Eyeballing an Asteroid

An ambitious attempt to place the first satellite into orbit around an asteroid suffered a major setback Sunday evening when the spacecraft’s rockets failed to fire to begin the complex maneuvers that would have put the craft in orbit on Jan. 10.

Mission controllers at the Johns Hopkins University Applied Physics Laboratory lost contact with the spacecraft for 27 hours before finally locating it with the help of NASA’s Deep Space Network antenna system.

The team said Tuesday that the Near Earth Asteroid Rendezvous spacecraft, or NEAR, was fully functional, but that they will not attempt to fire the rockets again until they are sure what happened Sunday.

Meanwhile, NEAR’s target, asteroid 433 Eros, sped past the spacecraft at 10:43 a.m. Wednesday at a distance of about 2,500 miles. The Johns Hopkins team is hoping to collect images of Eros, looking for tiny moonlets that might be orbiting it, and perhaps even making a color movie of its rotation as the 25-mile-long asteroid whips past. Those images should be made public today or Friday.

But the actual rendezvous will probably not occur until May 2000.

NEAR was designed to conduct more than a year of imaging studies that scientists expect will help explain how the solar system was created. More practically, the mission should help scientists prepare for the unlikely possibility that a similar asteroid might someday be found on a collision course with Earth.

Knowledge gained from studying Eros could teach us how to divert such a potential Earth killer. An asteroid only 6 miles wide is thought to have hit the Earth 63 million years ago, leading to the extinction of dinosaurs.

“It’s simply prudent to understand these objects in case someday we find one with our name on it,” said Carl Pilcher, NASA’s science director for solar system exploration.

The main goal, however, is to learn more about asteroids. The objects were created during the early stages of the solar system 15 billion years ago and have remained virtually unchanged ever since, providing a window into what conditions must have been like when the Earth was born.

“What we know about asteroids is very limited,” said mission manager Robert W. Farquhar of the Johns Hopkins laboratory in Laurel, Md. “It’s come from ground-based observations and quick fly-bys. But now, for the first time, we’re going to go into orbit around an asteroid and study it intensely for a year. We expect to get astounding information.”

Added project scientist Andrew Chang: “We can’t wait to get there and see what [the asteroid] will tell us.” But they apparently will have to wait a while longer.

Launched Feb. 17, 1996, NEAR is the first of seven missions to asteroids and comets that will continue through 2002. A relatively inexpensive Delta-2 rocket pushed it into an orbit around the sun that brought it back by Earth last Jan. 23 for a gravity boost that will let it intercept Eros.

A direct flight of the 1,775-pound craft to the asteroid would have required a much more expensive launch vehicle.

A year and a half after it was launched, NEAR flew within 753 miles of 253 Mathilde, the closest approach yet to an asteroid.

Pictures and other measurements made during the fly-by revealed that Mathilde was much less dense than scientists had thought, indicating that it is essentially a pile of closely knit rubble rather than a solid rock. Its density is a little more than the 1.3 grams per cubic centimeter of water, compared to the 3 to 4 grams per cc of solid rock. “If it were in the ocean, it would almost float,” Pilcher said.

Pictures also indicated Mathilde has large craters--suggesting an impact that should have smashed it to smithereens.

“Yet there it is, unshattered,” Pilcher said. “We need to learn how it dissipated the energy,” especially since scientists would probably try to shatter any asteroid nearing Earth.

On its Earth fly-by, NEAR swung within 335 miles of our planet.

At the beginning of this month, NEAR was in Eros’ orbit--well ahead of the asteroid, but with Eros catching up rapidly. The “burn,” or rocket firing, scheduled for Sunday would have speeded it up so that the asteroid would be gaining on it at only 750 mph. Later burns would have reduced the relative speed to 19 mph, allowing NEAR to be captured by Eros’ weak gravity.

Orbiting Eros Will Be Difficult

Maintaining an orbit once it is achieved will not be easy, because Eros, first discovered in 1898 by German astronomer Gustav Witt, is not round. It is shaped more like a brick--about 25.3 miles long, 9.1 miles high and 8.8 miles wide. It rotates every 5.27 hours.

Eros is a barren place with no atmosphere and no evidence of water. In its brief daytime, its surface is 212 degrees Fahrenheit. At night, it hits minus 238 degrees.

Its gravity is very weak. A 100-pound object on Earth would weigh an ounce on Eros. A rock thrown at 22 mph (major league pitchers hurl balls at 90-plus mph) would easily escape its gravitational pull.

Maintaining a stable orbit under such conditions is difficult. Were NEAR to enter a normal orbit in the same direction the asteroid revolves, the orbit would quickly become unstable, causing the craft to escape into space or crash into the surface.

Mission planners will seek to avoid this problem by inserting NEAR into a more stable counterclockwise orbit while the asteroid spins clockwise. Even so, thrusters on the craft will have to adjust the orbit every seven to 10 days to keep it stable.

But all of those plans were postponed Sunday when NEAR’s rocket engines shut down one second into a planned 18-minute burn and the spacecraft itself shut down, going into a preplanned “safe” mode that protects its electronic circuitry. The spacecraft had gone into a safe mode once before and was quickly rescued.

This time, controllers didn’t know how long the rockets had fired, and thus took a while to locate the spacecraft.

Mission planners are confident that NEAR has more than enough fuel to allow them to catch up with Eros once they have diagnosed its problems.

They could conceivably have fired the engines Wednesday and returned to the original schedule. But because the small team responsible for NEAR’s flight was exhausted by the task of recapturing it, they chose the other approach.

The mission can be followed at the Johns Hopkins lab Web site: https://near.jhuapl.edu

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A NEAR Miss

NASA scientists had hoped to achieve a first next month by placing a spacecraft in orbit around an asteroid, but a malfunctioning rocket engine has temporarily thwarted them. The Near Earth Asteroid Rendezvous spacecraft swept by asteroid 433 Eros on Wednesday, but it may not achieve an orbit around it until May 2000.

NEAR Spacecraft Configuration

NEAR was launched in 1996 by a Delta II rocket from Cape Canaveral, Fla. Powered by solar panels, it will orbit the asteroid for up to one year, studying its weight, size, shape, mass and magnetic field.

* Solar panels:

Four 1.8- by 1.2-meter gallium arsenide panels will produce up to 400 watts at NEAR’s farthest distance from the sun. Power is stored in a rechargeable nickel-cadmium battery.

* Scientific imaging equipment:

NEAR is equipped with an X-ray/gamma ray spectrometer, a near infrared imaging spectrograph, a multi-spectral camera with CCD imaging detector, a laser rangefinder and a magnetometer. Most electronics are mounted inside the craft.

Arranging the Rendezvous

NEAR was launched into a solar orbit that allowed it to sweep by Earth last January, gaining a gravitational boost to place it in the same solar orbit as Eros.

1. Satellite is launched

2. Satelitte swings by the asteroid Mathilde

3. Satellite swings by Earth for a gravitational boost.

4. Satellite passes Eros

*

EARTH: After a 2-year gravity assist, the saatellite passed by Eros on Wednesday.

Source and Images: Applied Physics Laboratory.

Researched by: Times staff.