Near Earth Objects, or NEOs come in 3 varieties. Apollo asteroids have
orbits that cross Earth's orbit, but whose average distance from the Sun
is more than Earth's. The orbital period for Apollo asteroids is greater
than or equal to 1 Earth year. Aten asteroids also cross Earth's orbit, but their
average distance from the Sun is less than Earth's so their year is
shorter than 1 Earth year. Finally, there are the Amor asteroids, which have
average distance from the Sun between the orbits of Earth and Mars, but
whose closest approach to the Sun is slightly outside Earth's orbit (1.017Ð1.3 AU).
Amors often cross the orbit of Mars, but they do not cross the orbit of
Figure 2 : On March 18, 2004, a
30-meter asteroid, 2004 FH, passed by the Earth only 42,600 km (26,500 mi)
away, about one-tenth the distance to the Moon. The rapid streak in the
picture is a meteor. (c) Wikimedia
How Many NEOs are there?
In 1900 there were only 21 known NEOs, all but one of
which were comets. By 1990 the number of NEOs detected had risen to 170,
the majority of which were asteroids. As of February 2011 there were 7,808
NEOs known. Of these, 1,198 were classified as Potentially Hazardous
There are far more small NEOs than large. On a
global scale it is the large NEOs, with diameters greater than 1 kilometre,
which pose the biggest threat. There are estimated to be around 1,000 such
NEOs. As of February 2011 astronomers had detected 822 of these large
A great place to see a map of the inner solar system, with color coded dots
for NEOs, is IAU Animations page.5,6
The Aricebo observatory in Puerto Rico and Goldstone in California,
frequently send and receive radar pulses bounced off of NEOs. The
received images do not look not very detailed by ordinary photographic
standards, but they are better than we can obtain with Earth-based or Earth
orbiting optical telescopes.
Figure 3. Asteroid 4179 Toutatis. Toutatis is roughly 4.5 km by
2.4 km by 1.9 km, and has a mass of roughly 5.0 x 1013 kg.
Radar image. Image credit: NASA/JPL/GSSR, 1996
While optical images of NEO asteroids tend to be dots moving against a
background of stars, radar images of NEOs can be quite detailed,
especially when high resolution radar images from Aricebo and Goldstone
can be combined into a 3-d model.
Asteroid 2000 PH5 Yorp makes a Sun-Earth horseshoe orbit co-rotating with Earth. As the asteroid
reaches alternate ends of the horseshoe every 100 years, it gravitationally bounces off
Earth and reverses direction. It is very slow-moving with respect to
Earth, and fairly close at the present time. This apparently aids the
construction of high-quality images.
Figure 4. a. Animation created from optical images of
asteroid 2000 PH5 Yorp, made by student Sharon McClure during closest
approach to Earth around 27 July 2004. (C) Armagh Observatory. b. Radar
image showing a rendering of asteroid 2000 PH5 Yorp. (Radar model animation via Wikimedia.) The shape
model was obtained by inverting high resolution radar images of the
asteroid obtained at Arecibo and Goldstone. The shape model is published in
Taylor et al., in Science, (Image credit: Patrick Taylor, Cornell
Radar images are interesting because they have to be constructed from the
returned signal. More distant parts of the asteroid return radio waves
later. Computer reconstruction of the object depends on rotation. By
catching the NEO images at different angles, a 3-d model of the asteroid
can be built up. The finest resolutions require 2 recieving antennas,
(usually at Goldstone and Aricebo), to compare speckle.
Figure 5. Binary asteroid 1999 KW4. Delay-Doppler radar
images obtained with Arecibo (left), Goldstone (center) and composite model
(right). Aricebo's larger antenna gives sharper resolution than Goldstone,
but using both together permits modelling at still better resolution. Image
Despite their weak gravity, several NEO asteroids have been found that
have satellites. 1999 KW4, shown above, is a binary, with one satellite.
Asteroid 1994 CC, shown below, is a triple system, with 2 satellites.
Figure 6. Radar imaging at
NASA's Goldstone Solar System Radar on June 12 and 14, 2009, revealed that
near-Earth asteroid 1994 CC is a triple system. Asteroid 1994 CC
encountered Earth within 2.52 million kilometers (1.56 million miles) on
June 10. Prior to the flyby, very little was known about this celestial
body. Image Credit: NASA/JPL/GSSR (Goldstone)
When a new NEO is detected, sometimes there are only hours between the
detection and its closest approach to Earth. Goldstone and Aricebo have
been very flexible about dropping other projects on short notice, to get
pictures of NEOs as they fly by. Asteroid 2010 AL30 appears to be a very
small asteroid, as well as a very
rapid rotator. This may account for the very grainy nature of the image.
Figure 7. "Goldstone 40 MHz (0.0375 microsecond) images of
2010 AL30. In each panel, increases from top to bottom and Doppler
frequency increases from left to right, so rotation is counterclockwise.
Time increases from left to right. The echo bandwidths and visible range
extents indicate that 2010 AL30 is a rapid rotator, which is consistent
with results from lightcurves obtained by Bill Ryan that yield a period of
~9 minutes. The entire sequence spans about one rotation by the asteroid.
The images were double sampled, so each row corresponds to 1.875 meters."
Image credit: NASA/JPL/GSSR
Contact binaries are asteroids that appear stretched, as if they were
about to be pulled apart by centrifugal force caused by rapid rotation.
Figure 8. "We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory
on six dates (15 - 21 September 2008), obtaining radar images and spectra. By combining
these data with an extensive set of new lightcurves taken during 2008-2009 and
with previously published lightcurves from 2005, we were able to reconstruct the
object's shape and spin state. 1996 HW1 is an elongated, bifurcated object with
maximum diameters of 3.8 x 1.6 x 1.5 km and a contact-binary shape."7
Image credit: NASA/JPL/GSSR
Figure 9. 2005 CR37 appears to be a contact binary:
1.8-km-long, extremely bifurcated. The asteroid's two lobes are round, with
regions of modest topographic relief, such as an elevated, 200-m-wide facet
(or crater). Since January, 1999, about 9% of NEAs larger than ~200 m
imaged by radar can be described as candidate contact binaries. Image
In the larger image below, asteroid 1999 JM8 appears to have 3 or 4
craters, that give it a vaugely face-like appearance. The small images in
the bottom row, may also show craters.
Figure 10. Asteroid 1999 JM8. Images from Goldstone and
Aricebo. (Credits did not indicate which frames were from which
observatory, or which were combined composites.) (c) Wikimedia. Image
Here is a NASA video of asteroid JL33. A crater appears in some
Figure 11. NASA/JPL/Goldstone video of asteroid JL33 Ð an
irregular, elongated object roughly 1.8 kilometers (1.1) miles wide. Image
Figure 12. Asteroid 1992 UY4. Images from
Aricebo, taken August, 2005. Movie. Image
Short Period Comets
Once, most of the known NEOs were comets, which are much brighter than
asteroids. The combination of high efficiency CCD cameras, radar, and
space-based infrared, UV, and optical telescopes has revealed a truer
picture of the near Earth space environment.
Figure 12: Comet Encke. Another example of the junk Earth
has not cleared from its orbit. Encke is a close-orbit comet with a period
of only 3 years. Its orbit crosses Earth's. (c) Wikimedia.
Pictures From Space Probes
We will get our best views of NEOs when we have dozens of space-based
telescopes in Solar orbit. Any pass within a few thousand kilometers of
an NEO can give more detailed images than the best radar, with the added
benefit of spectroscopy, to tell us about the NEO's composition.
10. "(29075) 1950 DA," Wikipedia, 18 September 2011
11. "Radar Images of near-Earth Asteroid (100085) 1992 UY4," L. A. M. Benner, M. W. Busch, S. J. Ostro, J. D. Giorgini,
A. A. Hine, J. K. Harmon, M. C. Nolan, R. Rose, R. F. Jurgens,
J. S. Jao, C. Magri, and J. L. Margot, NASA
Abstracts, 2011 October 10