Near Earth Objects

Near-Earth Objects are asteroids or comets that have orbits which may cause them to pass close to the Earth.

The study of NEOs is concerned with identifying objects in space whose orbits may put them on a collision course with Earth. This idea may seem like science fiction, but were such a collision to occur it could cause a global catastrophe. Scientific evidence now indicates that such events have probably happened many times in the past. One of the most widely accepted theories for the mass extinction of the dinosaurs is based on such a collision between an asteroid and Earth about 65 million years ago.

More recently, in 1908, a comet crashed into a remote area of Siberia known as Tunguska. The energy released in the collision was such that, had the comet crashed into a major city, it would have caused catastrophic damage. Searching for near-earth objects, perhaps rogue asteroids or comets, is therefore important for the future of civilization. Developing better techniques to find such debris is an important and exciting area of research at CIT Blackrock Castle Observatory.

Asteroids

Apart from the nine planets, the Solar system contains millions of other objects orbiting around the Sun, the sizes of which range from a few centimetres up to almost 1000 kilometres across (the asteroid Ceres is approximately 950 kilometres in diameter – that’s the distance from Dublin to Brussels!). These bodies can be categorised either as asteroids (sometimes called minor planets) or comets.

Asteroids are large chunks of rock and metal thought to be left over from the creation of the solar system. The majority of asteroids in the solar system are known to orbit around the Sun in a ring consisting of millions of such rocky bodies lying between the orbits of Mars and Jupiter. It is believed that the rocks in this ring are the remains of a planet that didn’t get the chance to form along with the other planets during the formation of our solar system. This is possibly due to the gravitational forces of the massive Jupiter preventing the rocks from clumping together under their own gravity thus allowing them to form a planet.

There are three main types of asteroid: C-type (which are dark and made mostly from carbon), S-type (these are grey and quite rocky) and M-type (which are by and large metallic). Asteroids do not generate any light themselves, so to detect them we rely on the light they reflect from the Sun. By analysing this reflected light scientists can determine the type as well as other physical properties of the asteroid.

First discovered in 1916, the asteroid Gaspra orbits the Sun near the inner edge of the main asteroid belt between Mars and Jupiter. This picture was obtained by the Galileo spacecraft during its approach to the asteroid on October 29, 1991. Gaspra is an S-type asteroid. (Image: NASA/JPL)

Comets

Comets, in general, are much smaller than asteroids and may be only a few kilometres in diameter. Unlike asteroids, which are made of rock and metal, comets are made largely of ice, concentrated into a dense core. There is also a relatively small amount of rock and dust embedded in the ice along with chemical compounds such as carbon dioxide, methane and ammonia which have been picked up by the comet as it travels through space. Comets are thought to originate from the edges of the solar system, and are probably remnants of the original cloud from which the solar system was formed. The distinguishing characteristic of comets in the night sky is their tail. The tail of a comet always points away from the Sun and is composed of charged particles and dust extending for millions of kilometres behind the comet. The tail points away from the Sun because it is pushed by the solar wind, which blows from the Sun into space.

Probably the most famous comet is Halley’s Comet. Named after the astronomer Edward Halley, this comet has an orbital period of between 74 and 79 years (usually 76 years is quoted). The most famous appearance of the comet was uring the Battle of Hastings in 1066 and is recorded in the Bayeux Tapestry housed in the Museum of Reading. The comet is located at the top right corner of the tapestry section shown to the left of this paragraph. In the last century Halley’s Comet was visible in 1910 and again in 1986, its next appearance will be in 2062.

Rogue Asteroids and Comets

Although most asteroids and comets remain in their initial orbits, a small fraction of them are ‘nudged’ out of their orbits, moving towards the Sun and the inner solar system. This nudge can arise from the gravitational forces exerted by a passing galaxy or star or as a result of the gravitational forces exerted by planets. The new orbits acquired by the asteroids can be very elliptical and some of them cross the orbits of other planets. Near-Earth Objects (NEOs) are those ‘rogue’ objects whose orbits cross that of the Earth. Identifying NEOs and determining their orbit is very important since such objects are likely to be on a collision course with Earth. Such collisions have taken place in the past with catastrophic results for life on the planet and more are certainly to take place in the future.

The importance of searching of identifying as many NEOs as possible is today generally accepted, with a number search programs funded around the world as well as the support of the amateur astronomical society.