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City Tech Research Team Casts Light on Asteroid Deflection
What Apophis might look like up close
In 2029 and again in 2036, the asteroid Apophis, discovered in 2004 and measuring at least 1,100 feet in diameter and weighing an estimated 25 million tons, will make two close bypasses of Earth at a distance of about 22,600 miles. While calculated to be statistically unlikely, it is possible that Earth’s tidal effects on Apophis as it passes might endanger the planet’s safety.
“A collision with an object of this size traveling at an estimated 30,000 to 40,000 mile per hour would be catastrophic,” according to NASA researcher and New York City College of Technology (City Tech) Associate Professor of Physics Gregory L. Matloff, whose long-term research interests include the best means to avert disaster should a collision be a distinct possibility. “Either destroy the object or alter its trajectory,” adds Matloff, who has concluded that diverting such objects is the wisest course of action to follow.
Space is filled with everything from dusty particles to chunks of ice and rock of different sizes left over from the formation of the solar system. While the dust poses no particular problem, it’s the bigger chunks of space debris that whiz by the planet – what are called Near-Earth Objects (NEOs) – that are of real concern.
We don’t always know that they’re coming that far ahead of time. On September 8, 2010, for example, two fairly small asteroids, discovered only four days before by NASA’s Minor Planet Center in Massachusetts, sped by the Earth – one within 154,000 miles of the planet’s surface and the second only 49,000 miles away. Both were small and NASA determined that there was no chance of a collision with the planet.
According to NASA’s Near Earth Object Program, NEOs are comets and asteroids nudged by the gravitational attraction of nearby planets into orbits that allow them to enter Earth's neighborhood. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.
The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process 4.6 billion years ago. The giant outer planets (Jupiter, Saturn, Uranus and Neptune) formed from an agglomeration of billions of comets, and the left over bits and pieces from this formation process are the comets we see today. Likewise, today's asteroids are the bits and pieces left over from the initial agglomeration of the inner planets that include Mercury, Venus, Earth and Mars.
Dr. Matloff favors diversion because blowing up an asteroid with, say, nuclear explosives risks creating another problem to worry about later on – the creation of debris that might bathe the Earth in a radioactive shower.
“One method of diverting asteroids that might impact the Earth,” says Dr. Matloff, “is the Solar Collector. Suggested by H. J. Melosh, I. V. Nemchinov and Yu. I. Zetzer in 1994, the Solar Collector would function by concentrating sunlight on an asteroid’s surface. For certain types of asteroids, an energized jet of evaporated material would be created. By Newton's Third Law, the reaction to this jet would alter the asteroid's solar orbit, hopefully converting an Earth-impact to a near miss.”
In 2007, Dr. Matloff worked with a team at the NASA Marshall Space Flight Center in Huntsville, Alabama, to investigate methods of deflecting the asteroid Apophis, when it makes those coming two close approaches to Earth. One parameter in modeling the efficiency of the Solar Collector is the penetration depth of concentrated sunlight on an asteroid’s surface. Matloff's literature search revealed that penetration depth for many terrestrial soils is around 100 microns or 0.000328084 feet (there are a million microns in a meter and one meter is equal to about three feet), which indicates that the Solar Collector would function well for an asteroid consisting of terrestrial soil.
But because asteroids are definitely not coated with terrestrial soil, Dr. Matloff approached Dr. Denton Ebel, meteorite curator at the American Museum of Natural History in New York City. Dr. Ebel graciously provided samples of rock and simulated soil (ground-up rock) from the Allende meteorite that impacted Mexico in 1969.
Using green and red lasers, optical transmission measurements (the fraction of light that passes through a material) of these samples were made at City Tech by Assistant Professor of Physics Lufeng Leng, a photonics and fiber optics researcher, and one of her students, Thinh Le. The research showed that optical transmission for Allende rock is less than 10 percent for a sample thickness of 0.000098425 feet for both laser colors. As the simulated soil-sample thickness was varied from 0.000098425 feet to 0.000164042 feet, very preliminary results revealed that transmission in green light fell from 2.35 percent to 0.59 percent and in red light from 5.78 percent to 0.79 percent. These experimental results indicate that light's penetration depth in this type of meteorite will not be dissimilar to the 0.000328084 feet approximate value measured for terrestrial soils by A. Ciani, K.-U. Goss and R.P. Schwarzenback in 2005.
In a related study, the team narrowed the laser beam as much as possible and scanned the surface of the thin-section Allende rock sample. Transmission differences between dark "matrix" and mineral-rich "chondrules" in red light indicated that lasers on a space probe positioned near an asteroid could be applied in determining surface composition, as transmitted light varies with composition.
“To my knowledge,” says Dr. Matloff, “this is the first experimental measurement of the optical transmission of asteroid samples. Dr. Ebel is encouraging other researchers to repeat and expand on this work, for meteorites like Allende and others.”
Dr. Matloff notes that the coming close passes of Apophis demonstrate that robotic and peopled missions to near-Earth asteroids should test the performance of the Solar Collector and other means of asteroid diversion in space. “At present,” he adds, “a debate is underway between American and Russian space agencies regarding Apophis. The Russians believe that we should schedule a mission to this object probably before the first bypass because Earth-produced gravitational effects during that initial pass could conceivably alter the trajectory and properties of the object. On the other hand, Americans generally believe that while an Apophis impact is very unlikely on either pass, we should conduct experiments on an asteroid that runs no risk of ever threatening our home planet."
These results were presented in a paper by Drs. Matloff and Leng and Thinh Le delivered on July 29, 2010, at the 73rd Annual Meeting of the Meteoritical Society held at the American Museum of Natural History and the Park Central Hotel.