Near-Earth asteroids (NEAs) are asteroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger, as well as being most easily accessible for spacecraft from Earth. In fact, some near-Earth asteroids can be reached with much less ΔV than it takes to reach the Moon. The most famous near-Earth asteroid is 433 Eros that was visited by NASA's Near Earth Asteroid Rendezvous probe.
A few hundred such near-Earth asteroids are known, ranging in size up to four kilometres. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometre in diameter at up to 2,000.
Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.
Some of the NEAs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the Kuiper belt, a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter.
There are three families of NEAs:
- The Atens, which have average orbital diameters closer than one astronomical unit (AU, the distance from the Earth to the Sun) and aphelia of greater than Earth's perihelion, placing them usually inside the orbit of Earth.
- The Apollos, which have average orbital diameters greater than that of the Earth and perihelia less than Earth's aphelion.
- The Amors, which have average orbital diameters in between the orbits of Earth and Mars and perihelia slightly outside Earth's orbit (1.017 - 1.3 AU). Amors often cross the orbit of Mars, but they do not cross the orbit of Earth. The two moons of Mars, Deimos and Phobos, appear to be Amor asteroids that were captured by the Red Planet.
Notice that all Atens and Apollos have eccentric orbits that cross the orbit of the Earth, making them potential threats to our planet while Amors do not cross Earth's orbit but some may come very close.
Also sometimes used is the Arjuna asteroid classification for asteroids with extremely Earth-like orbits. Near-Earth asteroid is a more restrictive term than near-Earth object.
The NEA threat
The general acceptance of the Alvarez hypothesis, explaining the Cretaceous-Tertiary extinction event as the result of a large asteroid or comet impact event, has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit.
The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on July 16, 1994, resulting in explosive impacts that would have been catastrophic on Earth. To be sure, Jupiter is far larger and more massive than the Earth and so undergoes far more impacts, but the event still provided an illustration that such things do happen and can be unimaginably destructive.
In fact on March 23, 1989 the 1,000-foot diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 400,000 miles passing through the exact position where the earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history.
Asteroids with a 1 kilometre diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometre objects happen every ten million years. Small collisions (but still potentially dangerous ones) occur a couple times each month.
Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid (29075) 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on December 31, 2000. Proper calculation of its orbit then demonstrated that it has a 1 in 300 chance of hitting the Earth on March 16, 2880. This probability is a thousand times greater than any other known asteroid threat, and 50% greater than all other known asteroid threats combined. (29075) 1950 DA has a diameter of a kilometre.
On March 18, 2004, LINEAR announced a 30 metre asteroid 2004 FH which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years.
It is difficult to determine the chances of its impact better than that. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect".
This effect was discovered by a Russian engineer named I. O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's spin orientation and rotation rate.
Projects to ameliorate the threat
Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the LINEAR which began in 1996. By 2004 LINEAR was discovering tens of thousands of objects each year and accounting for 70% of all asteroid detections. LINEAR uses two one-metre telescopes and one half-metre one based in New Mexico.
Spacewatch, which uses an old 90 centimetre telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis gear to search the skies for intruders. The project was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan.
The Spacewatch project has acquired a 1.8 metre telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimetre telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more.
Other near-earth asteroid tracking programs include Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey, Campo Imperatore Near-Earth Objects Survey (CINEOS), Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey.
"Spaceguard" is the name for these loosely affiliated programs, some of which receive NASA funding to meet a U.S. Congressional requirement to detect 90% of near-earth asteroids over 1 km diameter by 2008. A 2003 NASA study of a follow-on program suggests spending US$250-450 million to detect 90% of all near-earth asteroids 140 metres and larger by 2028.
Nonetheless, the fact that an impact of an NEA a kilometre or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating a nuclear weapon above the surface of an NEA would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. This is a form of nuclear pulse propulsion.
However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. In some ways, being struck with a loose cloud of smaller asteroids is worse than being struck with just one big one. This has led to a variety of other ideas for dealing with the threat:
- Setting up "mass drivers" on the object to scoop up dusty material and shoot it away, giving the object a slow, steady nudge.
- Flying a big sheet of reflective Mylar® to wrap itself around the asteroid, acting as a "solar sail" to use the pressure of sunlight to shift the object's orbit.
- Dusting the object with powdered chalk or soot to perform a similar adjustment, using the Yarkovsky effect.
Thinking on the matter continues - see Asteroid deflection strategies - and if there is no prospect of immediate action, the issue isn't going away, either.
An example of a recent asteroid impact
On June 6, 2002 an object with an estimated diameter of 10 metres collided with Earth. The collision occurred over the Mediterranean Sea, at approximately 34°N 21°E and the object detonated in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a medium-size nuclear weapon  (http://www.astro.uwo.ca/~pbrown/documents/flux-final.pdf). At that time India and Pakistan were at a heightened state of alert, ready to initiate a nuclear war with each other. If this asteroid impact had hit in this area the results might have been catastrophic.
- JPL Near Earth Asteroid Tracking program (NEAT) (http://neat.jpl.nasa.gov/)
There is an excellent article in the October 2003 issue of Scientific American regarding NEA's and long term strategies for protecting Earth from them.