Apophis Asteroid 99942

What Would It Be Like?

The impact of a 1 mile wide object

The impact of a 25 mile wide destroys the world!

Experts have been wary of asteroids since they came to the conclusion that one of them ended the Age of the Dinosaurs 65 million years ago. Scientists such as Stephen Hawking warn that their relatively close proximity presents grave dangers to humankind, a point of view supported in a number of recent books, such as William Burrows' The Survival Imperative: Using Space to Save Earth and British astronomer royal Martin Rees' Our Final Hour: A Scientist's Warning.

But others consider asteroids the next landscape for scientific discovery. "We're looking at the possibilities," says Kelly Humphries, a spokesman for NASA's Johnson Space Center. With NASA planning a moon-exploring spacecraft, Humphries says, "Anything robust enough to go to the moon is going to be robust enough for lots of missions."

In December, NASA astronaut Edward Lu told Space.com that plans under study include landing on an asteroid and retrieving rock samples for return to Earth before 2020.

And at NASA's Ames Research Center, lab chief Simon "Pete" Worden, a longtime advocate of such exploration, has set aside $10 million for designing small spacecraft that could visit asteroids, according to the Jan. 19 Science magazine.

The space agency does have a few asteroid missions already planned. In its just-released 2008 budget, NASA said it is studying a mission, dubbed the Origins Spectral Interpretation, Resource Identification and Security (OSIRIS) probe, to return rock samples from an asteroid.

In June, NASA will launch the Dawn mission to orbit the two largest asteroids, Ceres and Vesta, in the main asteroid belt between Mars and Jupiter.

And outside NASA, others also see asteroids' scientific potential.

"They are pristine in a way, vagabonds of the solar system, leftovers from the era of the formation of the planets," says American Museum of Natural History astrophysicist Neil deGrasse Tyson, host of PBS' NOVA scienceNOW, and author of the new book Death by Black Hole and Other Cosmic Quandaries. "And as for landings, they are low-hanging fruit, or low-hanging rocks, in this case, for space exploration."

Too close for comfort?

The International Astronomical Union has given identifying numbers to nearly 150,000 asteroids; about 5,000 are discovered every month. A mix of sand piles, dust balls, metal-rich rocks and burned-out comets, they mostly congregate in the asteroid belt between Mars and Jupiter.

Closer to home, NASA has, as of November, tracked 855 potentially dangerous Near-Earth asteroids. These pass within about 30 million miles of Earth, with a diameter of approximately 1 kilometer (.62 miles) or larger. Astronomers regard that size as the point at which impact with Earth would threaten civilization, says Richard Binzel of the Massachusetts Institute of Technology.

NASA operates a program, the Spaceguard Survey, to track this "cosmic shooting gallery," in the words of NASA scientist David Morrison, aiming to identify 90% of the estimated 1,100 civilization-buster Near-Earth asteroids lurking overhead before 2009. Congress has further told NASA to catalogue 90% of all Near-Earth asteroids less than 460 feet wide by 2020, and figure out ways to deflect any headed for Earth.

Tyson says such asteroids offer an intriguing array of midway points between the four-day trip to the moon and the six-month voyage to Mars.

"As steppingstones to Mars, (asteroids) are a really good way to learn to leave the comfort of the Earth-Moon system," says Binzel. "There are literally hundreds of Near-Earth asteroids that are probably easier to reach than the moon, in terms of the propellant you need to go there and back."

That's because asteroids have hardly any gravity. So fuel costs for blasting out of each one's "gravity well" are minimal. Eros, a hefty near-Earth asteroid, some 20 miles long by 8 miles across, has such light gravity that a person could toss a baseball off its surface and into orbit. In comparison, a rocket needs a 5,370 mph escape velocity to leave the moon.

And NASA's plans include building a rocket capable of sending astronauts to the moon, called Ares 1, which is scheduled to be ready for flight testing in 2014. The rocket designers aim to overcome the Earth's 39,600 mph escape velocity and deliver a 25-ton astronaut capsule to the moon, complete with the fuel needed to return. That capability should put a variety of asteroids within reach.

For something a bit sooner, Morrison will describe a Near-Earth Asteroid Trailblazing (NEAT) probe, low-cost landers designed to flit among nearby asteroids, scouting their surfaces, at a March American Institute of Aeronautics and Astronautics meeting.

"Landing on one would be more like docking with the international space station than a moon landing," says astronomer Daniel Durda of the Southwest Research Institute in Boulder, Colo. Astronauts would most likely "swim" over the surface of an asteroid, he says, so lightly do things fall on a typical one, which essentially has zero gravity.

A proposal to rein them in

Of course, there is also the impact threat to consider. In 1980, geologist Luis Alvarez suggested in the journal Science that a comet or asteroid impact ended the Age of Dinosaurs. Many researchers believe the impact landed off the Yucatan peninsula in the Gulf of Mexico, and fears that there will be another such mass-extinction event stock the cabinet of modern worries.

"From a practical point of view, some time in the future, one of these things is going to threaten Earth with an impact and we'll need to do something about it," Durda says. So why not visit one to get the hang of herding them? he asks.

The Harvard-Smithsonian Minor Planet Center predicts there will be more than 5,300 "close" asteroid encounters, within 18.4 million miles, by 2040.

One of the most interesting, Apophis, grabbed headlines three years ago because of the possibility that it would smack into Earth in 2036. Improved observations lower the odds to a 1 in 45,000 chance, Binzel says, "nothing to lose sleep over."

But the asteroid's close approach in 2029 to within 22,600 miles of Earth, closer than the moon, may offer an exploration opportunity.

In 2005, Lu and another astronaut, Stanley Love, proposed a "gravity tractor" design for deflecting Apophis and other asteroids from Earth. "Our suggested alternative is to have the spacecraft simply hover above the surface of the asteroid. The spacecraft tows it without physical attachment by using gravity as a towline," they wrote in the journal Nature.

Once in orbit and gravitationally bound to a dangerous asteroid, the space tractor would gently fire its thrusters to slowly "tug" the threatening rock onto a safer trajectory. Apophis, for example, would require a one-ton tug to orbit the asteroid for a month before its 2029 close pass by Earth to put it onto a safer path.

Are they hollow or solid?

One of the great uncertainties about asteroids is what they are made of, something that might make astronauts piloting robotic surveyors more likely than actual manned landings. Some, like Eros, appear to be fairly solid objects, based on their gravity, albeit intensely dust-covered ones. The slowly-rotating Mathilde, which the NEAR-Shoemaker probe flew past in 1997, appears three times less heavy than its size would indicate, suggesting it may be hollow. And the asteroid Itokawa is just a rubble pile, a surprise that explains the 2005 failure of Japan's Hayabusa probe to land there.

As Tyson says, asteroids are thought to mostly be leftovers from the era of planetary accretion 4.6 billion years ago in the solar system. Weathered by eons of orbits around the sun and impacts with other space rocks, they still offer clues to the ingredients of today's planets and moons. "Each asteroid is a piece in the puzzle of how the planets formed," Tyson says.

Broadly speaking, inhabitants of the main asteroid belt between Mars and Jupiter are thought to come in three flavors: dark carbon-rich "carbonaceous" ones that make up about 75% of the total, iron-rich "metallic" asteroids, and fairly bright "silicaceous" asteroids built from a mix of iron and sand.

But nobody knows for sure, Binzel says, which makes exploring asteroids an exciting prospect. A few are likely burned-out comets plying their retirement years in the placid depths of space. "Water or ice might be inside them," handy for space travelers, he says. "Others might have minerals that might be useful future resources."

(Space law still has a few wrinkles to iron out first though on mining asteroids, cautions Frans von der Dunk of the International Institute of Air and Space Law at Holland's Leiden University. The United Nations' Outer Space Treaty makes nations liable for mining companies and allows mining, he says, but stops short of defining property rights, making gold mines in space a legally dicey pursuit.)

"Asteroids have been a low priority for too long," says Burrows, The Survival Imperative author, who calls for long-term space colonies to serve as a refuge for humanity if there's a catastrophic collision. "People worry about terrorism, with good reason, but while it doesn't do to get over-excited, there are bigger threats."

Asteroid defense gets a hearing next month at an American Association for the Advancement of Science symposium in San Francisco. With new telescopes in Chile and Hawaii coming online, astronomers expect Near-Earth asteroids to turn up nearly 100 times more often than today's rate of discovery.

Asteroid scares may become more common, as a result, as presenters including Lu and Morrison will discuss, but the opportunities for exploration are expected to increase, as well.

"Hundreds of exciting and strange asteroids are nearby," Binzel says. "Certainly there is scientific interest."