290 Encyclopedia of the Solar System
warming by the Sun causes asteroids of all sizes to reradiate
their heat back into space. Because the asteroids are rotat-
ing, the reradiation does not occur in the same direction as
the incoming sunlight, resulting in a small force acting on
the asteroid. This force acts as a very gentle push on the
asteroid, which over many millions of years can cause the
asteroid to slowly drift inward or outward from its origi-
nal main-belt location. This is called Yarkovsky drift and is
especially effective on small objects; it may be particularly
important for supplying meteoroids to Earth. Cast away
fragments or drifting bodies that enter regions where res-
onances with Jupiter’s orbit are particularly strong, such as
the 3:1 Kirkwood gap, find that small changes in the semi-
major axis can result in large, exponential changes in other
orbital elements, in particular eccentricity, changing the or-
bit significantly on a short timescale. Thus, the effects of
chaotic regions are more than the sum of small changes in
motion over long periods of time. These regions ofchaotic
motionare associated with resonances with both Jupiter
and Saturn (Fig. 8). The two gas giant planets are believed
to play a significant role in directing meteoroids to Earth,
and presumably also many of the near-Earth objects.
Other objects evolve from Jupiter-family comets or
Halley-type short-period comets. Life in the Jupiter family
is not long-lived, as Jupiter imparts changes to the orbits on
timescales of 10^4 –10^6 years. Leaving Jupiter’s gravitational
sphere of influence, the soon-to-be near-Earth objects may
FIGURE 8 Dynamical resonances are regions where
gravitational interactions either deplete or protect asteroids from
changes in their orbit. (From Jim Williams, NASA/JPL.)
sometimes be perturbed by Mars and other terrestrial
planets and also affected by the influences of nongravita-
tional forces, such as volatile outgassing or splitting of the
cometary nucleus. These phenomena also contribute to or-
bital changes that result in planet-crossing orbits.4. Population
4.1 Search Programs and TechniquesOrganized, telescopic search programs for near-Earth ob-
jects operate worldwide. The search programs supported
by the National Aeronautics and Space Administration
(NASA) include the Lincoln Near-Earth Asteroid Research
(LINEAR) program, the Near-Earth Asteroid Tracking
(NEAT) system, Lowell Observatory’s Near-Earth Object
Search (LONEOS), the Catalina Sky Survey, and Space-
watch, the last two operated by independent teams at the
University of Arizona. International efforts and interests are
also strong at Japan’s National Space Development Agency
(NASDA) and a joint venture among the Department of As-
tronomy of the University of Asiago, the Astronomical Ob-
servatory of Padua in Italy, and the DLR Institute of Space
Sensor Technology and Planetary Exploration in Berlin-
Adlershof, Germany. Though the objectives of these pro-
grams are all similar, to inventory the objects in the vicinity
of Earth, each has its own design and approach. In the past,
when astronomers imaged the sky with photographic plates,
it was an eye-straining process to compare them and deter-
mine if something moved. Search programs today employ
digital imaging devices known ascharge-coupled devices
orCCDsthat cover large areas of the sky in a single expo-
sure. Typically a given area of sky is imaged and reimaged
3–5 times at intervals of 10 minutes to an hour. With digital
images, fast computers can compare the images, identify
and subtract all of the “uninteresting” objects that remain
fixed, leaving behind the tracks of a moving asteroid or
comet. By rapidly repeating this process for many patches of
sky throughout a night, nearly the whole sky can be scanned
in the course of about 2 weeks. Increasingly rapid and in-
creasingly sensitive search systems are expected to come on
line by the end of the decade.
When a near-Earth object is first discovered, astrono-
mers initially trace only a short piece of its orbit as mea-
sured over a few hours or even over a few weeks. With each
new NEO discovery, astronomers wish to assess whether
the object poses any immediate or future impact threat.
Orbit calculations for most objects can be made reliably for
many decades into the future, but of course if only a tiny
part of the orbit has been observed, the extrapolation into
the future becomes increasingly uncertain. Sometimes that
extrapolation shows that the Earth itself resides within the
overall uncertainty region for an NEO’s future position. If
the cross section of the Earth occupies 1/10,000th of this