Donald Yeomans '64 personifies NASA's insatiable pursuit of celestial knowledge.
By Dick Anderson
Donald Yeomans '64 has devoted the last 40 years to the study of comets and their celestial brethren. And when comet Shoemaker-Levy 9 collided with Jupiter in July 1994, "It showed rather dramatically that these things do run into planets," Yeomans says.
Hollywood took notice and spawned a pair of space-age disaster flicks that nearly collided in theaters in 1998. "The folks who did Deep Impact actually talked to us before they wrote the script," says Yeomans from his office at the Jet Propulsion Laboratory in Pasadena, California, "so they didn't get too much wrong." He adds, with a laugh, "The makers of Armageddon got everything wrong."
About 15 miles from the pitch meetings of Tinseltown, Yeomans wears two hats at JPL, a NASA laboratory managed by the California Institute of Technology. He's supervisor of the Solar System Dynamics Group, a 10-member team charged with charting the courses of the planets, their natural satellites, and more than 300,000 comets and asteroids. He also runs JPL's Near Earth Object Program, which computes the orbits over the next 100 years of the planet's astronomical neighbors. "NASA has taken the stance that it's worth $4 million a year to fund six different observatories to look full time for near-Earth objects," says Yeomans, who calculates the probability of impact with any object that gets too close."Once they're discovered, it's our responsibility to track them.
"We keep a short list of objects for which we cannot yet rule out an Earth impact in the future," Yeomans adds. "But as we get additional data, we will most certainly rule them out."
Indeed, the collision Yeomans is more focused on at the moment will be occurring in deep space. On January 12, NASA launched a spacecraft (Deep Impact) that is now on a collision course with the comet Tempel 1. When their paths intersect this summer, an 820-pound, refrigerator-sized impactor will crash into the comet at a speed of 10 kilometers per second, forming a crater, while a fly-by companion craft observes the collision and its aftermath. "This is what we live for," he says. "We're going out and smacking a comet to see whether its nucleus is solid ice, layered, or a collection of particles held together by little more than their own gravity."
For those whose science is a little rusty, comets are remnants from the formation of the outer solar system—the bits and pieces that didn't conglomerate into Jupiter, Saturn, or Uranus. "Comets are important scientifically because you'd like to know what were the building blocks of the outer planets," says Yeomans. "And because you can break down water into hydrogen and oxygen, which is rocket fuel, comets may be the watering holes and fueling stations for future interplanetary exploration."
In the days of ancient Greece and Rome, comets were thought to be apparitions—warning shots from an angry god. Centuries later, Sir Isaac Newton and Edmund Halley showed that comets actually orbit the sun, just like the planets, albeit with eccentric orbits. Halley's 1704 prediction that the same comet observed in 1531, 1607, and 1682 would return in 1758 was a huge step forward, Yeomans observes. (Yeomans's observations are detailed in his book, Comets: A Chronological History of Observation, Science, Myth, and Folklore, John Wiley & Sons, 1991.)
While comets were associated with meteor showers in the 19th century and thought to be relatively harmless, that theory was shattered in 1950, when astronomer Fred Whipple showed that comets were indeed solid bodies—"dirty snowballs," as he described them—with the very real capacity of running into Earth. A few years after Whipple's discovery, 12-year-old Donald Yeomans got a six-inch-diameter telescope as a Christmas present. "That got me started," he says.
Yeomans, a native of Rochester, New York, is the fifth in his family to attend Middlebury, following father George '33, uncle Edward '42, and older sisters Irene Yeomans Batal '59 and Jean Yeomans Lamson '62. Even though he majored in math, Yeomans's greatest influence at Middlebury was physics professor Ben Wissler. "Wissler would demonstrate the laws of trajectory by dropping objects in front of the room and shooting at 'em with a cork," Yeomans fondly recalls. "He was so good, I decided that astronomy was what I wanted to do."
Yeomans wrote his doctoral dissertation, at the University of Maryland, on how to account for a comet's rocketlike thrusts in predicting the object's future path. His work attracted the attention of the Goddard Space Flight Center, where he worked until migrating to JPL in 1976.
"We've been trying to sell a mission to a comet ever since I got here," he says. Except for a few large missions like Voyager I and II, space exploration tapered off in the post-Apollo era. Things perked up in the late 1980s, though, with missions to comets Halley and Giacobini-Zinner and the NEAR mission's landing on asteroid 433 Eros. In the last decade, missions have been smaller and more numerous.
When Deep Impact encounters Tempel 1 on July 3 (July 4 Greenwich Time), the impact may release enough dust to create fireworks visible to the naked eye over much of the Southwest. "While ground-based observations allow us to predict where the comet will be to within a few hundred kilometers, it takes observations from the spacecraft itself to zero in on the comet's nucleus," says Yeomans, who estimates Tempel 1 to be about 9 by 15 kilometers in diameter—or the shape of a "fat pickle."
The resulting pictures should be spectacular. When probes flew past Halley in 1986, the smallest visible object on the surface was about 100 meters in size. Deep Impact will have a resolution of about 2 meters from the fly-by spacecraft and about 15 centimeters from the impacting spacecraft.
Yeomans and his team are flying to Japan in September to participate in another comet mission. That's when Japan's Hayabusa mission is scheduled to meet up with asteroid 1998 SF36, match its orbit path, land on its surface, collect the ejecta (material thrown out of an impact crater) in a cone, and begin a two-year journey back to Earth for in-depth study of its elemental composition. "That's the Holy Grail of asteroid science—to find out which objects up there correspond to which meteorites we have down here," Yeomans says.
"I think we're in the second era of exploration," he adds. "We had Earth exploration in the late 15th and 16th centuries. When the histories are written for the 20th century and early 21st century, there will be a chapter on exploring the solar system."
Dick Anderson is a writer in Los Angeles. He agrees that Deep Impact seemed more realistic thanArmageddon.