Asteroids are the most numerous bodies in our solar system, with hundreds of thousands of them orbiting around the Sun in both belts and as individuals. They far outnumber our well-documented planets (and dwarf planets, to that matter) and are being studied by space agencies world wide, each of which are trying to shed some light on what historically were written off as simple floating rocks. However, asteroids are unique in the fact that they tell us much about the conditions of the universe post-big bang, how astrophysics effect space phenomena and how planets are formed, granting the scientific community great insight into our solar system’s origins and workings.
Structures of Asteroids
There are three types of asteroid: carbonaceous (C-type), siliceous (S-type) and metallic (M-type) variants, each corresponding to the composition of an asteroid, be that stony, stony-iron or iron. The composition of an asteroid – be that shape or material – is dependent on when and what it was formed from, as well as if it has undergone reconstruction post-collision.
Initially, at the dawn of the solar system, most asteroids were much larger than those now commonly found by astronomers, with sizes more consistent with a planet such as Mars and shapes varying wildly. However, the radioactive decay of elements within the asteroid rock melted these larger bodies, and during their fluid stage, gravity pulled them into spherical shapes before they cooled.
At this point, though, many smaller asteroids – which cooled more efficiently than their larger brethren – did not reach melting point and retained their uniform rocky-metallic composition and their initial irregular shape.
This process of asteroid formation can be seen vividly when contrasting many of the asteroids that modern scientists and astronomers are currently studying. Take the asteroid Ceres (Ceres was the first asteroid to be discovered and is now considered by some astronomers as a dwarf planet) for example – this is a large asteroid (it has an equatorial radius of 487km) and, in turn, is both spherical in structure and carbonaceous composition (C-class), as it was pulled apart easily and cooled slowly. However, if you compare Ceres to Ida, for example, which is a small asteroid (it has a mean radius of 15.7km), you find the latter is both irregular in shape (funnily, it looks like a potato) and heavily composed of iron and magnesium-silicates (S-class).
The majority of asteroids in our solar system are found in a concentration known as the main belt, which lies between Mars and Jupiter. This belt contains thousands of asteroids and takes roughly four and a half years to orbit the Sun on a slightly elliptical course and low inclination. Despite the fact that they all orbit in the same direction, collisions do occur at low velocities (for such large objects) and these cause the asteroids to be continuously broken up into smaller variants. Of this main belt, certain groups have been captured into peculiar orbits, such as the Trojan group of asteroids that fol low Jupiter’s orbit, or the Amor or Apollo groups, which cross the paths of Earth and Mars respectively and the Aten group, which sits inside Earth’s own orbit.
Asteroids collisions and craters
Evidence of asteroids collisions can be found everywhere. When asteroids collide with each other there are three main outcomes, each of which depends on the size of the impacting asteroid. If the incoming asteroid is 1/50,000th the size of the larger body then it will merely create a large crater, sending small fragments out into space. If the impact or is roughly 1/50,000th the size of the impacted, then the latter will fracture before breaking into rock and dust, before being pulled back together into a ball of rubble by gravity. Finally, if the incoming asteroid is larger than 1/50,000th the size of the other, larger asteroid, then it will immediately shatter into pieces and form a mini belt of smaller asteroids.
Very rarely, asteroids collide with the Earth, the most notable of which in the past 100 million years was the instigator of the Cretaceous-Tertiary extinction event that wiped out the majority of the dinosaurs 65.5 million years ago. However, there is evidence across the world of many other lesser-sized asteroids impacting the Earth, with their craters remaining a testament to their size. Importantly, their size is not directly represented by the size of the crater, which is roughly ten times the size of the impacting body. These impacts are postulated to have occurred infrequently over the Earth’s 4 billion year life span.
The asteroid finders
The Near Earth Asteroid Tracking (NEAT) program run at NASA’s Jet Propulsion Laboratory has one sole purpose, to find, explore and track near-Earth asteroids.
The Near Earth Asteroid Tracking program based at NASA’s Jet Propulsion Laboratory has discovered thousands of asteroids during its life span, both from its range of ground-based telescopes -a good example is the GEODSS (Ground-based Electro-Optical Deep Space Surveillance) telescope located on Haleakala, Maui, Hawaii – to its deep space probes, scanning space for new asteroids and other space phenomenon.
Its greatest achievement, however, has been its successful insertion of the NEAR (Near Earth Asteroid Rendezvous) Shoemaker space probe into orbit around the asteroid Eros in 2001, as well as landing upon its surface. This made it the first ever spacecraft to complete a soft-land (a landing where the probe is functional afterwards) on any asteroid.
The mission to Eros was primarily to return data on its composition, mineralogy, morphology, internal mass distribution and magnetic field. However, considering its success and time spent orbiting the asteroid, it was possible to also study its regolith properties (the loose material scattered over its surface), interactions with solar winds and spin rate.
This information was garnered with the spacecraft’s equipped x-ray/gamma ray spectrometer (used to measure the intensity of gamma radiation), near-infrared imaging spectrograph (used to measure and image the light properties of the near-infrared end of the electromagnetic spectrum), multi-spectral camera fitted with CCD imaging detector, laser rangefinder and magnetometer (measures the strength and/or direction of a magnetic field). Indeed, thanks to this wealth of information, we now have more first-hand data on Eros than any other asteroid.