Hubble Astronomers Make Plans to Observe Comet Hyakutake
FOR RELEASE: March 21, 1996
PRESS ADVISORY
HUBBLE ASTRONOMERS MAKE PLANS TO OBSERVE COMET HYAKUTAKE
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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