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Hubble Astronomers Make Plans to Observe Comet Hyakutake


FOR RELEASE: March 21, 1996


Serval teams of astronomers are planning to use NASA's Hubble Space Telescope to observe comet Hyakutake (officially designated as C/1996 B2) as it barrels past Earth and on toward the Sun. The combination of Hubble's high resolution and Hyakutake's close approach to Earth will allow for the most detailed study of a comet's nucleus since the spacecraft flybys of Comet Halley in 1996. Hubble's instruments will also be used to perform sensitive ultraviolet spectroscopy of the molecules evaporating from Hyakutake's icy nucleus. Hyakutake is one of the most active comet to pass this close to the Earth in over 400 years, and probably will become the brightest naked-eye comet in two decades, so astronomers say that this opportunity is unlikely to be repeated during Hubble's remaining lifetime. (The observatory is planned to operate until the year 2005).

Researchers will view the first Hubble comet images on March 26th and, allowing time for reviewing and preparing the data, hope to have an image which NASA and the Space Telescope Science Institute will make available to media representatives no later than March 28. A media advisory will be issued to announce availability of the image.

Comet Chase

On March 25 the comet will whisk within 9.5 million miles of Earth, zooming along at 93,000 miles per hour (45 times faster than a speeding bullet). Viewing the deep-space visitor will be especially tricky because the comet's position is relatively poorly determined and Hubble's instruments can only view small portions of the sky. Special efforts will be made to point Hubble at the right place at the right time; then Hubble will automatically track the speeding visitor like a skeet shooter following a catapulted clay pigeon. At its closest approach, on March 25, the comet will be moving along at nearly one arcsecond per second, which means that it will cover the width of the full Moon in about 40 minutes.

Comet Imaging and Ultraviolet Spectroscopy

Dr. Harold Weaver of The Applied Research Corporation, Landover, Maryland will lead a team taking Wide Field and Planetary Camera 2 (WFPC 2) images of the comet on March 26, near the time of the comet's closest approach to Earth. Astronomers expect to see details as small as four miles across with Hubble. If the icy nucleus, the heart of the comet, is about the size of Halley's comet (Halley is potato shaped, 10 miles across at its long axis), then Hubble will have a chance to resolve the nucleus. Weaver also hopes to see jets of dust in the Hubble images.

Weaver's team will also obtain ultraviolet spectra of the comet's coma, using both the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS). (Spectrographs separate light [ultraviolet light from the comet in this case] into its constituent colors.) These spectra should be at least ten times more sensitive than previously obtained ultraviolet spectra of comets and, hopefully, will reveal new molecules which have previously been inaccessible.

Magnetic Fields

On April 2nd, as Hyakutake hurtles away from Earth and on toward the Sun, John Brandt of the University of Colorado, and co-investigators, will use the WFPC 2 to observe Hyakutake's near-nuclear region to reveal the shape of the plasma and presumably the magnetic field. The comet captures the magnetic field out of the solar wind, a stream of subatomic particles from the Sun. Near the nucleus, the field should wrap around in a hairpin configuration, and also should show a pileup region on the sunward side of the nucleus. The region to be probed by the Hubble images will be the most detailed look at the near-nucleus plasma since the spacecraft flybys of comets Halley and Giacobini-Zinner in 1985-86.

Water in the Comet

On April 3rd and 4th a team of scientists, lead by Dr. Michael Combi of the University of Michigan's Space Physics Research Laboratory, will use three of Hubble's instruments, as well as coordinated ground-based telescope observations, to study water photochemistry on the comet, which is important for interpreting a variety of observations of comets.

The investigation is designed to make measurements simultaneously of hydrogen, hydroxyl, and oxygen in the coma (or atmosphere) of comet Hyakutake. (Hydroxyl radicals are made up of a one hydrogen and one oxygen atom). These are the most abundant constituents in the comet's coma, being produced when ultraviolet light from the Sun breaks apart the water molecules that are evaporated from the comet's nucleus. A water molecule is made up of two hydrogen atoms and one oxygen atom.

The key part is the measurement of the expansion speeds of the hydrogen atoms, which takes special advantage of Hubble's GHRS. Images of the hydrogen coma will also be made using the WFPC 2. The FOS will be used to detect hydroxyl. Ground-based observations will be made during the same time period to measure visible red light emissions of hydrogen and oxygen. Speeds can be measured using the same Doppler principle which enables police radar to measure a car's speed.

The Mystery of Diatomic Carbon

Another spectroscopic program, by Michael A' Hearn of the University of Maryland, will map the comet's emission in the light of the diatomic carbon (C2) molecule. Optical light from C2 is one of the strongest emissions in comets, but astronomers have not yet been able to tell where the C2 is coming from. (C2, itself, is probably not present in the nucleus. The C2 is likely produced by the breakdown of a more complex carbon-bearing molecule in the nucleus.) By simultaneously observing both the ultraviolet and visible emissions from C2, the team hopes to gain insight into the origin of this important cometary molecule.

Comet Discovery

The comet was discovered on January 30th, 1996 by Yuji Hyakutake (pronounced "yah-koo-tah-kay", with equal emphasis on all four syllables), who is an amateur astronomer from southern Japan. At that time the comet was 170 million miles from Earth. Hyakutake perhaps made a previous visit to the inner solar system some 10,000-20,000 years ago.

Backyard Observing Tips

During the last week of March, comet Hyakutake will be easily visible as it passes near the Big Dipper. The comet should remain bright through at least mid-April. The best way to view the comet will be with binoculars, though it should be visible to the naked eye on a dark clear night. On March 27th the comet passes very close to the North Star, Polaris and so is visible all night long from the U.S., Canada, Europe, and Japan through the last week in March. Before March 23rd, the comet can only be observed after about 10 pm. After March 25th the comet should be visible shortly after sunset.

What is a Comet?

Comets are denizens from the uncharted depths of space beyond Pluto. Icy remnants of the early solar system, comets follow a very elliptical orbit around the Sun that usually carries them beyond the outer planets. The heart of a comet is the solid "nucleus", which is typically a few miles across and consists of ices, dust and rock. For most of its existence a comet nucleus is in a frozen, quiescent state. This changes dramatically as it makes its death-defying plunge toward the Sun. The Sun's radiation warms the nucleus, causing the ices to turn directly from a solid to a gas (sublimate) and escape the surface of the nucleus. This venting creates both the coma (or atmosphere) surrounding the nucleus that extends from thousands to millions of miles, and also the tail of material that generally streams out in the opposite direction from the Sun.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

Contact: Don Savage
         NASA Headquarters, Washington, DC
         (Phone: 202-358-1547)

         Ray Villard
         Space Telescope Science Institute, Baltimore, MD
         (Phone: 410-338-4514)

         Cheryl Gundy
         Space Telescope Science Institute, Baltimore, MD
         (Phone: 410-338-4707)

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