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NEOs and PHAs; The Spaceguard Survey
Near-Earth Objects (NEOs) are asteroids and comets perturbed by
the gravitational attraction of other planets, mainly Jupiter.
These perturbations send NEOs into orbits which allow them to
potentially encounter the Earth. A subset of NEOs are the
potentially hazardous asteroids (PHAs). These make especially
threatening close approaches to the Earth (NASA, 2003). At least
681 PHAs are known. According to NASA (2003), "By monitoring
these PHAs …we can better predict the close-approach statistics
and thus their Earth-impact threat."
NASA in cooperation with the US Air Force in the late 1990’s
implemented a Spaceguard Survey program designed to identify and
catalog by the end of 2008 the orbital characteristics of all
comets and asteroids greater than 1 km in diameter and in an
orbit that crosses the Earth’s orbit (NASA Spaceguard Survey).
There are estimated to be around 1000 such objects (Cooke,
2004). What about objects less than a kilometer in size but
still large enough to kill millions of people? Surveys planned
for 2010 and beyond will be able to detect objects as small as
150 meters about the size at which the Earth’s atmosphere offers
us nearly full protection (Cooke, 2004).
The Spaceguard Survey program consists of a number of NEO search
programs involving public and private groups. They use varying
sized professional telescopes coupled with sophisticated
electronic imaging equipment to routinely survey the sky for
asteroids and comets. The techniques used are beyond the scope
of this essay, but these details may be found at the NASA
Spaceguard Survey web link (Yeomans).
The Minor Planet Center operates at the Smithsonian
Astrophysical Observatory in Cambridge, MA, on the campus of
Harvard Observatory. It functions under the auspices of the
International Astronomical Union (IAU) and is responsible for
the naming of minor bodies in the Solar System and "for the
efficient collection, (computation,) and dissemination of
astrometric observations and orbits for minor planets and
comets" (MPC). It is the official international clearing house
for such data, and it provides an almost hourly updating of the
orbits of important astronomical objects, particularly those
with potential harm to the Earth.
Can We Prevent an Impact?
No! We have no technology and nothing on the books that can
prevent an impact (Morrison). Fortunately, no known asteroid is
on a collision course with the Earth. By the end of the decade,
we should have identified a significant fraction of all those
asteroids and comets that come close enough to the Earth to be a
future hazard. We need to develop a lead time of at least 10-15
years for an impact warning. Hopefully, our warning will be
decades, if not centuries, allowing us enough time to determine
if there will be an actual impact, develop evacuation plans for
smaller impacts, and develop a means to divert a globally
threatening large asteroid (Cooke, 2004).
Suggestions for diverting an incoming global killer include
attaching an enormous solar sail or a gigantic rocket drive to
the asteroid’s surface to nudge it into a non-threatening orbit.
A nuclear explosion on the asteroid’s surface could alter its
trajectory by using the propellant effect of the vaporized
material to shove the asteroid in a direction away from the
Earth (Burns).
Conclusion
There is a lot to do. We have to find as many of the threats as
possible, and we have to develop a well thought out warning
system if there is an eminent threat. Nothing of the kind has
been developed. What should MPC do if it identifies an asteroid
with a high probability of impact? What is the chain of command
for notification? Who will develop the technology to divert a
really big one coming in? NASA? The world’s nations working
collectively? There is lot to think about.
References
The Barringer Meteorite Crater. The History of the Crater.
Becker L, Poreda RJ, Basu AR, Pope KO, Harrison TM, Nicholson C,
Iasky R. Bedout: a possible end-permian impact crater offshore
of Northwestern Australia. Science 2004; 304: 1469-1476.
Burns PR. How dangerous are Earth-crossing objects? At: http://www.pibburns.com/catastro/impacts.htm.
Cooke B. Killer impact. Astronomy 2004; December: 38-43.
Foster KR. Risk and public Policy: courting disaster. Science
2005; 307: 1205.
Hamilton CJ at: http://www.solarviews.com/eng/asteroid.htm.
Harris A. The Chelyabinsk meteor explosion. Letters, coauthor reply. Skeptical Inquirer November/December 2013; 37(6): 32.
MadSci Network at: http://www.madsci.org/posts/archives/may98/894579086.Es.r.html.
Minor Planet Center (MPC) at: http://cfa-www.harvard.edu/iau/mpc.html.
Moore, Sir Patrick, General Editor. Oxford Astronomy
Encyclopedia, Oxford University Press, 2002, New York, pages
31-32; 83.
Morrison D. Defending the Earth against what? Overview of the impact hazard.
NASA Near Earth Object Program.
NASA Spaceguard Survey at the end of 2007.
Schultz S. Sky survey lowers estimate of asteroid impact risk.
At: http://www.eurekalert.org/pub_releases/2001-11/pu-ssl110701.php.
Tagle R, Claeys P. Comet or asteroid shower in the late Eocene?
Science 2004; 305: 492.
Yeomans D, Baalke R at: http://neo.jpl.nasa.gov/programs/.
Essay posted
1 May 2005
References revised 18 February 2014
Minor text revision 25 February 2014
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