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Finding and detecting Near Earth Objects is a difficult task. Due to their often large distance from Earth NEOs are small objects in a big sky and simply finding the largest ones is challenging. Because we cannot predict from which direction new objects may come from, we most also observe the whole sky. Modern NEO search programs are doing just this and are employing the latest detectors and advanced computers to complete the task.
First Observations
Near Earth objects are distinguished from stars and galaxies in the sky by their motion. Because they are so far away stars and galaxies appear to have relatively fixed positions and only move in the sky due to the rotation of the Earth. If a telescope with the stars only objects that are moving relative to the Earth and stars will move in images taken through the telescope. These objects will either be planets, moons or asteroids or comets.
In a series of images taken of an area of the night sky NEOs will change position as they move along their orbits. Objects that move the most in images are generally the closest to the Earth. Identifying the orbit of a NEO involves estimating its distance from Earth and its velocity from telescope images and then calculating all its possible orbits using advanced computers. However, because the NEO may have not moved far over the course of a single night its speed and direction cannot be determined exactly at first. Follow-up observations of the NEO on subsequent nights will be needed to calculate its exact orbit. For this reason search programs publish the details of their first observations on the internet to allow other astronomers to follow-up NEO discoveries.
Size and Type of NEO
Identifying the size and type of Near Earth Object is also difficult. In general the brighter an NEO appears to be (described as its magnitude) the closer and larger it is, however, this will also depend on how much sunlight the surface of the NEO reflects, a property known as its albedo. Some types of asteroid, and extinct comets, can be very dark and reflect little light and thus appear to be much smaller than they really are.
Closer to the Sun than about Jupiter's orbit active comets can be identified in telescope images by the presence of a gas and dust tail. This makes them appear as hazy looking objects. However, identifying the type of Near Earth Asteroid can only be achieved by examining the colour of the asteroid in visible and infra-red light. The intensity of different colours, known as the spectrum, of an asteroid relates to minerals present on its surface and allows the type of asteroid to be identified. C-type asteroids, for example, are carbon-rich and usually have a low albedo (i.e. they are dark).
Identifying the type of NEO is important, particularly for smaller objects, since the density and strength of the object will determine how much of a hazard it represents. Strong, dense NEOs such as iron asteroids, for example, will survive passage of the atmosphere to collide with the surface. Weak NEOs such as comets may break up in the atmosphere and cause air blasts.
Observational Difficulties
There are many difficulties in observing Near Earth Objects. Dark objects, such as C-type asteroids and comets are not surprisingly more difficult to locate than high albedo asteroids such as S-types. Small NEOs are also obviously more difficult to observe than large ones.
Other difficulties are less obvious. Position in the sky is a particular problem. Whereas most asteroids have orbits that lie close to the plane of the Solar System, comets can come from any direction in the sky. Observing comets thus involves searching the whole sky rather than just the region around the ecliptic plane. Some asteroids pose even bigger problems since they have orbits smaller than the Earth's and so come from out of the Sun. These include many of the Aten asteroids and they can only be observed from space telescopes since they are always on the daytime side of the Earth.
Active comets prove to be particularly difficult to observe. Although they can be easy to find the atmosphere and tail of the comet makes it difficult to identify its exact position and the size of the nucleus within. Active areas with jets of gas also make it difficult to predict exactly how the orbit of a comet will change, particularly when the nucleus is spinning, since the jets will change the comet's orbit.
Detailed Observations
Perhaps the most detailed method for examining NEOs from the Earth is using radar. Observations of NEOs using radar can allow the size, shape and even the spin of objects to be identified accurately. Such techniques provide information which would be essential in overting a collision. Radar, however, is most effective for objects close to the Earth.
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