Astronomers have discovered several thousand asteroids that approach the Earth's orbit. These bodies are referred to as Near-Earth Asteroids (NEAs) or more generally, Near-Earth Objects (NEOs), since some of them are comets. The biggest two NEOs are 1035 Ganymede, at roughly 40 kilometers across, and 433 Eros, at about 30 kilometers. Eros comes within about 0.1 A.U. of Earth's orbit, but doesn't ever cross it. Asteroids that actually cross Earth's orbit represent the greatest danger of hitting us in the near future. The biggest of these Earth-crossing asteroids would cause a global disaster if they actually hit Earth, but that's unlikely to happen within the next million years.
Asteroids may be the greatest (and most predictable) threat to the Earth's ability to support life. Astronomers estimate that an object larger than a kilometer will impact the Earth every 500,000 years, and smaller asteroid impacts will take place at even greater frequencies. While most of the smallest asteroids burn up in the Earth's atmosphere or hit the surface as small bits of gravel, any asteroid bigger than about 50 meters will make it to the planet's surface and form a meaningfully large crater. Objects this size hit the Earth about once every 1,000 years. This is the size of the object that caused the Tunguska explosion in 1908. Impacts such as that in Chelyabinsk in 2014 happen every decade or so. Smaller objects, about 10 meters across, hit the Earth's atmosphere a few times each year.
The volume of space around the Earth has a constantly changing population of potentially dangerous debris. Asteroids don't last very long in Earth-crossing orbits. They generally survive only ten million years or so before hitting the Earth or being deflected into new orbits by Earth's gravity. The supply of NEOs is constantly replenished by similar orbit changing deflections made by other planets, and asteroid fragments are regularly thrown out of the main belt by the gravitational force of massive Jupiter. Still other NEOs are remnants of burnt-out comets that have lost their ices and are no longer active. Currently, no object is known to have an orbit that will cause it to cross Earth's orbit and hit Earth in the process, but surveys are still be conducted.
Sky surveys are programs of astronomical observations that detect and track the interplanetary bodies that may pose a threat. They all work in the same general way: a telescope takes multiple pictures of the same area of the sky, separated by several minutes or hours. Sophisticated search software compares consecutive images, and any object that changes position relative to the background stars is investigated further. The faster the object appears to be moving, the closer to the Earth it is. This is the same phenomenon that occurs when you look out the window of a moving car: nearby objects, like street signs, appear to flash past at high speeds. But things at a distance, like far-off scenery, look like they're moving more slowly.
Once a tentative identification of an object is made, further observations determine its orbit and other properties. Its apparent brightness is a complicated product of the object's albedo, distance, and size. Combining optical, infrared, and radar data can pinpoint the values for these properties. Radar detections can also be used to calculate the asteroid's spin rate and put together a shape model. Spectroscopy can be used to identify the composition of the asteroid and classify it according to spectral type.
LINEAR, or Lincoln Near-Earth Asteroid Research, is one of the most productive of the optical sky surveys. It consists of two telescopes in the New Mexico desert, run by astronomers from the Massachusetts Institute of Technology, with support from NASA as well as the U.S. Air Force. LINEAR scans the night sky an average of five times every unclouded night, looking for previously undetected NEOs. Its observations are concentrated along the ecliptic plane, where most NEAs are found.
Other sky surveys include Space watch, with two dedicated telescopes on Kitt Peak near Tucson, Arizona; the NEA Tracking (NEAT) program in Hawaii; the LONEOS survey at the Lowell Observatory in Flagstaff, Arizona; and the Catalina Sky Survey, also based in Arizona. In the past few year, many near-Earth asteroids have been discovered by the Pan-STARRS survey in Hawaii. A new telescope, the Large Synoptic Survey Telescope, is being planned as the next generation large sky survey. NASA supports all these programs to some degree, and the U.S. Air Force is also involved in some of them. However, the threat from space is not just a national defense issue, it is a global one.
For that reason, an international organization dedicated to detecting these potentially dangerous objects was created in 1998. Space guard is a private collection of observatories, astronomers, and amateur astronomers from around the world, including participants from Europe, Britain, the United States, Japan, and Australia. Space guard's goal is to find 90% of the NEOs larger than one kilometer across by 2008. That size limit was chosen because the impact of an asteroid a kilometer across would cause global devastation, versus merely regional disasters for smaller impacts. The total population of large NEOs (those one kilometer across or bigger) is estimated to be between 1,000 and 1,200.
The number of known NEOs has increased dramatically since the 1990s. The dramatic collision of Comet Shoemaker-Levy 9 into Jupiter in 1994 reinforced the threat of impacts from space in the public mind. The ecological threat was also established as an impact origin for the mass extinction that ended the cretaceous period became more widely accepted. To evaluate the future risk, the best approach is to continue the survey programs that discover and catalog Earth-approaching asteroids, until we have registered all those larger than 500 meters across. Calculating their future orbital trajectories will determine the likelihood of a major threat in the coming decades or centuries. In the longer term, the ability to fly spacecraft to these asteroids and slightly alter their orbits could safeguard civilization against a global disaster.
At the time of this writing, much media attention is being given to the NEA 99942 Apophis. In 2029 this 270m diameter rock will pass so close to the Earth that it will pass between the Earth and orbiting weather and communications satellites. All data shows there is no danger to life on Earth in 2029 (although there is a non-zero chance that orbiting satellites could get hit). There is a chance, however, that it could pass through a very specific gravitational "keyhole" that would cause Earth's gravity to adjust its orbit such that it will hit Earth in 2036. The probability this will happen is much much less than 1%, and even if this deflection does occur, plans are being made to be able to deflect Apophis as needed.