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There are several Near-Earth Object (NEO) discovery teams either in operation or in the
planning stages. The early efforts to discover NEOs relied upon the comparison of
photographic films of the same region of the sky taken several minutes apart. The vast
majority of the objects recorded upon these films were stars and galaxies and their images
were located in the same relative positions on both photographic films. Because a moving
NEO would be in a slightly different position on each photograph and the background
stars and galaxies were not, the NEOs appeared to ÒriseÓ above the background stars
when viewed with special stereo viewing microscopes.
All of the NEO discovery teams currently use so-called charged couple devices (CCD)
rather than photographs. These CCD cameras are similar to those used in camcorders and
they record images digitally in many electronic picture elements (pixels). The length and
width of CCD detector is usually given in terms of these pixels. A fairly common
astronomical CCD detector might have dimensions of 2096 x 2096 pixels. While the CCD
technology allows todayÕs detectors to be more sensitive and accurate than the older
photographic methods, the modern discovery technique itself is rather similar. Separated
by several minutes, three or more CCD images are taken of the same region of the sky.
These images are then compared to see if any NEOs have systematically moved to
different positions on each of the separate images. For a newly discovered NEO, the
separation of the objectÕs location from one image to another, the direction it appears to
be traveling, and its brightness are helpful in identifying how close the object was to the
Earth, its size and its general orbital characteristics. For example, an object that appears
to be moving very rapidly from one image to the next is almost certainly very close to the
Earth. Sophisticated computer-aided analyses of the CCD images has replaced the older,
manual stereo microscope techniques for all the current NEO search programs.
Not surprisingly, those discovery teams who search the largest amount of sky each
month will have the most success in finding new NEOs. How much sky each telescope
covers per month will depend upon a number of factors including the number of clear
nights available for observing, the sensitivity and efficiency of the CCD detector, and the
field of view of the telescope. Wide field of view telescopes can cover more sky per given
time than telescopes with narrower fields of view. It is also important for search teams to
extend their searches to greater and greater distances from the Earth or, in other words, go
to fainter and fainter limiting magnitudes. A 6th magnitude star is roughly the limit of a
naked eye object seen under ideal conditions by someone with very good eyesight. A 7th
magnitude star would be 2.5 times fainter than a 6th magnitude star and an 8th magnitude
star would be 6 times fainter than a 6th magnitude object (2.5 x 2.5 = 6.25). A difference
of 5 magnitudes is a brightness difference of nearly 100 (2.5 x 2.5 x 2.5 x 2.5 x 2.5 is equal
to about 100).
In terms of the discovery efforts for NEOs, NASA's current goal is to discover at least
90% of all NEOs whose diameters are larger than 1 kilometer within 10 years. To meet
the NASA goal, the rate with which new objects are discovered will necessarily be largest
in the first few years. This is because during the latter years of the 10-year interval, more
and more "discoveries" will actually be of objects that have been previously found.
Currently, the best estimate of the total population of NEOs larger than one kilometer is
about 1000. The progress toward discovering 90% of this population can be monitored
under the web page entitled ÒNumber of NEOsÓ within the section on Near-Earth
Objects.
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