Inner Planet Crossing Asteroids The Aten Amor and Apollo Families

Aten asteroids have orbital periods of less than one year and orbits that average less than 1 AU and therefore may overlap the Earth's orbit.The original Aten asteroid, 2062 Aten, with a diameter of 0.56 miles (0.9 km), was discovered in 1976 by Eleanor Francis Helin, a researcher at the Jet Propulsion Laboratory. 2062 Aten has an orbit slightly smaller than Earth's but more eccentric, and its orbit crosses Earth's orbit.There are now about 221 known asteroids that have an orbital period of less than one year and average less than 1 AU from the Sun.These are all called Aten asteroids.

Apollo asteroids have orbits that overlap the Earth's orbit, as Aten asteroids do, but Apollo asteroids have periods longer than one year. More than 1,300 Apollo asteroids are known. The largest Apollo asteroid is 1685 Toro, with a diameter of about 7.5 miles (12 km). There are about 13 Apollo asteroids with diameters larger than three miles (5 km). One curious Apollo asteroid also shows characteristics of a comet: Asteroid 4015 Wilson-Harrington was discovered on November 15, 1979, by Eleanor F. Helin at the Palomar Observatory. This asteroid was later discovered to be the same object as a short-period comet discovered by Albert Wilson and Robert Harrington, also at the Palomar Observatory.When Wilson and Harrington observed it, it had a comet-like tail, but when Helin observed it, it was gas-less. 4015 Wilson-Harrington is one of the enigmatic asteroids that has a gassy coma when at perihelion and therefore bridges the gap between asteroids and comets. It has an orbital period of four years, 107 days and is about 1.5 miles (2.5 km) in diameter.

Amor asteroids, the third class of asteroids that most endanger the Earth, orbit near but outside the Earth's orbit. Amor asteroids all have perihelia between 1.0 and 1.3 AU and are named after 1221 Amor, the first such discovered. There are more than 1,200 known Amor asteroids. 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 Mars's gravity field. Deimos has a strange, shiny surface, but Phobos, the larger of

Mars's moon Phobos, showing its large Stickney crater, is thought to be a captured Amor asteroid. (NASA/JPL)

Mars's moons, appears to be very similar to other Amor asteroids (see image below).

433 Eros, another Amor asteroid, is one of the most elongated asteroids known, with estimated dimensions of 20.5 X eight X eight miles (33 X 13 X 13 km). Eros is one of only three NEAs a diameter above six miles (10 km). It orbits the Sun in 1.76 years at an inclination of 10.8 degrees to the ecliptic. Its orbit carries it close to the Earth, with a perihelion distance of 1.13 AU; its aphelion (farthest distance from the Sun) is 1.78 AU. The closest approach of Eros to Earth in the 20th century was on January 23, 1975, at approximately 0.15 AU. Previous close approaches occurred in 1901 and in 1931. Because of its repeated close encounters with Earth, Eros has been an important object historically for refining calculations of the mass of

Mars's moon Phobos, showing its large Stickney crater, is thought to be a captured Amor asteroid. (NASA/JPL)

the Earth-Moon system and the value of the astronomical unit. The asteroid rotates in 5.27 hours and has an albedo of 0.16, bright among asteroids. Eros is thought to be compositionally varied: One side appears from spectrographic inspection to have a higher pyroxene mineral content and the other a higher olivine mineral content.

Because of Eros's close approaches to Earth and its interesting surface appearance it has been the subject of a space mission culminating with an actual spacecraft landing. The NEAR Shoemaker mission craft orbited the asteroid and then touched down on February 12, 2001. As shown in the upper color insert on page C-7, the image shows a photograph of Eros, along with the orbital path and landing route of the mission and the rotational motion of the asteroid.

Far from being a bare rock, this asteroid has its own regolith, the rocks and dust that form the "soil" on rocky celestial bodies. The crater shown in the figure on page 72, Psyche, is the largest on Eros. A large boulder perched on the crater wall illustrates Eros's unusual gravity: Because of its elongated shape the regions with lowest gravity on Eros are not necessarily in the bottoms of craters. The boulder appears to rest on the sloping wall of the crater instead of rolling down to the floor. These and the other NEAR Shoemaker mission craft photos were groundbreaking in the understanding of asteroids; these missions have provided data detailed enough to create an image of what standing on the asteroid might be like. Daytime temperature is about 212°F (100°C), the boiling point of water on Earth, while at night the temperature falls to —238°F (—150°C). Gravity on tiny Eros is of course weak: A 100-pound (45-kg) object on Earth would weigh about an ounce on Eros.

The largest asteroid that is a potential Earth hazard is 4179 Toutatis. This asteroid is nearly one mile (1.6 km) long. Its orbit makes it particularly dangerous, since it lies almost perfectly in the same plane as the Earth's, inclined from it by less than a half degree. No other hazardous asteroid larger than 0.6 mile (1 km) has been found that moves around the Sun in an orbit so nearly coplanar with Earth's. The more coplanar the orbits, the more likely a collision. Toutatis's orbit extends from just inside Earth's orbit to a point deep within the main asteroid belt. On September 29, 2004, Toutatis passed within just four times the distance from Earth to the Moon. For a previous close approach, a computer-generated image was made of the Earth as it would appear from Toutatis, shown in the

Crater Psyche, photographed from a range of 62 miles (100 km), is 433 Eross largest crater. (NASA/JPL/NEAR/Johns Hopkins University Applied Physics Laboratory)

lower color insert on page C-6, demonstrating graphically how close the objects come.

A meteorite a half mile (1 km) in diameter striking the Earth at 13 miles per second (21 km/sec), near the slower end of the possible range of meteorite velocities, will create a crater with a diameter of about 15 miles (25 km). The estimated population of asteroids with diameters larger than one kilometer in Earth-crossing orbits leads to an estimate of impact likelihood on the Earth of about three impacts per million years. Mercury has fewer asteroid impacts than the Earth but more cometary impacts. Impacts are also harder on Mercury because objects are moving faster as they near the Sun. Mars is being struck by a population of asteroids whose orbits do not cross the Earth's, so Martian cratering and Earth's are not as closely related as Earth's and Mercury's.

Another family of main belt asteroids is called the "shallow Mars-crossers." There are about 20,000 of these bodies. They are probably pieces left over from planetary accretion (the accumulation of smaller bodies into a larger body) of Mars, and they have a 50:50 chance, over the age of the solar system, of either hitting Mars or being thrown out of the solar system.

Earth Orbit Apollo Asteroids Nasa
Amor, Aten, and Apollo asteroids have orbits that approach or cross Earth's orbit.

Most of the near-Earth asteroids are in orbits that are unstable over the long term. In the section "Asteroids in the Inner Solar System," on page 63, two simple examples of stable orbits are described, but there are other, far more complex possibilities. Recently Paul Wiegert at the University of Western Ontario and his colleagues Kim Innanen and Seppo Mikkola have shown that four asteroids share the Earth's orbit in a most strange resonance. The asteroids 3753 Cruithne, 1998 UP1, 2000 PH5, and 2002 AA29 share the Earth's orbit while orbiting, not in an ellipse, but in a horseshoe pattern. The simplest version of this kind of orbit involves the asteroids switching from just inside to just outside the Earth's orbit whenever they come near, so that the asteroids never pass the Earth. The asteroids fall just outside the Earth's orbit and slow down, but just before the Earth overtakes them, they swing across the Earth's orbit, drawn by its gravity and the Sun's, into an orbit closer to the Sun. In this closer orbit, they speed up, but just before they lap the Earth on the inside, they are drawn behind the Earth across its orbit into an orbit outside the Earth, and they slow down again. In this way, they never collide with the Earth and, in fact, never pass the Earth.The asteroids follow a similar horseshoe pattern, but they constantly spiral around the Earth's orbit as they go.

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