What is an Asteroid?

There has been some discussion in the news recently about the near-miss of an asteroid.

An asteroid can simply be described as a minor planet that is larger than 1m and can reach diameters in the hundreds of kilometers.  They differ from comets in their composition.  A comet is typically made of ice while an asteroid is a rocky, carbonaceous and metallic.  Objects smaller than a meter in diameter are typically referred to as meteoroids that compose the majority of the meteors that fall to Earth.

Sometimes asteroids fall to Earth.  The famous Chelyabinsk meteor that fell to Earth in February 2013 was a 20m wide near-Earth asteroid.

Chelyabinsk Meteor (Image Credit: Marat Ahmetvaleev)

Most asteroids reside in the region of our Solar System between Mars and Jupiter.  A group known as 'Jupiter Trojans' sit in Jupiter's stable Lagrange points 60° ahead and behind of the planet's orbit.

There are a class of asteroids known as 'Near Earth Objects' that may pose a risk to our home planet.  A (NEO) is typically defined as any body in the Solar System that has a perihelion (closest approach to the Sun) inside Earth's orbit or 30% the distance from Earth to the Sun outside Earth's orbit.  This includes more than ten thousand asteroids, more than a hundred comets and a bunch of meteoroids and spacecraft.

Most large (>1km) near Earth asteroids have been cataloged with some still waiting to be found.  Large asteroids like this may cause global damage if they were to hit the Earth.

Asteroid 2012 TC4 has been in the news this week.  It is a small asteroid (10-20m in diameter) that passed about 50,000km from the Earth this morning.  To visualize how close this is, consider that the Moon is between 350,000-400,000km away depending on where in its elliptical orbit it is.

2012 TC4 Close Approach (Image Credit: NASA)

You can see many asteroids from your backyard with binoculars or a telescope.  The asteroid, Vesta, can actually sometimes be seen with the naked eye in excellent dark sky conditions when it is near opposition.  It is quite large (~525km diameter) and relatively high albedo (reflectivity).  The best trick for finding asteroids is to use the Pluto method.  Determine the location of the asteroid using mapping software.  Sketch or photograph the region under observation.  Repeat the next day and see which 'star' in the region moved during that 24 hour period.  That will be your asteroid.  I still have not had the opportunity to find and track an asteroid but I hope to soon.  Tracking the orbit of Neptune last year was my first foray into witnessing orbital motion in the outer Solar System.

Vesta (Image Credit: NASA Dawn Spacecraft)

The Dawn spacecraft was sent in a mission to explore the asteroid belt.  Using ion thrusters it was able to enter and leave the orbit of asteroids.  It visited Vesta (above) in 2011 and moved onto the dwarf planet Ceres in 2015.