Will We Be Able to Deflect an Earthbound Asteroid?
Natalie Wolchover, Life's Little Mysteries Staff Writer
Date: 16 August 2011 Time: 12:19 PM ET
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| Artist's concept of a catastrophic asteroid impact with the early Earth. An impact with a 500 kilometer (310 mile) diameter asteroid would effectively sterilize the planet. The Earth may have experienced such gigantic impacts in its youth, but fortunately today there are no projectiles this large to threaten our planet. CREDIT: Don Davis/NASA |
For any single human being, there are bigger things to worry about than death by space rock. For the long-term survival of humankind, on the other hand, asteroids pose a real danger.
A 6-mile-wide asteroid that struck off the coast of present-day Mexico 65 million years ago induced ecological changes that wiped out the dinosaurs. Inevitably, an Earth-shaking chunk of space debris will strike again.
However, so many earthly worries exist that a cosmic one which, at any given moment, is infinitesimally small doesn't garner much attention — or government funding. Several scientists who study asteroid hazards agree: Humankind probably won't start readying its planetary defenses until we know the danger is real. We'll need evidence that a large asteroid is actually headed here. [When Space Attacks: 6 Craziest Meteor Impacts on Earth]
It depends. "Human beings can solve any technical problems that are put in front of us," said Daniel Durda, senior planetary scientist at the Southwest Research Institute in Boulder, Colo., and an expert on asteroid collisions. "It's the social and political issues that we struggle with." Rusty Schweickart, former NASA astronaut and founding member of the B612 Foundation, a nonprofit organization dedicated to protecting the Earth from asteroid strikes, concurred: "The geopolitical realities are daunting. The technical issues are easy by comparison."
The asteroid Apophis was discovered on June 19, 2004. It will fly within 18,300 miles of Earth on April 13, 2029, but poses little risk of impact.
CREDIT: UH/IA
CREDIT: UH/IA
The main culpritsThough we know very little about the composition of asteroids — necessary information for determining their masses, and knowing how to knock them off course — we do know where most of the large, nearby rocks actually are. NASA's Spaceguard Survey tracks the paths of all near-Earth asteroids (NEAs) in Earth's neighborhood that are larger than 1 kilometer (0.6 miles) in diameter. [The 7 Strangest Asteroids In The Solar System]
Knowing an asteroid's location is step one in determining if it's on a crash course for Earth, and fortunately, none of the big ones pose a threat at this time. If one of these known asteroids were found to have a greater than 1 percent chance of striking Earth, astronomers would figure it out at least a decade beforehand.
"A large impact of something 1 km across — that's a bad enough scenario that it would motivate people to take this seriously," Durda said. According to Clark Chapman, another senior scientist at the Southwest Research Institute, the international community would probably band together and plan a mission to divert the path of the asteroid. "This would probably require matching its orbit with a series of spacecraft equipped with bombs," Chapman said. Getting the deflection mission off the ground would cost on the order of $10 billion.
Though we don't have all the technology worked out yet, we do know how to build nuclear devices, and we have already successfully sent spacecraft to and from asteroids. "A decade out, given the technology it would take to do a deflection, I think we could respond in time," Durda told Life's Little Mysteries, a sister site to SPACE.com.
David Morrison, interim director of the NASA Lunar Science Institute and an expert on impact hazards, thinks we might need more time. He believes it would take two missions to deflect an asteroid. The first would rendezvous with the target asteroid and figure out what it's made of, the second, which would be specially tailored to the asteroid based on the first mission, to knock it off course. "One decade would be the minimum, but that would mean sending the deflection mission before we had characterized the target, which would be a bit risky. Twenty years is more realistic as a minimum," Morrison wrote in an email.
Schweickart says 15 years of warning of a coming impact would be a safe bet. Along with the time it takes to assemble a launch vehicle, launch, fly to and rendezvous with an asteroid, you also need "time enough for the deflection itself to accumulate enough change in the [asteroid] orbit for it to miss the Earth impact. Post deflection will require anywhere from say 3 to 10 years for the orbit change."
Smaller targets
There are dangers involved with asteroids under the 1-kilometer threshold, however.
"Anything more than 300 to 400 meters (983-1,312 feet) in diameter can cause continental-scale damage," said Stephen Wolters, a research fellow at Caltech who studies near-Earth asteroids at NASA's Jet Propulsion Laboratory.
NASA has begun locating and tracking smaller asteroids, but there are thousands still at large. If one of these were on course to strike Earth, sky surveys would give us no more than a month of warning. In that case, said Morrison, deflection is probably not an option. Instead, "you respond more like the way we handle hurricane warnings," evacuating people from the area most likely to be struck.
Eventually we will track all asteroids larger than about 200 meters (656 feet) wide. The day scientists realize that one of these mid-size rocks has a chance of hitting us, and that we have enough time to do something about it, serious political strife will likely ensue, the experts say. [Will Asteroid Apophis Hit Earth in 2036? NASA Rejects Russian Report]
Uncertainties abound
"If we had a decade of warning on a smaller-scale asteroid, 200 meters across, I think that's down in the area where we would be much more prone to arguing about possibilities and we wouldn't actually respond to it," Durda said.
Such an asteroid collision probably wouldn't cause a global catastrophe, just a national or continental one, he explained. This means that perhaps not every country would care to pitch in to deflect it, and others might be opposed to any action if it puts them at greater risk.
"The big challenge will be the international implications of where it will occur, what the range is — usually orbital uncertainties mean there's a track across the Earth where it's most likely to hit," he said. "Then, if you decide to deflect the asteroid, where does that track move across the Earth's surface before movingoff the surface? Now you're taking an act of God and turning it into an act of litigation, where you've moved the impact point potentially into countries that had been safe."
He continued, "It's these very issues that will lead to all kinds of discussion and argument and inherent delay in taking any physical action."
Given orbital uncertainties, another issue is how probable an impact must be before we decide to take action. "The world economies cannot afford to protect against all low-probability hazards," Chapman said. A 1-in-1,000 chance of a collision, for example, will probably be ignored, and according to Schweickart, they already are. "There are already some near-Earth objects with impact probabilities greater than 1-in-1000 and no one is the least excited," he wrote in an email. A 130-meter-wide NEA called 2009FD, for example, has a 1-in-435 chance of impacting Earth in the year 2185.
"For [the case] of a 500-meter-wide near-Earth asteroid, at a probability of more than 1 percent, I think we would take the threat very seriously," Morrison wrote. "Below that I don't know."[The Greatest Mysteries of the Asteroid Belt]
Out of left field
There's one form of cosmic debris that no amount of international cooperation can do much about.
"There's always a possibility of a long-period comet coming from nowhere and giving us almost no warning," Wolters said. Not only do such bodies come from the outer reaches of the solar system, where we can't see them, "these comets can come from high inclination orbits where we're not looking for things. You might only have a few months notice."
Fortunately, experts estimate that long-period comets comprise only about 1 percent of all the space rocks that pass by Earth. "The good news is that they're only a tiny fraction of the overall hazard," Durda said
http://www.space.com/12645-asteroid-deflection-doomsday-earth-capability.html
http://www.space.com/12645-asteroid-deflection-doomsday-earth-capability.html
Asteroid-impact avoidance
From Wikipedia, the free encyclopedia
Asteroid mitigation strategies are "planetary defense" methods by which near-Earth objects could be diverted,
preventing potentially catastrophic impact events. A sufficiently large impact would cause massive tsunamis or
(by placing large quantities of dust into the stratosphere, blocking sunlight) an impact winter, or both. A collision
between the Earth and a ~10 km object 65 million years ago is believed to have produced the Chicxulub Crater
and the Cretaceous–Tertiary extinction event.
While the chances of such an event are no greater now than at any other time in history, there is a very high
chance that one will happen eventually, and recent astronomical events (such as Shoemaker-Levy 9) have drawn
attention to such a threat, and advances in technology have opened up new options to prevent them.
Detection efforts
[edit]What do we need to know?
Almost any deflection effort requires years of warning, allowing time to build a slow-pusher or explosive device to deflect the object.
An impact by a 10 km asteroid on the Earth is widely viewed as an extinction-level event, likely to cause catastrophic damage to the biosphere. Depending on speed, objects as small as 100 m in diameter are historically extremely destructive. There is also the threat from comets coming into the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months.[1]
....
U.S. Representative George E. Brown, Jr. (D-CA) was quoted as voicing his support for planetary defense projects in Air & Space Power Chronicles, saying "If some day in the future we discover well in advance that an asteroid that is big enough to cause a mass extinction is going to hit the Earth, and then we alter the course of that asteroid so that it does not hit us, it will be one of the most important accomplishments in all of human history."[edit]
Because of Congressman Brown's long-standing commitment to planetary defense, a U.S. House of Representatives' bill, H.R. 1022, was named in his honor: The George E. Brown, Jr. Near-Earth Object Survey Act. This bill "to provide for a Near-Earth Object Survey program to detect, track, catalogue, and characterize certain near-Earth asteroids and comets" was introduced in March 2005 by Rep. Dana Rohrabacher (R-CA).[4] It was eventually rolled into S.1281, the NASA Authorization Act of 2005, passed by Congress on December 22, 2005, subsequently signed by the President, and stating in part:
The U.S. Congress has declared that the general welfare and security of the United States require that the unique competence of NASA be directed to detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to the Earth. The NASA Administrator shall plan, develop, and implement a Near-Earth Object Survey program to detect, track, catalogue, and characterize the physical characteristics of near- Earth objects equal to or greater than 140 meters in diameter in order to assess the threat of such near-Earth objects to the Earth. It shall be the goal of the Survey program to achieve 90% completion of its near-Earth object catalogue (based on statistically predicted populations of near-Earth objects) within 15 years after the date of enactment of this Act. The NASA Administrator shall transmit to Congress not later than 1 year after the date of enactment of this Act an initial report that provides the following: (A) An analysis of possible alternatives that NASA may employ to carry out the Survey program, including ground-based and space-based alternatives with technical descriptions. (B) A recommended option and proposed budget to carry out the Survey program pursuant to the recommended option. (C) Analysis of possible alternatives that NASA could employ to divert an object on a likely collision course with Earth. The result of this directive was a report presented to Congress in early March 2007. This was an Analysis of Alternatives (AoA) study led by NASA's Program Analysis and Evaluation (PA&E) office with support from outside consultants, the Aerospace Corporation, NASA Langley Research Center (LaRC), and SAIC (amongst others).
Collision avoidance strategies
[edit]Nuclear weapons
Detonating a nuclear explosion above the surface (or on the surface or beneath it) of an NEO would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. This is a form of nuclear pulse propulsion. Even if not completely vaporized, the resulting reduction of mass from the blast combined with the radiation blast and rocket exhaust effect from ejecta could produce positive results.
Another proposed solution is to detonate a series of smaller nuclear bombs alongside the asteroid, far enough away as not to fracture the object. Providing this was done far enough in advance, the relatively small forces from any number of nuclear blasts could be enough to alter the object's trajectory enough to avoid an impact. The 1964 book Islands in Space, calculates that the nuclear megatonnage necessary for several deflection scenarios exists.[15] In 1967, graduate students under Professor Paul Sandorff at the Massachusetts Institute of Technology designed a system using rockets and nuclear explosions to prevent a hypothetical impact on Earth by the asteroid 1566 Icarus. This design study was later published as Project Icarus[16][17][18] which served as the inspiration for the 1979 film Meteor.[18][19][20]
[edit]Kinetic impact
The hurling of a massive object at the NEO, such as a spacecraft or another near-Earth object, is another violent possibility. A small asteroid or large mass in a stable high-Earth orbit would have tremendous kinetic energy stored up. With the addition of some thrust from mounted rockets (plasma or otherwise), it could be used like a stone from a slingshot to deflect the incoming threat.
An alternative means of deflecting an asteroid is to attempt to directly alter its momentum by sending a spacecraft to collide with the asteroid.
The European Space Agency is already studying the preliminary design of a space mission able to demonstrate this futuristic technology. The mission, named Don Quijote, is the first real asteroid deflection mission ever designed.
In the case of 99942 Apophis it has been demonstrated by ESA's Advanced Concepts Team that deflection could be achieved by sending a simple spacecraft weighing less than one ton to impact against the asteroid. During a trade-off study one of the leading researchers argued that a strategy called 'kinetic impactor deflection' was more efficient than others.
[edit]Asteroid gravitational tractor
Main article: Gravitational tractor
One more alternative to explosive deflection is to move the asteroid slowly over a time. Tiny constant thrust accumulates to deviate an object sufficiently from its predicted course. Edward T. Lu and Stanley G. Lovehave proposed using a large heavy unmanned spacecraft hovering over an asteroid to gravitationally pull the latter into a non-threatening orbit. The spacecraft and the asteroid mutually attract one another. If the spacecraft counters the force towards the asteroid by, e.g., an ion thruster, the net effect is that the asteroid is accelerated towards the spacecraft and thus slightly deflected from its orbit. While slow, this method has the advantage of working irrespective of the asteroid composition or spin rate – rubble pile asteroids would be difficult or impossible to deflect by means of nuclear detonations while a pushing device would be hard or inefficient to mount on a fast rotating asteroid. A gravity tractor would likely have to spend several years beside the asteroid to be effective.
[edit]Ion beam shepherd
Main article: Ion Beam Shepherd
Another "contactless" asteroid deflection technique has been recently proposed by C.Bombardelli and J.Peláez from the Technical University of Madrid. The method involves the use of a low divergence ion thruster pointed at the asteroid from a nearby hovering spacecraft. The momentum transmitted by the ions reaching the asteroid surface produces a slow but continuous force that can deflect the asteroid in a similar way as done by the gravity tractor but with a lighter spacecraft.
[edit]Use of focused solar energy
H. Jay Melosh proposed to deflect an asteroid or comet by focusing solar energy onto its surface to create thrust from the resulting vaporization of material, or to amplify the Yarkovsky effect. Over a span of months or years enough solar radiation can be directed onto the object to deflect it.
This method would first require the construction of a space station with a system of gigantic lens and magnifying glasses near the Earth. Then the station would be transported toward the Sun.
[edit]Mass driver
A mass driver is an (automated) system on the asteroid to eject material into space thus giving the object a slow steady push and decreasing its mass. A mass driver is designed to work as a very low specific impulse system, which in general uses a lot of propellant, but very little power.
The idea is that when using local material as propellant, the amount of propellant is not as important as the amount of power, which is likely to be limited.
Another possibility is to use a mass driver on the moon aimed at the NEO to take advantage of the moon's orbital velocity and inexhaustible supply of "rock bullets".
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