Hale-Bopp was an unusually bright comet that swung near Earth in the late 1990s, reaching its closest approach to the planet in 1997. It was most spectacular in the Northern Hemisphere and visible to the naked eye for about 18 months.
Hale-Bopp was probably one of the most viewed comets in history. Popular media mentioned the comet frequently. It provided quite the sky show, being 1,000 times brighter than Halley's Comet at the time of its discovery, NASA stated. Its twin blue-and-white tails were easily visible even from light-polluted areas such as Chicago.
Sadly, there was a tragic footnote to the appearance of Hale-Bopp. About 40 people who were part of the "Heaven's Gate" cult in San Diego committed mass suicide as the comet came close to Earth.
A blob by M70
The comet was found independently by two amateur astronomers, Alan Hale in New Mexico and Thomas Bopp in Arizona. At the time of its discovery, Hale-Bopp was the farthest comet ever to be discovered by amateurs, according to NASA.
Both of the amateurs trained their telescopes at nearby globular cluster M70 on July 23, 1995. Hale, who had a Ph.D. in astronomy but had a job running an educational company, had been looking at the same area recently and was surprised to see a new blob in the sky.
"As soon as I looked," he said in a 1997 Time magazine interview, "I saw a fuzzy object nearby. It was strange, because I'd looked at M70 a couple of weeks earlier and the object hadn't been there."
Almost simultaneously, Bopp saw the same object in the sky. Both men sent their observations to the International Astronomical Union's Central Bureau for Astronomical Telegrams. Hale recalled looking at the sky a few hours later and seeing the object had moved. In his mind, he told Time magazine, it was definitely a comet.
Astronomers were astounded at how bright the comet, officially designated C/1995 O1, appeared — even from its great distance away. They guessed that it could make a brilliant show when it arrived close to Earth, but predictions are always messy when it comes to comets. They are balls of ice, snow and dirt that erupt in ways that are hard to anticipate.
"It's been kind of nerve-racking to sit through all those months wondering if the comet would fizzle," Hale told Time.
The comet's closest approach to Earth was about 120 million miles (193 kilometers). (By comparison, the sun is 93 million miles, or 150 million km, from Earth). Astronomers still had the Hyakutake comet fresh in their minds at the time as the previous one passed by in 1996. Hale-Bopp was many times farther away than Hyakutake, but still appeared much brighter.
'Traffic jam on the Internet'
In its months-long approach to Earth, descriptions of the comet drummed up interest in newspapers as well as the Internet, then a young but growing public phenomenon.
"There are a multitude of websites containing information on Hale-Bopp; they are attracting so many visitors that they are causing a traffic jam on the Internet," Scientific American wrote in March 1997.
"A Hale-Bopp homepage set up at NASA's Jet Propulsion Laboratory was logging more than 1.2 million visitors a day over Easter weekend and had set up two mirror sites."
It wasn't only amateurs watching the show, of course. Professional telescopes swung to watch the comet as it approached. One NASA study that included the Hubble Space Telescope reported that there were two ices in the heart of the comet completely separated from each other.
Hubble's measurements indicated the comet's nucleus is "huge," NASA wrote, at 19 to 25 miles (30 to 40 kilometers) across.
What surprised the science team the most was an outburst that saw dust streaming from the comet's nucleus in quantities at least eight times as much as the average.
"The surface of Hale-Bopp's nucleus must be an incredibly dynamic place, with 'vents' being turned on and off as new patches of icy material are rotated into sunlight for the first time," stated Johns Hopkins University astrophysicist Harold Weaver, who led the study at the time.
Heaven's Gate was an end-times cult led by Marshall Applewhite, a former music professor who advocated sexual abstinence and who had undergone castration. Members believed that their bodies were merely containers that could be abandoned in favor of a higher physical existence, according to an article at History.com. They believed that an alien spacecraft was following Hale-Bopp.
In late March 1997, Applewhite and 38 followers drank a lethal cocktail of Phenobarbital and vodka and lay down to die, convinced that they would leave their physical bodies, enter the alien spacecraft and pass through Heaven's Gate into a higher existence. The bodies of 21 women and 18 men were found lying in bunk beds in a mansion in suburban San Diego, Calif. They wore matching clothes and new sneakers
"According to material the group posted on its Internet site, the timing of the suicides were probably related to the arrival of the Hale-Bopp comet, which members seemed to regard as a cosmic emissary beckoning them to another world," the New York Times reported in March 1997.
Next visit in thousands of years
For the most part, Hale-Bopp represented an exciting time for astronomy, but the fireworks came with a hint of wistfulness for astronomers, as well. Hale-Bopp's last appearance had been about 4,200 years before, and it won't return to the inner solar system for thousands of years. Scientists scrambled to do as much science as possible during the comet's brush by the planet.
Astronomers at the European Southern Observatory released pictures of the comet in 2002, five years after Hale-Bopp got closest to Earth. The comet was then 1.2 billion miles (2 billion km) from Earth, about halfway between the orbits of Saturn and Uranus.
"The large 'dirty snowball' nucleus of ice and dust (probably about 50 km diameter) continues to be active, despite the very low temperature where it is now. This is quite unusual for a comet," ESO stated, based on observations from La Silla Telescope.
The comet's spectacular appearance 16 years ago may be overshadowed by another comet bearing down on the inner solar system in early 2013. Comet ISON could be brighter than the full moon when it arrives close to Earth later this year.
— Elizabeth Howell, SPACE.com Contributor