I wonder how many of the people watching Comet Hyakutake cruise by our corner of the solar system last month bothered to wave and shout: "Hi, Mom!" Or more precisely: "Hi Great- Great- . . . Great-Grandma!"
Certainly, many thousands of people came out to have a look--tens of thousands at Griffith Observatory alone, according to director Ed Krupp, who said it was a clear case of "mad comet disease." Three thousand even showed up on Oscar night.
To be sure, Comet Hyakutake didn't look much like Aunt Millie. From my backyard, it was just a patch of fuzz, like a star someone had tried to erase. Its nucleus is only about a mile across.
Still, we owe our lives, quite literally, to comets. People (like all living things) are mostly carbon and water. And virtually all the carbon and water on Earth, scientists think, came here by way of comets. More than half of the carbon, it is surmised, came from a single, colossal impact.
"We basically owe our existence to collisions of asteroids and comets with the Earth," said comet expert Steve Ostrow of NASA's Jet Propulsion Laboratory in Pasadena.
When Earth first formed, the story goes, it was simply too hot to hold onto carbon and water, which evaporated away. Comets formed out beyond Pluto from the crumbs left over when the planets solidified. Far from the sun, it was cool enough for carbon-rich compounds to congeal, along with frozen water.
The result was a halo of dirty snowballs, called the Oort cloud, where comets cruise for billions of years. Occasionally, one gets kicked out of its parking orbit and comes hurtling toward the planets. As these mountain-sized crumbs get closer to the sun, the ice inside starts to turn to gas, causing pressure to build. The comet blows off steam. We see these gases as a glowing tail. No wonder our early (human) ancestors called them "hairy stars."
But if comets are hairy stars, says UC Berkeley geophysicist Raymond Jeanloz, they are raining tons of dandruff into the solar system--dust that the solar wind blows off the surface.
Comet Hyakutake started to crumble even as it passed us in March, shedding chunks and forming mini-comets in its tail, said comet expert Beatrice Mueller from her perch at the National Optical Astronomy Observatories at Kitt Peak, Ariz.
As it sweeps toward the sun, Hyakutake's surface will get quite hot. More chunks will break off. At its closest approach, on May 1, it will be a flying baked Alaska, toasty on the outside, frozen in the middle.
It should brighten up again next week--especially if its wispy gas tail is joined by a trail of larger dust particles knocked off the nucleus by the impact of sunlight. But as JPL's Stephen Edberg points out, "Comets are unpredictable."
The core itself is blacker than night, coated with organic sludge that is darker than the darkest velvet. It's the stuff we're made of. As Hyakutake swung by, astronomer Michael Mumma of NASA's Goddard Spaceflight Center was able to detect several complex organic carbon compounds boiling off the core that had never been seen in comets before.
These pint-sized interplanetary visitors have had a disproportionate impact on Earth--a fact that seemed to strike people where they live when Comet Shoemaker-Levy smashed into Jupiter in the summer of 1994. The impact set off a volley of nuclear-scale explosions. That could easily happen here.
It may have happened before. If the paleontologists are to be believed, it was just such a comet that killed off Tyrannosaurus rex 60 million years ago.
Ostrow thinks the big role of small bodies is ironic. "Human history and human destiny are intimately tied to [comets and asteroids]," he said.
Goddard astrophysicist Carey Lisse, one of the scientists who discovered X-rays crackling unexpectedly from the comet recently, finds the scenario more than ironic.
The dinosaurs had a "pretty good run of 60 or 80 million years," he said. "We've only been here for 3 million. We'd better know what's out there."
After Hyakutake slips beneath the horizon this summer, it won't be back again for at least 17,000 years. If this was a second coming--and no one knows for sure--then the last time Hyakutake was here, our (human) ancestors were still making paintings on the walls of caves, Krupp said.
Meanwhile, another monster comet, Hale-Bopp, is on the way, expected to make an appearance April 1, 1997. Stay tuned.