Amor Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering area between the Earth and just outside of Jupiter's orbit are the location of Amor asteroids.

Amor asteroids are near-Earth asteroids with perihelions outside of Earth's orbit, i.e. they never cross the orbit of Earth). However, they can cross the orbit of Mars (and in some cases, Jupiter), so it is believed that Phobos and Deimos may have been Amor asteroids captured by Mars. These class of near-Earth asteroids are named after the first asteroid defined to be an Amor asteroid, 1221 Amor.

Amor asteroids are defined by three things:

1. It must have an orbital period of greater than one year. Since Kepler's third law of planetary motion says that the square of the period of the orbit in years must equal the cube of the semi-major axis of the orbit in AUs, the semi-major axis must be greater than one AU.
2. To be a near-Earth asteroid, recall that the asteroid must come within 0.3 AUs of Earth's orbit. This the is the closest Venus and Earth can theoretically get.
3. To be an Amor asteroid, it cannot come closer to Earth than Earth's aphelion because it cannot cross any part of Earth's orbit. Earth's aphelion is 1.017 AU.

In reality, the third definition trumps the first definition since obviously, 1.017 AU is greater than 1.0 AU. By these definitions, for an Amor asteroid, the semi-major axis must be greater than 1.017 AU  and the perihelion of the asteroid must be between 1.017 AU and 1.3 AU. There are 3729 known asteroids that fall into this category, 580 of which are numbered, and 75 with proper names. The most-well known Amor asteroid is 433 Eros which is the first asteroid to be orbited and landed on. The spacecraft NEAR Shoemaker visited and flew by twice before landing in 2001.

433 Eros rendering from NEAR Shoemaker visit

via Astronomy Picture of the Day



 

Amor asteroids can be further subdivided into four subgroups:

• Amor I are Amor asteroids with semi-major axes between Earth and Mars, i.e. 1.017 AU
• Amor II have semi-major axes between 1.523 AU and 2.12 AU. The average semi-major axis for main asteroid belt asteroids is 2.12 AU. They have perihelions of less than 1.523 AU as all Amor asteroids must have. Again, they may impact Mars.
• Amor III have semi-major axes between 2.12 AU and 3.57 AU. They may be considered main belt asteroids, but have high enough eccentricities to bring them within 0.3 AU of Earth. Some may have high enough eccentricities to bring them close to Jupiter (5.2 AU). Amor asteroid 5370 Taranis is actually a Jupiter crosser.
• Amor IV have the most extreme eccentricities since their semi-major axes are greater than 3.57 AU but still have perihelions between 1.017 AU and 1.30 AU. Their aphelions are greater than 5.2 AU (the size of Jupiter's orbit) and some may even have aphelions nearing Saturn (9.6 AU). All these asteroid not only cross the orbit of Mars, but also cross the orbit of Jupiter. So far, no Saturn crossers in this subgroup have been discovered. Only two Amor IV asteroids have been discovered: 3552 Don Quixote and (85490) 1997 SE5.

Again, these are asteroids that we do not have to worry about as they do not come closer than 0.017 AU of Earth. But they will be concern for any future crewed missions to Mars and beyond.

Note: The Moon is 384,400 km or 0.00257 AU, so there is no danger of these asteroids impacting the Moon, either.