Johns Hopkins Gazette: March 10, 1997

Hal Weaver Eyes
Prize in the Skies

Emil Venere
News and Information
Astronomers have been treated to an amazing display of two spectacular comets over the past year, and the grand finale is coming in early April.

That's when comet Hale-Bopp, the largest and one of the most unusual comets ever observed by astronomers, will peak in brightness as it makes its closest approach to the sun.

Taken alone, the two cometary visitations are exciting. But when you throw in 1994's extraordinary observations of a comet actually colliding with the planet Jupiter, it just doesn't get any better, said astrophysicist Hal Weaver.

"This is really a golden age of cometary astronomy," said Weaver, who specializes in research on comets and their connection to the origin of the solar system.

Hale-Bopp was discovered in July 1995 by amateur astronomers Alan Hale of Cloudcroft, N.M., and Thomas Bopp, of Glendale, Ariz. Only a few months later, an amateur Japanese astronomer peering through a pair of binoculars spotted comet Hyakutake, which peaked last spring.

Since then Weaver has been hopping from one project to another, juggling scientific papers about both comets and stealing away to various meetings to talk about his work.

"When baseball season starts this spring, I don't know what I'm going to do," said the astronomer, who has two sons, Alex, 8, and Eric, 5.

Not that he's complaining.

Quite the opposite. Hopkins astronomers are celebrating Hale-Bopp's arrival with ambitious science projects; Weaver trekked to Hawaii last week, observing the comet with an infrared telescope on the windy summit of Mauna Kea, a 796-foot dormant volcano. Next month other Hopkins scientists, led by astrophysicist Paul Feldman, will travel to White Sands, N.M., launching a rocket that will rise above the Earth's atmosphere to observe ultraviolet light from Hale-Bopp.

"We will enhance our knowledge because it's a particularly bright comet, and we are looking at it with new technology, so it's a big leap of understanding," said Feldman, who is chairman of the Department of Physics and Astronomy.

Comets bombarding the Earth billions of years ago may have provided vital water and organic compounds necessary for the genesis of life.

"But I don't promise definitive answers to the very large questions, like, Where did life come from?" Feldman said. However, considering that cometary bombardments probably have played an important role in Earth's evolution, Hale-Bopp's appearance is profound.

"It's obviously of great interest to have a comet that's this large coming in," Feldman said, noting that one compelling question has to be, "What's the probability that such a comet could hit the Earth?"

According to one estimate, a comet might hit Earth once every three million years. But there is no fear of that happening with Hale-Bopp; it will reach its closest distance to Earth, 122 million miles, on March 22.

Hale-Bopp returns to Earth only about once every 3,000 years, so astronomers are taking advantage of its rare visit.

Their goal: to learn and confirm important details about the composition and origin of comets; by doing so, they will help to uncover secrets surrounding the formation of the solar system, 4.6 billion years ago. They may even gather information about the original material that existed in space before the solar system was born.

"This is a unique opportunity," Weaver said. "We have never had the chance to examine a comet in this much detail over this large a range of distance from the sun."

Hale-Bopp was unusually bright when it was still a great distance from the sun, well outside the orbit of Jupiter. For that reason, it has given scientists their best view ever of the changes in a comet's solid nucleus as it gets closer to, and is progressively heated by, the sun. Those changes, in turn, provide information about the composition and structure of comets.

Scientists are interested in comets because of their place in cosmic history.

"We think that when you look at a comet today you are looking back into the past," said Weaver, a research scientist in the Department of Physics and Astronomy.

The solar system was born from a huge cloud of gas and dust, which coalesced into larger and larger bodies, eventually forming the planets. But many smaller, left-over bodies were driven to the outskirts of the solar system by the gravitational forces of the giant gas planets, Jupiter, Saturn, Neptune and Uranus.

There, far from the sun's warming influence, these comets, frozen "dirty snowballs" of ice and dust, preserved the original material from the solar system's birth. Studying comet Hale-Bopp might enable astronomers to "take a scoop out of" the original material from the interstellar cloud that condensed to form the solar system, Weaver said.

He led a team of astronomers who used the Hubble Space Telescope to observe the comet for a one-year period, ending in mid-October 1996, when Hale-Bopp came too close to the sun from Earth's perspective to be viewed safely with the space telescope. Looking too close to the sun could subject the telescope to dangerous heating.

For that reason, astronomers will have to use telescopes on the ground and on sounding rockets to observe the comet as it approaches its rendezvous with the sun.

During their yearlong study, astronomers learned surprising details about Hale-Bopp. Some of their findings contradict conventional thinking about how comets are put together. The astronomers observed ultraviolet light from the comet with Hubble and the International Ultraviolet Explorer satellite.

By examining Hubble images, researchers estimated that the comet's nucleus may be 30-40 kilometers (about 22 miles) in diameter. In comparison, the potato-shaped nucleus of Halley's Comet, considered to be a large one, had an equivalent diameter of about 10 kilometers.

"Hale-Bopp looks like it's a monster in that respect," Weaver said. The average comet is thought to have a nu-cleus of about 5 kilometers in diameter.

Another surprise is the way in which the comet would suddenly grow brighter and then return to its usual brightness within an hour or so.

"The surface of Hale-Bopp's nucleus must be an incredibly dynamic place, with new 'vents' being turned on, possibly triggered by the rotation of an icy region into sunlight, and then turned off again," Weaver said.

Also surprising is the way in which various types of ices are being vaporized. A well-accepted cometary model suggests that dust particles and various chemical compounds, such as carbon dioxide and carbon disulfide, are all contained inside frozen water. As the comet nears the sun, it heats up, vaporizing the water and releasing other material and dust particles that were contained in the ice. The dust is spewed off in a huge tail extending millions of miles, reflecting sunlight and brightening the comet.

But their observations have astronomers wondering about that model. They found that various chemicals have been vaporizing independently of water. While the vaporization rate of water increased more than 10-fold between April and October 1996, there was only a twofold increase in the rate of dust being released.

If the model were correct, water, dust and the other components should be released at relative rates.

"The poor correlation between the water and dust production suggests that much of the dust and water might be coming from physically different regions of the nucleus," Weaver said. "Similarly, carbon disulfide may be coming from yet another patch. Some of these effects have been observed in other comets. But Hale-Bopp will probably provide the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to comet Halley in 1986."

Of comets
and asteroids

Comets and asteroids are similar in that they are both believed to be left over from the formation of the solar system, about 4.6 billion years ago.

A swirling cloud of gas and dust initially produced a rocky material, which coagulated into larger and larger bodies that merged to form the planets.

But not all the material went into building the planets. Some bodies, called asteroids, inhabit a vast belt located between the orbits of Mars and Jupiter. Because of their relative closeness to the sun, they formed hot, vaporizing lighter substances, such as water, leaving behind dried-out rocky hulks.

But other leftover bodies, called comets, were formed from material located much farther from the sun, outside the orbits of Uranus and Neptune. Because they coalesced so far away, comets were born cold, at temperatures as low as minus 260 degrees Celsius, or about minus 435 degrees Fahrenheit.

Therefore, they have been preserved over billions of years, perhaps harboring some of the original material from which the solar system formed, whereas the planets have undergone major changes over time from geological, thermal and atmospheric processes.

Comets have been called "dirty snowballs" because they contain dust mixed with frozen compounds, including water, carbon dioxide and carbon disulfide. But asteroids are like the dirt in the snowball, without the icy components.

Comets come from two regions in the outer solar system. One region is a disk of bodies called the Kuiper belt, located outside the orbit of Neptune. The other region, called the Oort cloud, is much larger and may extend to 9 trillion miles from the sun, nearly halfway to the nearest star, Alpha Centauri. Comets in the Oort cloud sometimes end up in orbits that take them into the inner solar system when they are jostled out of place by the gravitational field of a star, or a giant molecular cloud, from which new stars form. Scientists believe that is what happened to comet Hale-Bopp at least 30,000 years ago. It takes roughly 3,000 years to complete one orbit, which extends from outside the orbit of Pluto to about 85 million miles from the sun.

Another class of comets, called "short-period" comets, take less than 200 years to complete an orbit around the sun and are thought to have originated in the Kuiper belt.

Comets and asteroids do occasionally hit the Earth, but asteroids are considered more dangerous than comets. That's because the asteroid belt is so full that they collide with one another, throwing off debris that can end up in unstable orbits, hitting the innermost planets, including Earth.

Go back to Previous Page

Go to Gazette Homepage