Two hundred million miles from Earth, just beyond the orbit of Mars, an odd object about twice the size of Manhattan tumbles end over end, revolving around the sun at average speeds of more than 40,000 miles per hour. Shaped like a lumpy, pockmarked potato, with an enormous crater at its middle— where, 4 billion years ago, it was almost riven in two— asteroid 433 Eros demands our attention for two reasons. First, it is one of the oldest objects in the solar system and therefore holds many clues about the earliest days of Earth. Second, it has 800 or so cousins nearby, one of which could someday put out the lights on our home planet.
Orbiting 62 miles above Eros, the NEAR probe caught this image of a 6-mile-wide, saddlelike depression at the asteroid's center.Photo by NASA/Johns Hopkins University Applied Physics Laboratory
Five years ago, as Congress voted to spend a mere pittance to search for asteroids headed our way, NASA launched the Near Earth Asteroid Rendezvous mission. Its journey will end the second week in February. As you read this, the NEAR-Shoemaker probe is in a tight orbit around Eros, setting up for its final maneuver: a soft landing near the asteroid's south pole. No one is certain that a touchdown is possible. Indeed, if things go badly, the NEAR-Shoemaker probe will plow into Eros at about six miles per hour and be destroyed. But its cameras will be shooting photos all the way to ground zero.
Eros was born of material left over after the sun coalesced out of the solar nebula 4.5 billion years ago, and it carries inside it a trove of information about how the planets formed. The solar system began as a swirling cloud of gas and dust. Particles clumped and formed into chunks of rock. Eventually the chunks clumped into small planetesimals, which clumped into planets. As the planets grew in mass, the heat they generated melted their building blocks into their component minerals. The heavy metals, such as iron, nickel, and gold, gradually settled to the interiors and cores of the planets, while lighter ones rose toward the surfaces. This primordial matter no longer exists in its original form on Earth, or on any of the other planets. But asteroids never joined this planet-forming roundup and never reached a large size, so they never melted. They remain intact, unaltered— pristine versions of what Earth once was. By studying them, and the meteorites they have spawned, we lift a veil on our planet's earliest days.
Eros belongs to a population of rogue rocks that broke away from the main asteroid belt between Jupiter and Mars. Apart from the moon, these near-Earth asteroids are our closest galactic neighbors. Every one of them follows a unique, irregular orbit that brings it disconcertingly close to Earth. The asteroid believed to have caused the dinosaurs' extinction was just such an object. In this millennium, there is a 1 percent chance that another near-Earth asteroid will wallop Earth. The NEAR mission is valuable precisely because we are learning to do some tricky maneuvers: steering a spacecraft into orbit around a very small mass that has very little gravity, and then landing the craft on what is no more than a big rock. Eros is not thought to be much of a threat: The closest it has come to Earth is 14 million miles. But its relatively large size makes it a great place to practice getting near enough to an asteroid to blow it up before it gets a chance to collide with Earth.
And apparently, practice is needed. NEAR, designed and built by the Johns Hopkins University Applied Physics Laboratory, was originally scheduled to rendezvous with Eros on January 10, 1999. In preparation, its main thruster was set to fire for 15 minutes on December 20, 1998, to slow the craft down. Thirty-seven seconds into the burn, at 5:23:01 p.m. eastern standard time, contact was suddenly lost. "I went home that night thinking it was all over," remembers Andy Cheng, NEAR's chief scientist. But after 27 hours of silence, NEAR called home.
The mission team discovered that a software glitch had caused the trouble: The space probe's computer thought that the thruster had started up too vigorously and so shut itself down as a defensive measure. Because of the shutdown, NEAR began to tumble out of control. "Its solar panels weren't pointed at the sun, and its antenna wasn't pointed anywhere," Cheng says. After eight hours, backup guidance systems took over, and NEAR stabilized. Its solar panels turned toward the sun, and its batteries began to recharge.
NEAR's thrashing about, however, left it approximately 2,000 miles off course. The very next step, taken two days later, was to navigate NEAR back into position for a flyby of the asteroid. "We wanted to get pictures of this thing, even if they were bad pictures," remembers mission director Robert Farquhar.
The team rescheduled orbital insertion for February 14, 2000, more than a year behind schedule. This time, everything went as planned. NEAR cruised toward Eros at a leisurely 22 mph and slipped into orbit 207 miles above the asteroid's largest crater, Psyche.
In the year since, the spacecraft has been worked down to three miles above the surface, where it has cruised over the south and north poles and photographed every square foot of Eros's terrain. Previously, astronomers knew little more about Eros than that it had a highly reflective surface. NEAR's instruments have filled in the details: The ground is blanketed with a layer of fragmented rocks and soil as much as 300 feet deep, strewn with a million huge boulders as big as houses, and scarred by a hundred thousand grooves, ridges, and craters more than 50 feet wide.
Eros has been battered mercilessly by other asteroids for the last 4.5 billion years, and Cheng and his colleagues think that at some point early in its history it must have broken off from a much larger asteroid. Measurements from NEAR's laser range finder show that Eros is solid, with a uniform density— not, as once thought, a scattered pile of rubble held loosely together by gravity.
The reddish tint of rocks and soil in this false-color image of the crater Psyche shows where iron atoms were stirred up by micrometeorites and solar wind. Photo by NASA/Johns Hopkins University Applied Physics Laboratory
NEAR's X-ray and gamma-ray spectrometers show that Eros's chemical makeup is similar to that of the chondrites, the most common, primitive meteorites found on Earth. Eros, then, "is a touchstone," says NASA astrophysicist Jacob Trombka, who leads the NEAR spectrometry team. "We have chondritic meteorites on the ground, which we think reflect the primitive chemistry of the solar system." NEAR confirms that asteroids contain the materials— such as iron, magnesium, silicon, aluminum— that gave rise to all the planets.
Soon, NEAR's stream of data may stop for good. On February 12, the craft will begin a "controlled descent" onto Eros, starting from about 21 miles above the surface. Slowing to six miles per hour, the craft may "run out of fuel on the way down, because we'll be hovering for about an hour and thrusting the whole time," says Farquhar. "The main goal is to try to get as many high-resolution pictures as we can, at very low altitudes."
The maneuver is rather difficult— more good practice. Someday, Farquhar says, we may have more urgent reasons for such a stunt— like landing a bomb.
"If we pull this thing off," he says with a bit of swagger, "as far as NASA is concerned, we will have walked on water."
Hard RockFlybys
Eros is the only small object ever orbited by a man-made craft, but other probes have studied asteroids and comets:
• The International Cometary Explorer flew through the tail of Comet Giacobini-Zinner on September 11, 1985, at 47,000 miles per hour, taking measurements of charged particles and electromagnetic waves. The Explorer discovered water and carbon monoxide molecules in the tail, confirming that a comet is what Carl Sagan called a "dirty snowball in space." The probe then visited Halley's comet on March 28, 1986, passing between the sun and the comet at a distance of 17 million miles.
• Galileo, on its way to Jupiter, studied 951 Gaspra on October 29, 1991, from a distance of 1,000 miles. The rocky, egg-shaped asteroid was the first viewed at such close range by a spacecraft. Galileo then encountered 243 Ida on August 28, 1993, and discovered Dactyl, a tiny moon orbiting the 36-mile-long, 14-mile-wide asteroid.
• NEAR passed 753 miles from 253 Mathilde— a rock the size of Rhode Island— on June 27, 1997.
• Deep Space 1 flew just 16 miles from 9969 Braille, a near-Earth asteroid, on July 29, 1999, and discovered a magnetic field, proof that Braille was once part of a larger body with a core.— K.A.S.
When the Big OneComes
Early on November 3 last year, NASA and the International Astronomical Union announced a 1-in-500 chance that asteroid 2000 SG344 could strike Earth on September 21, 2030. The tiny asteroid, about 100 to 200 feet wide, would generate a blast equivalent to 750 times that of the bomb dropped over Hiroshima. Late that same day, astronomers calmed the rising concern after determining that 2000 SG344 would miss Earth by at least 3 million miles. Someday, however, we won't be so lucky.
If we see it coming, we'll have two options: push it off course, or blow it up. Time will be critical to making the choice. "Realistically, we'll probably have hundreds, or thousands, of years," says astronomer David Rabinowitz of Yale University, a coinvestigator on the Near Earth Asteroid Tracking project, which is scanning the skies to spot potentially dangerous asteroids. "In that case, a very small rocket striking the surface could do the trick. Asteroids have chaotic orbits that are very sensitive to initial conditions. If you change where they are now, they'll end up in a completely different place later on."
If the danger were more imminent— say, in 50 years— nuclear bombs could come in handy, with one caveat: Breaking a large asteroid into several large chunks might not be much better than leaving it untouched. Rabinowitz believes that "smashing an asteroid up into bits is a good idea if the bits are the size of gravel" and therefore small enough to burn up harmlessly in Earth's atmosphere. "If we had to do it now, we'd need nuclear weapons," he says. "But I don't like the idea of showering Earth with radioactive pieces of asteroid." A few hundred years into the future, figures Rabinowitz, "there will probably be less-radioactive ways to go about it."
One possibility— now just a concept— would be landing a "mass driver" on the asteroid. "You could have a device that would continuously 'eat up' the asteroid and throw the matter off the surface at a very high speed," Rabinowitz says. "For every action there is an equal and opposite reaction, so these successive 'thrusts' would gradually push the asteroid out of the way." — K.A.S.
NEAR has landed! For an update, see www.discover.com/mar_01/featlanded.html
Check out the Near Earth Asteroid Rendezvous Web site at near.jhuapl.edu.
NASA's Near Earth Object Program home page contains the latest NEAR news and links to NEO sites, including the NEO page at Harvard's Center for Astrophysics at cfa-www.harvard.edu/iau/NEO/TheNEOPage.html.
