Foiling Perils From Outer Space
Huge reflective sails could divert killer asteroids, City Tech scientists say.
This is not science fiction: A 25 million-ton asteroid that's 90 stories tall really is hurtling toward Earth. On Easter Sunday, April 13, 2036, it could smash into our planet somewhere between Kazakhstan and Venezuela. It's named Apophis after the Egyptian god of darkness and the void.
If it plunged into the Pacific, Apophis would spawn tsunamis that could hammer the West Coast with 50-foot waves for an hour or more. If it hit land, it could kill tens of millions of people; NASA says it would strike with 68,000 times the force of the atom bomb that leveled Hiroshima.
By comparison, when a meteor a fifth its size exploded in the air over unpopulated Siberia in 1908, it flattened 80 million trees over 800 square miles (larger than New York's five boroughs and Westchester County, combined). Its shock wave broke windows and knocked people off their feet hundreds of miles away.
Panic spread when Apophis was first spotted in 2004, for the chances of collision initially were put at 1 in 37. NASA's recent calculations now peg the threat at 1 in 45,000 or less.
Heave a sigh of relief but beware of the unknown. When Apophis passes Earth in 2029 and heats as it falls toward the sun, it could calve into smaller pieces or emit a tail, which would act like a rocket and change its direction unpredictably. If Apophis or its fragments enter one of two "keyholes" in space, there could be impact when it returns seven years later.
Apophis - although a wake-up call for planetary security - is almost beside the point. Comets and asteroids have hit Earth before, and a big one most likely triggered the mass extinction that snuffed out the dinosaurs 65 million years ago. NASA predicts a 1 in 100 chance that an asteroid at least 140 meters (459 feet) in diameter will smack down within 50 years with enough force to obliterate a state or a coastline. Apophis or some other hunk of rock - the name really doesn't matter, for the threat is equally dire. But it can be averted.
Gregory Matloff, a New York City College of Technology assistant professor of physics and a NASA consultant, favors deflecting asteroids with space-based solar sails. These are sheets of reflective metal less than a tenth the thickness of a human hair. A solar sail 50 meters (164 feet) on a side could travel alongside a large asteroid for a year, continuously focusing the sun's rays on it, burning off part of the surface and creating a jet that could steer the asteroid away.
And, Matloff says, there's no better asteroid to try this on than Apophis, since we know it's coming and have ample time to prepare a mission to divert it.
Solar sails also could keep satellites in position without fuel, power missions across the solar system faster than rockets and provide limitless electricity by converting sunlight to microwaves and transmitting them to Earth.
Matloff, who has theorized about solar sails for more than 30 years, is part of a dynamic team of City Tech physicists who are refining the science behind solar sails in a steady stream of papers and conference presentations.
Together and individually, Matloff and physics professor Roman Kezerashvili have explored possible materials, thicknesses and construction techniques for solar sails.
During the summer, Kezerashvili and assistant professor Justin F. Vázquez-Poritz galvanized the International Academy of Astronautics conference in Aosta, Italy, which focused on missions to the outer solar system and beyond. They described how the Einsteinian curvature of space-time would affect the steering of solar sails.
"Everyone in the room got excited," says Les Johnson, deputy manager of the Advanced Concepts Office at NASA's Marshall Space Flight Center in Huntsville, Ala. "They're the first to assess how general relativity might affect close-approach [to the sun] solar sails. With Roman's nuclear physics background and Justin's relativity and string theory background, they're looking at this from the side. They're covering things that those who have worked the field haven't thought about and really need to."
Meanwhile, assistant professor Lufeng Leng, a photonics and fiber optics researcher, joined Matloff in a paper that suggests using the lowest-tech optical device - a lensless pinhole camera - to monitor the health of solar sails after they're deployed. Now, firing up the lasers in her lab, Leng measures the optical properties of meteorite samples, seeking a better understanding of how light interacts with regolith (the loose rocky, icy or dusty surface covering of celestial bodies). This is a first step toward building a more accurate model of how a solar collector could deflect an Earth-threatening asteroid or comet.
This is impressive work for just four members of a 14-person physics department, not to mention a department that is only three years old and from an institution that awards more associate degrees than bachelor's degrees.
"Everyone thinks that City Tech is a teaching institution, not a research institution," says Kezerashvili, the physics department chair. "But as I see it, we're a physics department with excellence in teaching and in research. Our faculty publishes in the most respected journals."
Unlike conventional chemical rockets that roar into space on pillars of fire and smoke and burn an ever-dwindling supply of fuel with each course correction, solar sails require no propellant once unfurled in space. That gives them greater range than any existing rocket without in-flight refueling, which has never been attempted.
Solar sails function like boat sails, but instead of wind, they're driven by photons from the sun. A photon is a unit of light, a particle that lacks mass but packs electromagnetic energy and momentum. When photons hit the sail, they bounce off, transferring their momentum and pushing it.
If sails directly face the sun, ceaseless photon bombardment would continuously accelerate them, speeding them across the solar system and beyond. If sails are angled, they will spiral toward or away from the sun, allowing them to be steered, just as a boat tacks by shifting its sails in relation to the wind.
"Solar sails are the only way we can take the first steps into interstellar space," says NASA's Johnson, who formerly managed interstellar propulsion for the agency and has co-authored two books with Matloff. They also hold a 2003 patent that marries a photon-driven solar sail with an electrodynamic tether system that both generates propulsion to steer the spacecraft and draws power from a planet's magnetic field, which can be used to perform orbital maneuvers.
"We looked at fusion, fission, electric propulsion and more, and what we could build in the near term," Joshson says. "The only two options are solar sails and nuclear fission, and fission is too expensive and complex. Solar sails wouldn't be. You could go further and further out with sails before you hit a technical barrier. They wouldn't get you to a star any time soon - maybe in a thousand years - but they'd get you there."
But what happens to an exquisitely thin sheet of metal as it flies through space, ceaselessly scorched by the sun and cosmic radiation? That's where Kezerashvili, a theoretical physicist, comes in.
While talking with Matloff, Kezerashvili says he realized that "solar radiation not only pushes solar sails, it also destroys the materials from which they're built." He scribbled equations on the blackboard. Matloff said no one else had thought about this, and that led to a series of papers in the Journal of the British Interplanetary Society and other journals.
They considered what would happen on a microscopic level to a twin-walled, hydrogen-inflated solar sail made of the metallic element beryllium, with walls 10 to 20 nanometers thick (one nanometer is 3 to 10 atoms wide). Beryllium, while toxic to mine and handle, became their favored material, for it is three times lighter than aluminum and has a high melting point. They investigated how harsh solar radiation could degrade beryllium, affect the sail's structural integrity and allow hydrogen to escape, which would deflate the sail.
"For materials science, all of this is very well known, but from a cosmic point of view, it was something new," Kezerashvili says.
Their other papers looked at topics including sail thickness, the performance of a single-layer sail, the relative merits of different metallic films, and whether it's wise to mitigate the electrostatic pressure that ultraviolet radiation causes when it ionizes the sails, creating a positive charge. (An ionized sail will deflect protons hurled by the solar wind, which transfer their momentum and increase the sail's speed; but too much electrostatic pressure can rupture a hollow-body sail. The trick is finding the right balance of thickness, material and amount of inflation gas.)
Vázquez-Poritz, versed in string theory, brought a different perspective to what he calls "the theoretical playground." (An amalgam of quantum mechanics and general relativity, string theory is the leading candidate for a "theory of everything" that would describe all matter and the interactions of the four fundamental forces: gravitational, electromagnetic, weak and strong.)
Newton said that gravity is the force between two masses, like two molecules or the sun and Earth. Einstein said that objects move in curved trajectories and accelerate not because there's gravitational force but because they follow the curvature of space-time (for as he conceived it, space and time are not different, but a single geometric manifold, and traversing it is like a marble rolling across the curves of a rumpled sheet).
"Newtonian gravity was enough to get man to the moon, but the closer you get to the sun, the stronger the curvature of space-time and the less you can ignore Einstein," Vázquez-Poritz says.
Suppose you want to unfurl a solar sail where it will get the fastest start for a trip across the solar system. Since the force of photons diminishes the farther you get from the sun, just as your perception of a flame's heat lessens the farther away you pull your hand, you'd want to open the sail extremely close to the sun - say at .05 astronomical units (or AU, where one AU is the distance from the sun to the Earth; the first planet, Mercury, orbits at .39 AU).
Why is getting a fast start so important? Consider this: NASA launched Voyager 2 in 1977 to study the heliosphere (the area covered by our sun's solar wind, which starts at 1 million mph), the termination shock (where the solar wind slows below the speed of sound, which is 1 mile in 5 seconds) and the heliosheath (where pressure from the interstellar medium causes the solar wind to form a comet-like tail). It took 30 years for Voyager to reach the heliosheath, some 80 to 100 AU away, and it will take at least as long to reach the 200 AU heliopause (where the solar wind stops).
But, Kezerashvili and Matloff calculated, a solar sail deployed at .05 AU could speed to the heliopause in just 2½ years. In 30 years, it could explore a vast swath of the Oort Cloud - a reservoir of long-period comets and other space debris - that stretches from 1,000 to 50,000 AU away or farther.
"You ask me why do I want to come so close to the sun with a solar sail?" says Kezerashvili. "I want to know what happens beyond the solar system, and if I launch something, I should know the result during my lifetime."
He and Vázquez-Poritz considered Kepler's third law, a mathematical formula conceived in the early 1600s that describes the relationship between the longer orbital periods of planets far from the sun and the shorter orbital periods of planets close to the sun. In two papers, they argue that deviations from Kepler's law occur when the curvature of space-time and solar radiation pressure act simultaneously on a solar sail-propelled satellite. In short, if you open the sail close to the sun and don't account for Einstein, you might miss your target by more than 1 million miles.
"This could be an ideal way to test the effects of general relativity," Vázquez-Poritz says. "With a solar sail, we could measure effects that otherwise would be too small to measure."
He adds that solar sailing could thus become the second technological application of general relativity. The first is the Global Positioning System, which calculates your position by detecting minute differences in timing between signals from different orbiting satellites.
Soon after Kezerashvili and Vázquez-Poritz presented their papers at the International Academy of Astronautics in Italy this summer, Matloff presented one on a different topic that he co-authored with undergraduate Monika Wilga. Call it celestial hitchhiking.
Before humans can travel far from the protection of the Earth's magnetic field, they have to figure out how to shield themselves from potentially lethal cosmic rays. This high-energy, high-mass radiation is a major obstacle to a two- to three-year mission to Mars. Shielding needs to be heavy and launching it from Earth would be expensive – "at $10,000 a pound, you're talking about $50 to $60 billion in launch costs," Matloff says.
But, he wondered, why not hitch a ride on a passing asteroid, much as a hermit crab climbs into an empty seashell?
Wilga, his Astronomy 1 and 2 student two years ago, is now heading into a physics major at Queens College and aims for a master's in astronomy or astrophysics. She searched the Web for known near-Earth objects (NEOs, meaning asteroids and comets) with certain characteristics, including crossing the orbits of Earth and Mars and being in our neighborhood between 2020 and 2100. A few fit the bill.
"There's a trade-off, of course," Matloff says. "A ship must perform more powered maneuvers to rendezvous with the NEO. Also, total flight times for NEO-shielded missions may be a bit longer than for unshielded planetary transfers."
The symposium committee selected all of the City Tech papers for submission to IAA's journal, Acta Astronautica.
This hitchhiking notion flows naturally out of Matloff's work on deflecting threatening asteroids. He favors a two-sail system. Over the course of a year or more, a large parabolic collector sail would reflect sunlight onto a smaller, maneuverable thruster sail that would concentrate an intensely hot beam of light on a point on the asteroid's surface. Just as a child might use a magnifying glass to focus sunlight to set a dried leaf on fire, the thruster would vaporize rock, creating a controllable jet whose velocity would enable scientists to steer the asteroid safely away.
Leng is now in City Tech's photonics lab, conducting experiments aimed at building a mathematical model of how that idea would work. Her research - still at what she calls "a very preliminary stage" - starts with meteorite samples borrowed from the American Museum of Natural History, where Matloff also is a Hayden associate in astrophysics.
Working with undergraduate math major Thinh Le, she directs differently colored laser beams at the samples. Since these 30-micron (.00117-inch) thin meteorite sections are embedded in epoxy glass, she has to account for how much energy the glass reflects and absorbs. "Basically, we measure the light intensity before it hits the sample and measure it again after to get the transmission coefficient," or the amount hitting the meteorite. Working with meteorites of different thicknesses, "we can get a bunch of curves and begin to build up a model."
A crucial question is how far the light penetrates below the surface, for a beam that penetrates too deeply will simply heat the asteroid, while a beam that penetrates just the right amount - perhaps a thousandth of a millimeter - would produce a steerable jet. It all depends on better understanding the penetration depth of electromagnetic radiation (like light) in NEO regolith.
Leng says that if she hadn't joined the City Tech physics department, she might never have ventured into space research. "My past projects are all photonics, fiber optics and communication, so this is very exciting for me and my students. When they hear the word 'space,' wow! It's amazing. They are fascinated by the idea."
Scientists propose several ways to deflect Earth-bound asteroids and so far NASA has not settled on a preferred method, according to Robert B. Adams, who headed the team at the agency's Advanced Concepts office that studied asteroid-deflection methods in 2007. Matloff was their solar sail expert.
"The solar collector is definitely on the table," says Adams. So are ideas including a nuclear explosion away from the asteroid, a kinetic impactor that would ram into it and a gravity tractor, which would hover near the asteroid and use the gravity that naturally occurs between them to pull the asteroid slowly off its course.
"The solar sail hasn't received as much attention, but it's a good application with NEOs because it gives you more control over which way your thrust is generated," Adams says.
He added that it's probably wise to have a number of options available, because, under a 1988 congressional mandate, NASA is cataloging a dizzying number of asteroids and comets that are approaching Earth. Roughly 1,000 are wider than 1 kilometer (.6 mile); an estimated 21,000 are wider than 140 meters (459 feet), big enough to wipe out a coastline.
"We see a lot of asteroids after they've flown by," Adams says. "If they're coming from the inner solar system out, they're difficult to spot because they're coming from the sun. It's unnerving to see one fly by close to us, and we didn't know it existed."
For example, there was only a 21-hour warning before an SUV-size asteroid called 2008TC3 exploded in a 1-kiloton (1,000-ton) fireball high over Sudan's Nubian Desert on Oct. 7, 2008. Within an hour of discovery by astronomer Richard Kowalski at the NASA-sponsored Catalina Sky Survey, NASA's Jet Propulsion Laboratory had predicted the time and location of that "small impact event," one of several that occur each year.
NASA notified agencies ranging from the National Security Council to the Department of State, which alerted Sudan. By the time the asteroid entered the Earth's shadow 19 hours after discovery, 26 observatories worldwide had reported 570 positional measurements. It was the first test of NASA's NEO Program and planetwide cooperation in this area – and it was sheer luck that this asteroid didn't target a populated area, for there would have been scant time to evacuate.
Space resources may prove invaluable in combating climate change and restoring Earth's econological balance, according to a forthcoming book by Johnson and Matloff, Paradise Regained: The Regreening of Earth, which is illustrated by C Bangs.
One much-discussed idea is using kilometers-wide solar sails to collect solar radiation and beam it down to Earth in the form of microwaves. Another idea is positioning a huge sail - or trillions of two-foot-diameter sails - to cast a shadow on Earth, uniformly reducing sunlight across the planet by a fraction in order to negate the heat gain caused by greenhouse gases.
But for the moment, dreams of solar sailing remain just that.
The first serious effort to mount a solar sail mission arose in the 1970s, after Battelle Memorial Institute scientist Jerome Wright came up with what would have been a headline-grabber.
Knowing that Comet Halley - arguably history's most famous comet - would be streaking past Earth in 1986, Wright calculated a flight path for a solar sail-powered scientific mission. If launched in 1981, the craft could conduct a prolonged, fly-along study of the comet, he wrote. (The comet achieved rock-star status when it appeared in 1759, right on the schedule that English astronomer Edmund Halley had predicted in 1705. Through a close reading of history, Halley identified it as the first known periodic comet, meaning it flies by Earth regularly, in this case about every 75 years.)
Bruce Murray, then JPL's director, ordered an engineering study and pitched the idea to NASA management. In late 1976, the agency green-lighted design work. Murray turned to Louis Friedman to run the lab's Halley Comet Rendezvous-Solar Sail Project; four years later, they would join with astronomer Carl Sagan to form the nonprofit Planetary Society, which promotes space exploration.
Friedman, now the society's executive director, recalls that his team first considered a one piece, 800-by-800-meter (about a half-mile on a side) solar sail. Then they opted for a "heliogyro" design with eight blades, each 7.5 kilometers long. Like a helicopter's rotor, the blades would spin for control and stability. Despite its gargantuan size, the team reasoned that the heliogyro could be deployed more easily than the square sail by using naturally occurring centrifugal force to unroll the individual blades as the craft spun.
"To be honest we were overreaching at that time," Friedman says. "Halley was a once-in-a lifetime opportunity, so you wanted to go for it. We should have focused on step-by-step technological development as we're trying to do now. Had we achieved a solar sail flight - any flight - we would be using that technology today. It would have caught on and been a great asset in planning planetary missions, even doing sample-return from asteroids and comets."
In the end, NASA scrapped the heliogyro in favor of solar-electric propulsion, which provides thrust through magnetism and electricity generated by a spacecraft's solar panels. But rising cost estimates scuttled that system as well. Ultimately, NASA failed to send a mission to Halley, although Soviet, Japanese and European spacecraft did fly by the comet.
Even if the agency had stuck with the heliogyro, it never would have made the rendezvous because deployment required an operational space shuttle - and that program was way behind schedule. Although Enterprise, the prototype shuttle, rolled out in 1979 and proved that the stubby-winged craft could glide and land,
Columbia, the first operational vehicle to lift into space, didn't fly until 1981 - and Halley wasn't on its task list.
That pretty much ended NASA's interest in solar sails until early in this century, when it commissioned contractors to build two differing prototypes of 400-square-meter sails that would fit into a suitcase-sized box during launch. The two prototypes were tested in the world's largest space simulator, but never made it off the ground. When President George W. Bush directed NASA to concentrate on sending astronauts to Mars, the agency eliminated solar sails and many other science projects.
President Barack Obama is reviewing the agency's goals and the spotlight has been on manned missions. His U.S. Human Space Flight Plans Committee recently recommended that Mars should be the ultimate, but not the first, destination. Rather, the United States, with international and commercial partners, should either return to the moon or take a "flexible path" to points in the inner solar system.
Whether solar sails will become a priority is not known, but NASA is considering another test. In 2008, the agency cannibalized earlier large prototypes to build two versions of NanoSail-D (D for demonstration), a 100-square-foot sail weighing under 4 kilograms (8.8 pounds). It hired the SpaceX firm to launch it, but the vehicle failed to reach orbit. Now it's contemplating a 2010 launch for its twin. Although NanoSail-D would deploy in space, the intent was not to work as a true solar sail; rather, it would test using the sail as an atmospheric drag break to slow down a satellite as it re-enters the atmosphere after a mission.
Others are pursuing solar sail technology more aggressively.
The Japan Aerospace Exploration Agency, whose adventuresome projects get scant attention in the U.S. media, became the first to deploy thin-film solar sails in a 2004 suborbital flight. A second test followed two years later.
Meanwhile, in 2005 The Planetary Society used $3.5 million in private funds ($2 million from Cosmos Studios and the rest from space enthusiasts) to contract with Russia's Space Research Institute to build and launch Cosmos 1. It was designed to be the first solar sail to be deployed from Earth's surface. The New York Times Magazine called it one of the most innovative ideas of the year. Cosmos 1 had eight triangular blades, each 50 feet long, arranged like an umbrella, but each blade could pivot independently. The intent was modest: By proving that the pressure of photons would raise its orbit, it would clear the path for substantial missions. However, the military Volna rocket, launched from a Russian submarine in the Barents Sea, malfunctioned, and Cosmos 1 was lost.
"We won't do Cosmos 1 again," Friedman says. "But motivated by NanoSail-D, we've become enamored of a smaller and lower-cost craft. We're studying the possibilities and will raise private money to do that. This is the way to the stars."