Apollo 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 the area around the Earth is the location of Apollo asteroids.

The third and final class of near-Earth asteroids are the Apollo asteroids. The first asteroid to be determined to be an Apollo asteroid is, surprisingly, 1862 Apollo. Okay, maybe not that surprising. Apollo asteroids are defined by three things:

1. They all have semi-major axes of greater than one AU
2. Their perihelions are all less than 1.017 AU (the distance from the Sun to the Earth at aphelion).
3. They have the potential to collide with the Earth, i.e. they are all Earth-crossing asteroids.

There are only 5766 known Apollo asteroids, though more are being found. Of those 5766, 832 are numbered. Remember that a numbered asteroid has a known orbit and can have its position predicted at different times.

The largest known Apollo asteroid is 1866 Sisyphus which has an average diameter of 8.5 km and a mass of approximately 7.7e18 kg (7.7 followed by 17 zeroes for non-math people), 773,900 times smaller than the Earth. Even though it is much smaller, if it even did hit Earth, everyone on Earth would have a really bad day. Luckily, it has been predicted that its closest approach will be on November 24, 2071 (set your calendars) and will only be 0.11581 AU away from Earth (still farther than the Earth-Moon distance). The Chicxulub asteroid which is believed to have killed off the dinosaurs 65 million years ago was comparable in size.

The Chelyabinsk meteor on February 13, 2013 was also a Apollo asteroid. It was estimated to only be 20 meters in diameter with a mass of 13,000 metric tons (13 million kilograms), again much smaller than 1866 Sisyphus.

Though Aten asteroids have the potential to cross Earth's orbit, Apollo asteroids are the ones with which we should be the concerned. Every single Apollo asteroid has the potential to collide with Earth as all of them cross the Earth's orbit. Apollo asteroids, by far, make up the majority of known near-Earth orbits (which is called an observational bias - comment if you have a question on this). As technology improves, possibly hundreds if not thousands more near-Earth asteroids, and by default, Apollo asteroids, will probably be discovered.

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.

Asteroid 3753 Cruithne

3753 Cruithne is an asteroid that has an orbital period of 364 days and a semi-major axis of 0.998 AU, very similar to Earth. Therefore, it has a 1:1 resonance with the Earth, even though it does not orbit the Earth - Cruithne is NOT a moon of Earth. 

Globedia (note: this website is in Spanish)

 



But unlike Earth, its orbit is much more eccentric with a perihelion of only 0.484 AU (between the orbits of Mercury and Venus) and an aphelio of 1.51 AU (at the orbit of Mars). As shown in the next graphic, it has a normal, elliptical orbit.



"Orbits of Cruithne and Earth". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons -

However, when you compare the orbit of Cruithne with a stationary Earth, the orbit takes on a strange shape - it has a kidney bean shape relative to Earth. Again, this does not mean that it orbits the Earth and is another moon of Earth. What the kidney bean shape reveals is how the position of 3753 Cruithne compares to the Earth.

"Horseshoe orbit of Cruithne from the perspective of Earth". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons -

 

Next time, I'll be posting about an even stranger orbital path relative to Earth - a horseshoe shape.

Asteroid 2002 AA29

Time Lapse view of 2002 AA29 moving among distant galaxies

Paul Wiegert, Astronomy Dept, University of Western Ontario



 

Much like 3753 Cruithne, 2002 AA29 is an Aten asteroid with a semi-major axis of about one AU and an orbital period of approximately one year. However, the eccentrictiy is much lower (0.012 - almost circular), even lower than Earth's eccentricity.  Its perihelion is only 0.988 AU and its aphelion is 1.012 AU, keeping it very close to Earth's orbit.  But this object is never in danger of colliding with Earth. It is locked into a 1:1 resonance with Earth, and this resonance is very stable. Its inclination, orbital tilt with respect to the ecliptic, is 10.739°.

What makes this asteroid unique is its orbit with respect to the Earth. When it is just inside Earth's orbit, it is travelling faster than Earth and so will get farther and farther ahead of Earth until it will almost lap Earth. At this point, Earth's gravity will slow down 2002 AA29 which will cause it to move to a higher orbit. This is a consequence of the conservation of angular momentum. When an object is moving slower in a circular (or elliptical) orbit, it has to move farther away. This is just like Kepler's Second Law of Planetary Motion. As a planet moves closer to the Sun, it is moving faster. Farther away, it moves slower.

 

Now, since the Earth is moving faster in a lower orbit, 2002 AA29 lags farther and farther behind Earth until eventually Earth catches up from behind and almost laps the asteroid. Earth's gravity now accelerates the asteroid, causing it to move to a lower orbit, and the orbital dance continues. Each part of this dance (2002 AA29 moving farther and farther ahead of Earth and almost lapping Earth; lagging farther and farther behind till Earth almost laps it) takes approximately 95 years. Because the Earth and 2002 AA29 only approach each other and the orbits never cross, there is not danger of the asteroid hitting us.



Overhead view of the orbit of 2002 AA29 with repect to the Earth's (Erde) orbit

"2002aa29-orbit-3" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -

Inclination of 2002 AA29 to Earth's orbit

"2002aa29-orbit-2" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -

 



This speeding up and slowing down create a unique shape. Each of the loops in the image below is the position of AA29 with respect to the Earth's orbit every year. The whole series of loops has a shape like a horseshoe with the Earth being in the gap at the two ends of the horseshoe.

 



 

"2002aa29-orbit" by JPL/NEO, Daniel Arnold - NASA - Jet Propulsion Laboratory - Near Earth Object Program, labeling and size of bodies modified by Daniel Arnold

Licensed under Public domain via Wikimedia Commons -