The Polar spacecraft, so-named because it orbits Earth's poles, was launched on Feb. 24, 1996 and is part of the International Solar-Terrestrial Physics program. It's primary mission is the study of Earth's aurora. But beginning on March 27th, one of the instruments on Polar will take on extra duty tracking Comet Hale-Bopp. The Goddard Space Flight Center in Greenbelt, Maryland will host a press open house on Polar's Hale-Bopp results on Monday, April 7th, 1997.
This instrument, a wide-field visible and ultraviolet wavelength camera operated by Dr. Louis A. Frank of the University of Iowa, is ideally suited to the task. Scientists interested in the chemistry of the comet have the most to learn from the molecules and atoms in the comet that are detectable only in the ultraviolet part of the spectrum. But this part of the spectrum is largely blocked by Earth's atmosphere--and thus lies beyond the capabilities of all the ground-based observatories that are tracking the comet as it passes through the inner solar system.
Even the Hubble Telescope will not be able to view the comet, though for entirely different reasons. Hale-Bopp will appear be so close to the Sun that if the Hubble Telescope tried to view the comet, its optical and electronic systems would most likely be destroyed. Polar runs that risk, too, but the Polar cameras can view near the Sun because the super-polished mirrors of its telescope prevent large amounts of sunlight from being scattered into the extremely sensitive sensors of its cameras. Dr. John Sigwarth, a research scientist at the University of Iowa, is in charge of this delicate imaging operation.
"It's very exciting," says Frank, the Carver/James A. Van Allen Professor of Physics at the University of Iowa, of the venture. He is confident that the instrument, known as the Visible Imaging System, will provide a wealth of data on the nature and origins of Hale-Bopp. There may even be a few surprises. Comets have been known to fragment or even split in two as they approach the Sun.
Hale-Bopp was discovered on the night of July 22, 1995 by Alan Hale, an astronomer at the Southwest Institute for Space Research in Cloudcroft, New Mexico, and Thomas Bopp, an amateur astronomer in Glendale, Arizona. The two independently spotted the inbound comet when it was still out beyond the orbit of Jupiter.
Hale-Bopp's orbit is almost exactly perpendicular to the ecliptic, the plane of Earth's orbit around the Sun. After passing near the orbit of Jupiter last year, the comet then climbed above the ecliptic as it journeyed into the inner solar system. On March 22, the comet will be at its closest to Earth--actually farther than the Sun itself, 122 million miles versus 93 million miles away. A week later, on April 1st, the comet will be at perihelion, its closest approach to the Sun. Even though the comet will pass near the orbital path of Earth as it descends below the ecliptic later in April, Earth itself will be on the other side of the Sun at the time.
Hale-Bopp's current orbit suggests that it was last in our neighborhood some 4,200 years ago--when Stonehenge was still in use--and won't be back around for another 3,000 years or so. The comet has an icy core that astronomers estimate to be about 25 miles in diameter, as big as a mountain, or four times the size of Halley's Comet.
For more information concerning the observing plans for the VIS, please visit the Polar Hale-Bopp Web page.
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