Though he has rejected the creationist views of his youth, he believes in the wholeness of the universe. Asteroids, says Gehrels, were created when gases and dust particles were compacted by the gravity of the sun. Some of these dustballs became big enough to generate substantial gravity of their own, drawing in more asteroids and eventually forming the planets. The atoms that make up his body, Gehrels says, were part of the asteroids that bonded together to form Earth 5 billion years ago. He believes an asteroid is the closest humans can come to their own origin.
Gehrels, Rabinowitz and Scotti work the darkest 18 days of the month, each taking a grueling six-night shift.
Here is the scene on the fifth afternoon of a recent shift: Gehrels emerges from his nap in the small dormitory on the mountaintop. He is bearded, skinny, with a heavily lined face; he wears a blue overcoat, brown pants tucked into his socks and scuffed brown wingtips with holes in the bottom, as if he has hiked up and down the mountain too many times. The first thing the astronomer does is look up.
Gehrels walks to the Steward Observatory, a cinder block building shaped like a can of shaving cream. He clips up two flights of stairs to Spacewatch, which could pass for a giant water heater.
Gehrels` crew has attached the world`s largest charge couple device
(CCD)-a piece of electronic equipment similar to that found in many home video cameras-to their telescope, enabling them to find Earth killers 200 million miles away. This CCD picks up images a million times as faint as those the eye can see and relays them to a computer.
Gehrels heads down one flight of stairs, into a dark, windowless room with tile floors, a fuse box on the wall, water pipes running overhead. It feels like a basement. It is mission control.
From his computer screen, Gehrels can see the heavens drifting by:
brilliant stars, elliptical galaxies, splashy star clusters. ``There is a beautiful galaxy,`` he says. ``And another one.``
To find asteroids, Gehrels programs the telescope to sweep the same square of sky three times.
Stars and galaxies-so large and far away-don`t move at all between sweeps. Asteroids, however, are much closer, and the computer picks up their movement relative to the stars. Most asteroids orbit the sun in the main asteroid belt, between Mars and Jupiter. But some are pulled into elliptical orbits because of Jupiter`s immense gravity, bringing them across Earth`s path.
The movement of these near Earth asteroids is the most dramatic from sweep to sweep. The computer is programmed to highlight them.
The odds of discovering an Earth approaching asteroid on any given night are slim. Each sweep of Spacewatch covers only a narrow sliver of sky-equivalent to 15 full moons side by side. In a month, Gehrels and his crew can cover only 1/3000th of the entire sky.
(On nights when the moon is full, and the sky is too bright to search for dim asteroids, another of Gehrels` colleagues, Robert McMillan, uses Spacewatch to hunt for planets around other stars. No luck on that front, either.)
Rabinowitz was on duty last year when the almost 10-yard asteroid swept between the moon and Earth-three days into the Gulf War. ``On TV, missiles are flying,`` he recalls. ```What the hell`s this? It can`t be a missile. But what is this flying through the sky?` I had a sense that night of being bombarded.``
Strange things do happen on the mountaintop.
Some nights, usually near the end of his shift, Gehrels hallucinates.
``I imagine all these maidens, running up the mountain, tearing their garments on the thickets, just to be with this great astronomer,`` he says.
``I am the only astronomer on this mountain who never locks his door. Because I keep waiting for it to happen.``
The University of Arizona pays the salaries of the three observers. To operate Spacewatch, Gehrels says he needs $300,000 to $400,000 a year, which he scrapes together from many sources.
About 15 percent comes from private contributions-$15, $20, $30 at a time. ``I have one man who sends $1,000 twice a year,`` he says, ``with a condition that I don`t tell his wife.``
A few years ago, desperate for funds, Gehrels wrote to Liz Taylor, offering to name an asteroid after her in exchange for a contribution.
``She never responded,`` he says.
The combined operation is so bare bones, says Gehrels, that ``I am the one who cleans the john here once a month.``
Gehrels would like $180,000 for a more powerful telescope. But he is pessimistic. And he thinks increased interest in programs such as his will only hurt him. More colleagues will compete for money from the same small pot. NASA officials say only a few million dollars will be needed to upgrade existing telescopes and pay for scientists to staff them. Gehrels would love to see Congress appropriate money for an international asteroid search effort- he`d love to get some of it. But that may be only slightly more likely than an asteroid impact.
On the last day of Gehrels` shift, the mountaintop is covered in thick fog. Observing that night is like trying to see through a jar of mayonnaise.
But around 8 p.m., the heavens clear briefly. Gehrels climbs the catwalk to the top of the dome, throws open the hatch and pokes his head into the night sky. The wind howls, and his hands freeze on the metal railing. He`s wearing just a light jacket and sandals.
A half moon arches almost directly overhead. Orion`s belt shines.
``Jupiter`s rising,`` he yells down excitedly.
Tucson glows to the east, beyond the pitch black desert. Without Spacewatch, he can see no far galaxies, no star clusters, no Earthbound asteroids.
But he knows they`re out there. He knows they`re coming.