'String of pearls' comet near collision with Jupiter


Faster than a speeding bullet and more powerful than an arsenal of hydrogen bombs, fragments of Comet P/Shoemaker-Levy 9 are streaking toward a series of titanic collisions with the planet Jupiter beginning this weekend.

Astronomers have never seen anything like it before, and from Baltimore to Beijing and the South Pole, they are gearing up to record the seven-day barrage, which will occur more than a half-billion miles away.

The 2-million-mile-long squadron of 21 fragments, each composed of loosely packed ice and rock up to 2 miles wide, is rushing toward Jupiter's southern hemisphere at nearly 40 miles per second.

The first -- one of the smallest -- will plunge into the gaseous planet just before 4 p.m. EDT tomorrow, and every major telescope on Earth, in orbit or touring the planets has spun toward Jupiter to watch. The last fragment will hit July 22.

"We're all excited about it," said astronomer and comet specialist Harold Weaver, 40, at the Space Telescope Science Institute in Baltimore. A similar comet striking the Gulf of Mexico 65 million years ago is believed to have ended the reign of the dinosaurs, and another like that "might ultimately wipe us out."

Thousands of amateur astronomers around the world hope to spot the fireballs the largest fragments may produce as they bore into the dense Jovian atmosphere, or at least some hint of a change in the planet's banded cloud patterns.

Despite their excitement, scientists are urging the media not to expect too much, recalling the public's disappointment with the appearances of Comet Kohoutek in 1973, and last August's Perseid meteor "storm." Both were duds compared with their advance billing.

"Nobody should go out and buy a telescope," said Hubble astronomer Keith Noll, 35. Backyard astronomers are unlikely to see anything. In fact, only one of the comet's fragments will strike when Jupiter is visible from the eastern United States -- about 10:49 p.m. Saturday.

A 'challenging' observation

Many of the professionals doubt that they'll see any startling effects from the collisions, despite estimates that the largest fragments will release energy measured in millions of megatons.

"That's why we're using the largest telescopes in the world," said astronomer Heidi Hammel, 34, of the Massachusetts Institute of Technology, who will lead a team of astronomers photographing the collisions with the $1.5 billion Hubble Space Telescope.

As a photographic spectacular, she said, "Most of us think this is not going to be a big deal. It's a very challenging observation."

Jupiter, after all, is 534 million miles away from Earth, and 89,000 miles across. The biggest of the comet's "bullets" may be no more than 2 miles wide.

But "in terms of the science that may come out of it, that is a big deal, and that's why so much has been mobilized on this," she said.

Shoemaker-Levy 9 is believed to have been a single, 3-mile-wide comet orbiting the sun when it was drawn into an elongated orbit around Jupiter in the 1960s. It broke apart in 1992 when it passed too close to the planet, and was discovered in March 1993 by Arizona comet hunters Eugene and Carolyn Shoemaker and David Levy.

Batteries of sensors

Astronomers soon calculated that the chain of fragments was headed for a collision in July 1994.

Scientists will observe the collisions, from Earth and from space, in all portions of the electromagnetic spectrum:

* Radio telescopes will scan for changes in emissions from the planet's radiation belts.

* Infrared sensors will seek out fireballs and seismic waves rippling through its atmosphere.

* Ultraviolet detectors will look for auroras similar to Earth's "northern lights."

* Cameras will watch for weather changes near the impact points, and light flashes reflected off Jupiter's moons and faint dust ring.

* Spectrographs will split light from the planet to detect and measure any chemical debris from the comet. They also will identify any chemical compounds splashed up from Jupiter's poorly understood gaseous interior.

At stake are potential new discoveries about the composition of comets and Jupiter, and the physics of high-energy impacts.

"This is the first time we've been able to predict when such an impact was going to occur on another body, prepare for it, make predictions, and see if they're verified," Dr. Hammel said.

"Scientists like to take a system and poke it, and see how it reacts to their poke," she said. "This is the first time we [astronomers] have been able to do that," she said.

Hubble offers best views

The refurbished Hubble Space Telescope is expected to provide the best views, the first to be released late Saturday or early

Sunday. Hubble scientists have been busy with last-minute preparations, despite the distraction of an announcement Tuesday of an undisclosed number of layoffs at the institute due to NASA budget cuts.

Their nerves also were rattled last week when Hubble unexpectedly put itself into a "safing" configuration that closed the telescope's aperture door. It took five days for engineers to fix the problem, traced to computer software. Observations resumed on Monday.

"We were lucky it all happened last week instead of this week," Dr. Weaver said.

Beyond Jupiter's horizon

None of the Earth-based or orbiting telescopes will have a direct view of the collisions, because the impacts will occur on the "back" side of the planet, just over the Jovian horizon as seen from Earth.

Astronomers hope that a few of the fireballs will rise high enough to be visible over the horizon, or that the impact sites will become visible within a half-hour as Jupiter rotates and brings them around front.

Fortunately, the Galileo spacecraft en route to Jupiter, and now 150 million miles from the planet, will have a direct view. Due to antenna problems, however, those pictures may not arrive for months.

Uncertain effects

For all their preparation and predictions, though, scientists admit that they don't really know what to expect. There is no precedent and, for all their looking since the comet was discovered 16 months ago, they can't agree on how big the fragments really are -- the key to how big a punch they will pack.

They also disagree on how the fragments will behave as they approach Jupiter. Some say they will fall apart, yielding no more than a meteor shower.

Others predict a "soft catch," in which the fragments flatten out and make only a shallow penetration of the atmosphere. Still others believe that the comet will bore hundreds of miles into Jupiter's atmosphere before erupting in a fireball, but they disagree over just how deep and how explosive the hits will be.

No effect here this time

The debate is not entirely academic. For scientists interested in the possible effects of an object striking Earth, "it has a bearing on how big an [object] the earth's atmosphere can protect us from," said Dr. Mordecai-Mark Mac Low , a University of Chicago astrophysicist.

Scientists are confident about this, however: The collisions on Jupiter will have no effect on Earth; and Jupiter, though composed mainly of hydrogen, will not explode or ignite to become a second "sun," as science fiction novels such as Arthur C. Clarke's "2010" might suggest.

If such things were possible, they would have happened long ago. Collisions with comets are thought to have been quite common through the history of the solar system. Some believe that falling comets may have been the source of much of the Earth's water and perhaps even the organic chemicals that gave rise to life here nearly a billion years ago.

Extinctions on Earth

Scientists have found fossil evidence that large icy comets or rocky asteroids may have been responsible for several mass extinctions on Earth. A "small" one with the force of 2,000 Hiroshima A-bombs is thought to have caused the mysterious 1908 "Tunguska" explosion that leveled a forest in Siberia and incinerated hundreds of reindeer.

Shattered comets like Shoemaker-Levy 9 also may be fairly common. One estimate is that comets with a diameter of a few kilometers break up in Jupiter's gravity once every 80 years.


Telescopes at several sites will enable Marylanders to view Jupiter during the seven-day collision period.

* The Baltimore Astronomical Society will have at least a dozen telescopes for a star party Saturday at Downs Memorial Park in the 4900 block of Mountain Road, Pasadena. Solar observations and lectures will begin at noon. After dark and until about 11 p.m. there will be views of Jupiter, Venus, globular star clusters and nebulas.

* The University of Maryland will open its observatory near University Boulevard and and Metzerott Road in College Park from 8 p.m. to 10 p.m. Sunday through Wednesday. Astronomers will explain what appears on the main telescope's video monitors. Portable telescopes will be available.

* The Johns Hopkins University will open its observatory at the Bloomberg Physics Center, near the San Martin Drive entrance of the Homewood campus, from 8:30 p.m. to 1 a.m. Saturday through Friday. Call 516-6525 after 5 p.m. for weather updates.

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