Avoiding impacts (Our Solar System)

OUR SOLAR SYSTEM

Avoiding impacts

Our coverage of the 2004 Planetary Defense Conference continues. Day 3 brought discussions of orbital keyholes and "tractor beams."

by Bill Cooke
Source


Astronomers and policy makers are meeting this week in Garden Grove, California, to discuss what many might consider a far-out concept — protecting Earth from asteroid and comet impacts. At the 2004 Planetary Defense Conference, the first meeting on the topic sponsored by the American Institute of Aeronautics and Astronautics, presentations range from the need for better definitions of the types of objects likely to hit us to specific proposals for deflecting an inbound rock.

Posted February 24, 2004

Funding professionals and amateurs
Dana Rohrabacher, a Republican representative from California, delivered the conference keynote address on Monday, February 23. He began by comparing the likelihood of an asteroid strike to a good hand of poker, quoting a scientist who told him the chance of being dealt a royal straight flush in Las Vegas is the same as an asteroid striking Earth. "I have been dealt a royal straight flush in Vegas," Rohrabacher says.

Despite the likelihood of an impact, he says the U.S. government is spending a pittance to deal with the threat. Rohrabacher says he's satisfied with Spaceguard, the current international program to find 90 percent of all asteroids a half-mile across or larger by 2008. But, he stresses, "We need to build from here."

Rohrabacher has introduced two bills into Congress that will help extend our current detection capabilities. The George R. Brown Near-Earth Object Survey Act, named in honor of the late congressman, provides an additional $20 million in 2005 and 2006 specifically for the detection of Near-Earth Objects (NEOs) down to 328 feet (100 meters) in size.

Of more interest to amateur astronomers, the Charles "Pete" Conrad Astronomy Awards Act, named for the third man to walk on the Moon, would set aside $10,000 in both 2004 and 2005 for awards to amateur discoveries related to NEOs.

"We need to get people looking up," Rohrabacher says. "That's what the Pete Conrad bill is about."

Understanding the threat
Talks on the first day of the conference focused on defining risks and sharing what astronomers know about NEOs. David Morrison (NASA Ames Research Center) puts the risk of death from an impact at about 1 in 50,000 over a human lifetime, or in the same range as other natural hazards. An object half a mile across packs a one-million-megaton punch, enough to threaten the globe, but objects this large are rare. "Tunguska-class" objects, that is, those measuring about 195 feet (60m) across, strike Earth far more frequently. Indeed, the class itself refers to an object that exploded above the Siberian taiga in 1908 with an estimated force of 10 megatons or more. But Morrison estimates extending the search to objects even in the 328-foot (100m) range will require another $400 million.

Don Yeomans of the Jet Propulsion Laboratory (JPL) in Pasadena, California, pointed out that a Tunguska-class object passes within the distance from Earth to the Moon every three months. He equates asteroids to dark travelers on a highway with no stop signs and summed up the primary needs in asteroid detection as: "One, need to find them early; two, need to find them early; and three, need to find them early."

One of the biggest problems in planetary defense is that all asteroids are not created equal. NEOs are "uniformly unique," with structures ranging from very solid, rocky bodies to loose agglomerations called "rubble piles."

Steve Ostro (JPL) reported that radar studies of NEOs allow for a more precise determination of orbits than current methods do and that such studies provide, on average, a warning time of several centuries. Radar is limited, however, to objects that pass relatively close to Earth. Unfortunately, all but the smallest objects could be rubble piles. Such structures reduce the effectiveness of a missile strike or nuclear explosion, the solutions most loved by screenwriters, by a factor of a hundred. In the words of Keith Holsapple (University of Washington), "it's like punching a pillow."

Every six minutes, our planet moves its own diameter along its orbit. Any successful mitigation strategy must change the arrival time of a potential impactor by this amount. A talk by Paul Chodas and Steve Chesley, both from JPL, discussed how the impact risk of an inbound comet or asteroid changes as astronomers observe it over time. Early detection doesn't help much for a comet on a collision course, since jets of gas and dust begin to affect its position as it nears the Sun. Even with asteroids, they note, the probability of an impact must reach nearly 100 percent before astronomers can firmly establish where on Earth it will strike.

Clark Chapman of the Southwest Research Institute gave perhaps the conference's most eagerly anticipated talk. He related last month's bizarre sequence of events, between January 12 and 14, involving an object now designated 2004 AS1. An amateur brought the object, then known by its provisional identification of AL00667, to the attention of professional astronomers through an Internet message board, setting off a ten-hour crisis that rippled through the NEO community. A small set of relatively poor-quality observations gave the impression that an impact was imminent within days. The tale ended only when another amateur astronomer failed to find the inbound object at a position predicted by its apparent collision course.

The story highlights flaws in the current system — flaws these astronomers clearly want to see fixed — and underscores the importance of amateurs in identifying possible threats. For the first time, despite differences of opinion, serious talks about what risk level requires some sort of positive response, even if it's just notifying someone outside the NEO community, are taking place.


Posted February 25, 2004

Nuke it or tow it?
On the second day of the conference, discussions began in earnest on mitigating the impact threat posed by NEOs with the method preferred by Hollywood and some in the military: the "fast physics package," or nuclear weapon. At first glance, this seems to be the simplest and most straightforward way of deflecting a big object heading toward Earth. Two presenters, Vadim Simonenko of the Russian Center for Nuclear Research and Dave Dearborn of Lawrence Livermore National Laboratory, tried mightily — and at times very humorously — to make the case.

Increasing evidence suggests most asteroids have formed not as solid rock but as rather loose assemblages of rubble. Astronomers continue to debate the true nature of the rubble, but the fact remains that a loose collection of small rocks would act as sandbags to a bullet, effectively dissipating most of the explosion energy. Only a small portion of energy, therefore, would be available for changing an asteroid's trajectory. Nuclear weapons, however, would work well against rocky bodies.

Other presentations centered on using spacecraft to intercept and alter an asteroid's trajectory, the method preferred by many conference attendees and advocated by the B612 Foundation, a group established to demonstrate that current technology is capable of relocating an asteroid. The organization's goal is "to significantly alter the orbit of an asteroid in a controlled manner by 2015."

Talks given by astronaut Ed Lu of the NASA Johnson Space Center and former Apollo astronaut Rusty Schweickart outlined a simple concept. Their plan would use a low-thrust, nuclear-electric-powered spacecraft similar to that envisioned for the Jupiter Icy Moon Orbiter to tow or push an asteroid into a non-threatening orbit. Changing the asteroid's speed by just half-an-inch per second would, in ten years' time, divert it from a collision with Earth.

But the devil is in the details. A rubble-pile asteroid may not be strong enough to hold a tow cable, forcing the spacecraft to deploy something like a big claw to grapple onto a large portion of the object. The "tugboat" spacecraft must attach itself near the rotational pole of the asteroid and then change the asteroid's spin axis so it aligns with the rocket's thrust — not an easy task. For this to work as a defense, the plan requires detection of a colliding asteroid 15 or more years before it would hit.

Daniel Sheeres of the University of Michigan, another member of the B612 Foundation, closed the session with two presentations about how the weird shapes and extremely low gravity of NEOs can result in very funky orbits that are not intuitive. For example, the Moon is a relatively massive and roughly spherical body; and the more thrust a rocket engine produces, the longer the orbit it achieves. But this is not true of some asteroids, like Itokawa, where a weak thrust enables a vehicle to stay off the surface much better than a stronger thrust would. One short "hop" of thrust on Itokawa would keep a spacecraft off the surface for six months. This is very strange, to say the least, and a real hurdle for anyone trying to keep a spacecraft in a stable orbit around one of these objects.


Posted February 26, 2004

Keyholes, tractor beams, and panic
The third day of the conference began where the second left off, with more presentations about deflection techniques and spacecraft missions designed to employ them.

Al Harris of the Space Sciences Institute started the morning off with a simplified summary of equations that dictate how much an asteroid's velocity must change to divert it from a collision course with Earth. The bottom line is that other than sending everybody to caves until the "nuclear winter" has passed, there is not much we can do to mitigate an asteroid discovered months or even a couple years before impact.

The situation improves dramatically if we discover a threatening object ten or more years out. With this amount of lead time, we can choose either to deflect or disrupt the asteroid. Disruption scenarios have one major drawback: they turn a single colliding asteroid into a cloud of smaller fragments that will strike Earth. But if we break up an asteroid a decade or more before it's due to hit, any fragments will have dispersed well before the debris reaches us.

One interesting deflection concept takes advantage of an orbital "keyhole," a point in an asteroid's path where it receives its greatest boost from a planet's gravity field. A slight nudge to an asteroid approaching a keyhole can cause it to miss the point. This would reduce the gravity assist it receives from the planet and dramatically alter its trajectory. Celestial mechanics is a wonderful thing.

Mark Barrera of Aerospace Corporation outlined a mission design using the "all-out" nuclear option. His proposal was pretty detailed, with launch windows and schedules, use of multiple interceptors, and even a straw-man design of the kill vehicles. It was an excellent illustration of the thought and attention to detail the presenters have put into their talks at this conference.

Dan Durda of the Southwest Research Institute presented more details of the B612 mission. As discussed above, the B612 concept calls for a towline or grapple to tug the asteroid along, but no one was convinced rubble-pile asteroids are strong enough to hold up under even this gentle strain. Over dinner last night, however, astronaut Ed Lu proposed a solution to this issue - and I must say it is a most unconventional one.

The newly proposed plan calls for the B612 spacecraft to be about the length of its 100-yard-wide target asteroid and weigh about 20 tons. This relative equality in size would make it possible for the vehicle to hover 10 to 20 yards above the asteroid's surface and use its gravitational pull on the asteroid to gradually increase the speed. No wires, no grapples, and best of all, no need to worry about the composition of the asteroid. We're calling this the "Lu Gravitational Tractor Beam."

The afternoon consisted of talks dealing with human factors involved in planetary defense, particularly public reactions to an impending strike. A presentation by Al Harrison of the University of California, Davis laid to rest the popular notion that the public might panic in the event of an asteroid strike. His accounts of the heroic actions by persons on the street during the 9/11 attacks and in plane crashes and other disasters convinced the audience there's little chance of folks going "bonkers" before or after a Tunguska event. People pull together when the chips are down, and that is a great thing to be able to say about our civilization.


Bill Cooke is an astronomer with NASA's Marshall Space Flight Center in Huntsville, Alabama.