common natural hazards, such as earthquakes and severe storms. Nevertheless, there are many people who are interested in this problem. In 2003 NASA completed a study of these sub-km impacts and concluded that it was both technically feasible and cost-effective to mount an expanded Spaceguard Survey, with much larger telescopes, to search for these smaller asteroids. Alternatively, it is possible to use a different search strategy to find very small NEAs in the last week before impact. This much warning would allow us to evacuate the impact area, much as we evacuate people in the path of a large hurricane.
Radar is a very powerful tool for studying the properties of known NEAs and determining their orbits, but it is nearly useless as a search technique. To be located by radar, a NEAs must be relatively close to the Earth. Our radar telescopes can detect it only if it is close enough to be illuminated by the radar pulses transmitted toward it. Since a small object shines in the visible by reflected sunlight, its brightness drops off as the square of the distance: if you move it 10 times farther away, its brightness is reduced by a factor of 10x10, or 100. But if a radar target is moved ten times farther away, it brightness is reduced by the distance to the fourth power, or a factor of 10x10x10x10 = ten thousand. However, when an already known NEAs does come very close, the powerful planetary radar systems at Arecibo in Puerto Rice and at Goldstone in California can often obtain images of surprising detail and also detect binary companions or moons of NEAs.
Certainly you would be warned. The search for NEAs is an open and transparent process, with the latest discoveries posted on the Internet as soon as they happen, and the orbits of known asteroids updated daily when new data are available. It is a staple of Hollywood fiction that the government would keep such discoveries secret, since this makes for an exciting plot line, but this is a silly idea when it comes to NEA impacts. The sky is visible to anyone, and there are thousands of astronomers around the world who could see any incoming object. The government has no monopoly on the sky or on telescopes to observe the cosmos.
David Morrison, May 2013