asteroid 2005 YU55
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Figure 1. Radar image of asteroid 2005 YU55 was generated from data taken in April, 2010 by the Arecibo Radar Telescope in Puerto Rico. 2005 YU55 will make a close approach to the Earth, November 8, 2011. Youtube video. SOHO Satellite movie. November 7, Goldstone RADAR movie, via Sky and Telescope. Image credit: NASA/Cornell/Arecibo Additional credit: NASA/JPL/GSSR, 2011

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A Gallery Of NEOs (Near Earth Objects)

Figure 2. a. NASA Asteroid Watch Movie of 2005 YU55 from Goldstone data. b. NASA-JPL's Youtube video about 2005 YU55. Image credit: NASA/JPL/GSSR, 2011
By F. E. Harris

November 1, 2011 (updated November 9)

Introduction

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 (1.0171.3 AU). Amors often cross the orbit of Mars, but they do not cross the orbit of Earth.1

Asteroid_2004_FH animation
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 Asteroids.4 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 objects.

A great place to see a map of the inner solar system, with color coded dots for NEOs, is IAU Animations page.5,6

Radar Images

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.

Asteroid2000ph5movie asteroid2000PH5
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 University).

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.

asteroid1999KW4a asteroid1999KW4b
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 credit: NASA/JPL/GSSR

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.

TripleAsteroid1994CC Animation of Asteroid1994CC
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.

Asteroid2010AL30
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.

Asteroid8567_1996HW1
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

ContactBinary2005CR37
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 credit: NASA/JPL/GSSR

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.

Asteroid1999JM8 - a
Asteroid1999JM8 - b
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 credit: NASA/JPL/GSSR

Here is a NASA video of asteroid JL33. A crater appears in some frames.

Figure 11. NASA/JPL/Goldstone video of asteroid JL33 an irregular, elongated object roughly 1.8 kilometers (1.1) miles wide. Image credit: NASA/JPL/GSSR

Asteroid1999JM8 - a
1992uy4.jpg
Figure 12. Asteroid 1992 UY4. Images from Aricebo, taken August, 2005. Movie. Image credit: NASA/JPL/GSSR

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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.

Comet Encke
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.

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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.

Asteroid Steins movie
Figure 13. Rosetta Encounter with Asteroid 2867 Steins. Rosetta came within 800 km of asteroid Steins. The direction of Rosetta's OSIRIS camera changes during this video, to keep Steins in view. Credit: ESA 2007 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/ DASP/IDA9


References

1. "Near-Earth object," Wikipedia
http://en.wikipedia.org/wiki/Near-Earth_object
2. "NASA in Final Preparations for Nov. 8 Asteroid Flyby," JPL press release, October 26, 2011.
http://www.jpl.nasa.gov/news/news.cfm?release=2011-332
3. "ODDBALL ASTEROID CAPTURED ON NEW VIDEO COMPUTER SIMULATION," JPL
Press Release, November 27, 1996. http://neo.jpl.nasa.gov/images/toutatis.html
4. "How Many?," National Space Center Online.
http://www.spacecentre.co.uk/Page.aspx/186/HOW_MANY_/
5. "Map of all Inner Solar System Asteroids," Wikepedia, 29 July 2011, latest edit.
http://szyzyg.arm.ac.uk/~spm/neostorm.png
6. "IAU Minor Planet Center - Maps and Animations"
http://www.minorplanetcenter.net/iau/Animations/Animations.html
7. "ASTEROID RADAR RESEARCH," L. A. M. Benner, MS 183-601, Jet Propulsion Laboratory
http://echo.jpl.nasa.gov/
8. "Triple Asteroid System Triples Observers' Interest," JPL News and Features, August 06, 2009.
http://www.jpl.nasa.gov/news/features.cfm?feature=2259 http://www.jpl.nasa.gov/images/asteroid/20090806/neo20090806-516.gif
9. "Near-Earth Asteroid 2000 PH5 Observed by School Student at Armagh Observatory," Armagh
Observatory, 2004. http://star.arm.ac.uk/press/2000PH5.html
10. "(29075) 1950 DA," Wikipedia, 18 September 2011
http://en.wikipedia.org/wiki/Asteroid_1950_DA
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
http://echo.jpl.nasa.gov/asteroids/1992UY4/1992UY4.html
12. "GOLDSTONE DELAY-DOPPLER IMAGES OF 1999 RQ36," NASA Abstracts, September 21 - October 1, 1999
http://echo.jpl.nasa.gov/~lance/1999RQ36/1999RQ36.html
13. "Goldstone: Desert outpost performs radio imaging of close-passing asteroid 2005 YU55," Emily Lakdawalla, The Planetary Society Blog, Nov. 9, 2011
http://www.planetary.org/blog/article/00003253/
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