Asteroids

Young Astronomers Blog, Volume 28, Number 21.

When we think of our solar system, we typically think of our Sun, eight planets, and five dwarf planets. We often forget about the objects circling the Sun in the asteroid belt between the orbits of Mars and Jupiter. They are, of course, the asteroids.

Most of the larger asteroids were discovered during the first half of the 19^th century. The first four, Ceres, Pallas, Juno, and Vesta, were found in the first decade and caused quite a stir. For several years they were considered full-fledged planets bringing the total number of planets to eleven. Neptune and Pluto had yet to be discovered at the time. However, as more were found, they were reclassified as minor planets and are now generally referred to as asteroids.

Asteroids are thought to be left over material from the formation of the solar system. Most are found in the asteroid belt around 2 to 3 ½ astronomical units (AU) from the Sun. There are three types of asteroids based on their composition (and location).

• Stony (S type) are bright and composed of silicates and clay. They are found in the inner part of the asteroid belt.
• Metallic (M type) are relatively bright objects composed of nickel-iron and some stone. They are found in the middle of the asteroid belt.
• Carbon/Chondrite (C type) are dark and composed mostly of carbon. They are the most common, accounting for around ¾ of the asteroids, and are found near the outer part of the asteroid belt.

Not all asteroids are located in the asteroid belt.

• Trojans orbit ahead or behind some of the planets. Most are found in Jupiter’s orbit.
• Centaurs are found between the orbits of Jupiter and Neptune. Although, a 2013 study by the NASA satellite NEOWISE found that many Centaurs are more comet-like than asteroid-like.
• Other asteroids pass close to the Earth and are known as Near-Earth Some of these cross the Earth’s orbit and are classified as Earth-crossers. NASA tracks these and has identified around 10,000 Near-Earth asteroids.

Trans Neptunian/Kuiper Belt objects are typically not considered to be asteroids. These are icy objects also left over from the formation of the Solar System but are found outside of Neptune’s orbit.

The official terminology can be a bit confusing. For many years, asteroids have also been referred to as “minor planets”. Minor planets include any sizable object in the Solar System other than planets, moons, and comets. The definition got a bit more muddied with the demotion of Pluto when the IAU officially designated objects other than planets, dwarf planets, and moons, but including comets, as “small solar system bodies”. The IAU does note that the term minor planet is still acceptable.

The International Astronomical Union’s Minor Planet Center keeps track of all the minor planets (and comets) in the Solar System and assigns a number to each in the order they are discovered (e.g., 1 Ceres  and 4 Vesta). You can also find the number of asteroids on NASA’s website, which as of October 2020, has reached over one million.

There have been several spacecraft sent to explore various asteroids. A few performed flybys sending back the first close up images from the asteroid belt. These included a photo of Ida and its moon Dactyl taken by the Galileo spacecraft as it travelled toward Jupiter. Yes, even asteroids have moons.

The most elaborate mission was Dawn, which orbited both Vesta and Ceres. Dawn was launched in September 2007. Its Ion engine propelled it to Vesta, where it arrived in July 2011. Dawn became the first spacecraft to orbit an asteroid and remained in orbit until September 2012 when it departed Vesta for Ceres. Dawn entered orbit around Ceres in March 2015 becoming the first spacecraft to orbit two celestial objects. Dawn studied Ceres from various altitudes over several years before running out of fuel in 2018.

Vesta was found to be a “protoplanet” with a differentiated core, mantle, and crust. Although, it is not large enough to be spherical, and as such, is not considered to be a dwarf planet.

Ceres proved to be remarkably interesting. Ceres is large enough to be spherical and is classified as a dwarf planet. Dawn discovered two large “white spots” (Cerealia Facula and Vinalia Faculae) in the Occator Crater, which turned out to be composed of salt crust (sodium carbonate) left over from salty water that escaped to the surface.

Dawn also discovered Ahuna Mons, a cryovolcanic mountain rising above the surface of Ceres.

There have also been missions to collect samples from asteroids and return them to Earth. The Japanese missions Hayabusa and Hayabusa 2 collected samples from the asteroids Itokawa and Ryugu respectively.

The United States spacecraft Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) was launched in September 2016. It reached the asteroid Bennu in October 2018. In October 2020, OSIRIS-REx dipped down and picked up some samples from the surface of Bennu. OSIRIS-REx will return the samples to Earth sometime in 2023.