Thursday, March 28, 2013

A Proposal to Eliminate Smaller Asteroid Threats

In this oblique view, the path of near-Earth asteroid 
2012 DA14 is seen passing close to Earth on Feb. 15, 2013The news that a high-powered 50kW laser developed by a German firm (http://www.dailymail.co.uk/sciencetech/article-2259639/The-groundbreaking-Star-Wars-laser-shoot-drone-sky-TWO-MILES-away.html


Can knock down a drone from two miles away, and cut through a steel girder from 1kilometer away (yet accurate enough to hit a target the size of a mortar round) got me to thinking of using multiple similar lasers in an effort to deflect small (< 50m dia.) asteroids. These would be similar to the Chelyabinsk object that recently exploded over central Russia. The German defense firm Rheinmetall Defence that developed it – could conceivably get its name in lights.

Think then of several Russian and U.S. craft carrying these devices to intercept an oncoming small (‘city buster’) asteroid before it can wreak havoc. If the detailed effects and dynamics can be worked out before hand, I see no reason why the oncoming threat can’t be stopped. Or, would we rather spend $20 billion more on missile “defense” systems that have been shown to be useless?

These ruminations emerged after paging through Stephen Hawking’s magnificent book on mathematics: God Created the Integers. As I beheld the panorama of mathematics presented, from Euclid’s elements, to Archimedes’ achievement, to Rene Descartes and his analytic geometry and Isaac Newton’s Calculus, then to the stirring triumphs of Karl Friedrich Gauss, Bernhard Rieman, Henri Lebesgue (Lebesgue integrals) and other notables – I questioned how all this display of exactitude and reason could be for naught.


Surely, we as a species have developed this monument of the mind – this superb mathematics – for something other than to publish in obscure journals (read by maybe 200 specialists), or to teach proofs to endless brigades of college students! There has to be something more in all that fabric of math that actually delivers a powerful practical benefit! Why not use whatever kernel or residue (no pun intended) to deflect an asteroid? Why not put our greatest minds to work on it, as opposed to wasting their energies developing the next financial device - say based on a Gaussian Copula formula?

Of course, the solution may be much more complex than simply targeting a small asteroid using some high-powered lasers. It may well require using these lasers in tandem with other means, such as radiant energy directed at the surfaces of the asteroid to create an artificial YORP Effect. Then, say in the case of an irregularly shaped object, one might alter its spin axis as well as motion- trajectory, when used in conjunction with the lasers.


The YORP Effect, named after Yarkovsky-O'Keefe-Radzievskii-Paddack – or the physicists that discovered it, is a phenomenon that occurs when photons from the sun are absorbed by a body and reradiated as heat. In the process, two forces influence the object: one from the impact of the photons, providing a tiny push, and the other as a recoil effect when the object emits the absorbed energy. For small, irregularly shaped objects , YORP can cause measurable changes in motion.

In 2009, I attended a scientific conference (sponsored by the Dynamical  Astronomy Division of the American Astronomical Society) that featured a paper entitled; ‘Analytic Theory of the YORP Effect for Near –Spherical Objects. .At that time torques of the form:

dt = r x F dS

were considered, where r is the radius vector and F the force supplied. The element of asteroid surface area is dS.  The situations were confined to the cases therefore, where the impinging solar radiation was at right angles to the asteroid’s spin axis.  Three separate detections of the effect were announced, including for  a nearly spherical object (1998 KY), and on two more irregular objects, (1862 Apollo, and 25143 Itokawa).

In the case of the Apollo object the observed effect was approximately 3.0 x 10 -4 deg/day, vs. the theoretically –predicted YORP effect magnitude of 2.6 x 10 -4 deg/day. Earlier, Cornell graduate student Patrick Taylor and assistant professor of astronomy Jean-Luc Margot mapped the shape and located the spin pole of a 100-meter-diameter (about 300 feet) near-Earth asteroid called (54509) 2000 PH5 (abbreviated to PH5) between 2001 and 2005, using radar at the National Science Foundation's (NSF) Arecibo Observatory in Puerto Rico and NASA's Goldstone telescope in California.


On average, asteroids rotate every four to 12 hours. But the smallest asteroids (with a diameter of less than 10 kilometers, or about 6 miles) tend to spin either unusually slowly or unusually quickly -- and astronomers have long wondered why.

YORP could also explain why some asteroids come in pairs. Most asteroids are actually loosely bound clumps of rubble with very little internal cohesion, so an object with an increasing spin rate could eventually spin faster than its own strength and gravity can endure -- ultimately flying apart to form two objects.


Clearly, in the case of the Chelyabinsk object (which would have caused much more damage had it come in at a steeper angle) there is an internal stress limit beyond which it can be shattered, broken. Conceivably, the Rheinmetall Defence lasers could do this long before the object gets to Earth’s vicinity. Meanwhile, if an orbital analysis can be done, it's feasible to perhaps use other lasers (correctly placed) to provide a driving radiant pressure or force (actually, force per unit area) to ‘steer’ an oncoming object to a slightly different  path, enough to miss Earth - if even by a hair (geostationary satellite distance or less). Such a safe deflection can either be by resorting to multiple YORP effects initiated from different directions or destroyed using multiple 50kW lasers of the type Rheinmetall Defence has developed. (Another possibility is to use the lasers to sublimate mass from the asteroid to enhance the rate of deflection via the YORP effects. Some experiments on this have already been successful at Univ. of Glasgow).

No, I am not saying any of this will be easy! But as JFK once said in regard to the future manned Moon mission: “We choose to do these things not because they are easy but because they are HARD!”  And certainly, sparing a city the size of New York City or Moscow from utter devastation is worth as much as spending $1 trillion a year to thwart a bunch of robed Earth- terrorists from planting dirty bombs! (Yes, the probability of the asteroid hit on a big city is much less than a terror attack, but the consequences would be a million times worse if  the small asteroid strike occurred!)

  Of course, before all this can be done we need to get a superior detection system in place. This is where the ‘Sentinel’ system comes in which would provide our planet with an early warning system in the form of an infrared telescope to sight these celestial intruders long before they arrive at our ‘doorstep’. The cost will be in the billions, yes, but think of it as one year’s money already squandered on Afghanistan, but in this case used for a positive humane purpose.

Meanwhile, I pretend to be no asteroid –deflecting specialist or engineer (my field is solar physics). If, however, I’ve sown the seeds for an idea to stop these things, this blog will have been worth it!

See also:  http://neo.jpl.nasa.gov/news/news178.html

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