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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.

 

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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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