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Comet Hale-Bopp: What was it? Where had it been? Where was it going? Photos
Comets like Hale-Bopp, and the earlier well-known Comet Halley, are dirty snowballs with tails of blowing gases and interplanetary dust millions of miles long. They are like cosmic time capsules which share with us a rare glimpse of the origin of the Universe.
Hale-Bopp Comet photo by Fayetteville (NC) Observer astronomer Johnny Horne. Click the image for more Hale-Bopp photos by the photographer.
These exotic travelers – which seem to come out of nowhere, stay in Earth's sky for a time, then fly off into nowhere – actually were born faraway on the frozen fringe of our Solar System as it formed 4.6 billion years ago. They've changed little since then.
A cloud of comets
Many comets are still out there, revolving around the Sun in a vast swarm of comets called the Oort Cloud. The cloud lies at the far edge of our Solar System, about halfway to the nearest star.
Every now and then a comet is knocked or pulled out of the Cloud to fall into the Solar System in a closer orbit around the Sun. When we look at one of those closer comets, we are, in effect, looking backwards in time and seeing the physical and chemical conditions that existed at the time of the formation of our Solar System.
As a comet moves closer into the center of the Solar System, the Sun's heat releases gases from the comet, creating a cloud around the comet's core. Particles hurled out by the Sun, called solar winds, distort the cloud around the comet to create the comet's tail.
What's in the comet?
Scientists can't see Comet Hale-Bopp's core, but they estimate it to be about 25 miles in diameter. The big dirty snowball has huge chunks of ice and great gobs of tiny interplanetary dust particles mixed together.
Astronomers say that comets which have made trips near the Sun have developed a crust – a rigid envelope of debris and refrozen ice. As the comet returns to the vicinity of the Sun, pressure builds under the crust, material under the crust erupts, and a hole blows out through the rubble crust forming spectacular jets.
Pictures of Hale-Bopp show bright jets of light where gases and dust particles have been warmed by the Sun and exploded outward. Astronomers say Hale-Bopp is one of the most active comets they've ever seen.
In the year 4397
Comet Hale-Bopp is on a 2,400-year trek around our Sun. That means that, if Hale-Bopp survives this trip around the Sun without melting, it will be back around in about 2,400 years – sometime around the year 4397.
The famous Comet Halley, on the other hand, travels around the Sun every 76 years. It will be seen again in less than a century from now, in the year 2062.
Other well-known comets seen in recent years are Comet Hyakutake, which makes an appearance in the inner Solar System every 9,000 years, and Comet Kahoutek.
Because Hale-Bopp is so big and bright, scientists have an opportunity to take a long look at it, observing how gases are burning off the core. With infrared and radio telescopes, they hope to find molecules never seen before in a comet. Its tail is millions of miles long.
- Hale-Bopp's last trip near Earth was some 4,200 years ago. The great Pyramids were new 4,200 years ago. The Roman and Greek Empires were still in the future 4,200 years ago.
- Comet Hyakutake, which made a brief appearance in 1996, had a faint blue-green tail. It was discovered by amateur astronomer Yuji Hyakutake in Japan on January 30, 1996, using a pair of high-power binoculars. The luminous blue-green cluster of dots was closest to the Earth on March 25, 1996. A cluster of rock and ice, Hyakutake was the fifth-closest comet to Earth within this century. It also was one of the more visible from Earth's northern hemisphere. As Hyakutake cruised by the Sun and was warmed by solar energy, natural geysers erupted with great force through holes in the crust on the comet's surface.
- Comet Halley made an impressive, highly-publicized visit to the inner region of the Solar System in 1910, but its reappearance in 1986 was less spectacular. Even so, spacecraft from Earth flew out near Comet Halley to sample its gases and dust in 1986. Those probes showed that Halley really is a dirty snowball.
- Comet Kahoutek, traveling near the Sun in 1973, seemed to run out of gas. It turned out not to have as much gas for the Sun to boil off as astronomers had thought, so it couldn't throw off enough dust to make it appear bright in Earth's sky.
Who is Hale? Who is Bopp?
Alan Hale and Tom Bopp were the first people to spot one of the brightest comets to be seen from Earth this century.
As the comet approached the inner region of our Solar System on July 22, 1995, Hale was at home in southern New Mexico, examining the evening sky through his telescope, when he saw something that looked like a fuzzy dot among the stars. Bopp was four hundred miles away in the Arizona desert, stargazing with friends, when he saw the same fuzzy dot.
Hale was using a 16-inch reflector telescope. Bopp was using a homemade 17 1/2-inch telescope. Their discovery was made minutes apart. The most rewarding thing about discovering a comet may simply be having it named after you.
Neither Bopp nor Hale makes a living studying the skies, although Hale did write a book about their discovery. The men did not know each other beforehand. The outgoing Hale has a Ph.D. in astronomy. The more-reserved Bopp is an amateur astronomer who works for a large construction materials company. They weren't paid to look at stars that July night. Instead, they were doing something they love to do which turned out to make history. Comet Hale-Bopp mades friends of perfect strangers and wrote their names in scientific history.
Are space aliens near the comet?
Someone somewhere made up a story that either an unidentified flying object or a body like the planet Saturn is following the comet. The mystery object was said to be carrying aliens to Earth and they even may have been guiding the comet toward Earth. Photographs of something trailing in Hale-Bopps' wake turned out to have been modifed by computer. Astronomer Hale called the alien spacecraft stories "nonsense" and explained that the mystery object was merely a bright star in the background.
Could a comet hit Earth?
NASA artist depiction of object striking Earth
There is no chance of Hale-Bopp hitting Earth. It is a long way off as it goes around the Sun, coming only as close to Earth as 122 million miles away. Other comets have come closer to Earth in the past with no apparent physical effect on our planet.
Hale-Bopp is so spectacular because, even at its great distance – 122 million miles from our planet – it appears very bright to comet watchers on Earth. Part of the reason for its brightness is it's very large – the core is about four times as wide as the core of Comet Halley. Comet Hale-Bopp is huge.
On the other hand, comets or asteroids apparently do hit Earth sometimes. Once or twice every million years a large body dives into Earth's atmosphere and smashes into the ground or ocean with tremendous force.
Some 2,000 large Near-Earth Objects (NEO) cross Earth's orbit and, in theory, could hit us. While the threat is real, the odds are in our favor.
There have been close calls in recent years:
Of course, tiny objects hit Earth all the time. Most dunk in the oceans or crash on uninhabitated land. However, a meteorite smashed through the rear end of a Chevy Malibu in Peekskill, New York, on October 9, 1992. The car was totaled; no one was hurt.
- A half-mile-wide asteroid crossed Earth's orbit on March 23, 1989, about 400,000 miles from Earth. Our planet had been on that spot six hours earlier.
- The closest near miss was an asteroid about 30 feet wide which passed within 106,000 miles of Earth on January 17, 1991.
Scientists have had a hard time getting Congress to fund an early-warning system. A 1991 NASA study, requested by Congress, called for an international Spaceguard Survey system of ground-based telescopes surveillance cameras to detect Earth-threatening objects. Congress refused to pay for the $10 million-a-year project. NASA spends $1 million a year on smaller search programs which monitor about 10 percent of the sky per month.
What if astronomers could spot a big object six months before it might hit Earth? What could be done? With enough advance notice, military planners might:
NASA says in its Fact Sheet on Asteroid and Comet Impacts, "At present no asteroid or comet is known to be on a collision course with the Earth. The chances of a collision within the next century with an object 1 km or more in diameter are very small (roughly 1 in ten thousand), but such a collision is possible and could happen at any time."
- Melt it with lasers on Moon or Earth used to boil off layers, thereby redirecting it.
- Smash it to smithereens with rockets carrying heavy payloads to smash the object into bits and divert them off course.
- Blow it off course with a nuclear blast near the object.
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