|
HOW SCIENTISTS ARE STUDYING IT br /
As the orbit of Comet Hale-Bopp brings it closer to the Sun in late March, scientists will study or will have studied the large and bright comet using sounding rockets, spacecraft and ground-based observations. Below is a short description of what Goddard Space Flight Center scientists are doing:
Polar Spacecraft - NASA\'s Polar spacecraft will use three of its 11 instruments to study the Comet Hale-Bopp from March 25 to April 2 as the comet nears the Sun. Observation periods occur at 18-hour intervals when the spacecraft is near apogee (farthest point from the Earth), with two of the periods extending over three hours. The Polar instruments, the Ultraviolet Imager (UVI) and Visible Imaging System (VIS), will study the ultraviolet emissions from the comet and will VIS also will image the comet at selected wavelengths in the visible light. The Polar Ionospheric X-Ray Imaging Experiment (PIXIE) will look for X-rays in Hale-Bopp. However, it is not known whether the comet will be shining in the X-ray wavelengths detected by this experiment, which are much shorter and high-energy rays than those previously detected in comets. The Polar spacecraft, launched in February 1996, is currently located in a polar orbit 32,000 mile (apogee) by 3,200 mile (perigee) from the Earth. Polar is part of NASA\'s International Solar-Terrestrial Physics program which includes the Solar and Heliospheric Observatory and Wind spacecraft.
Sounding Rockets - University scientists also will use NASA sounding rockets to gather data about Comet Hale-Bopp. NASA Goddard\'s Wallops Flight Facility (WFF), Wallops Island, Va., will conduct four sounding rocket launches starting March 24 through April 5. The missions will be launched for NASA by the U.S. Navy at the White Sands Missile Range, N.M. The experiments, which will be launched shortly after sunset, are being provided by the University of Colorado, Boulder; University of Wisconsin, Madison; Southwest Research Institute, San Antonio; and Johns Hopkins University, Baltimore. The payloads, launched on two-stage Black-Brant IX rockets to altitudes of 175 to 240 miles, will observe the comet in the ultraviolet wavelengths of light for about five minutes before returning to Earth. The payloads will be recovered following a parachute descent at White Sands.
International Ultraviolet Explorer (IUE) - First observations of Comet Hale-Bopp from space were conducted with International Extreme Ultraviolet Explorer in September 1995. An extensive campaign of synoptic observations of Hale-Bopp were carried out between May and August 1996. Observations of hydroxyl molecules (chemical formula, OH) showed that the water production rate from Hale-Bopp far surpassed any other comet observed at this distance from the Sun. IUE observations ceased in September 1996 when the IUE observatory was turned off after 19 years of operation.
Ground Observations - Dr. Michael J. Mumma of Goddard and his science team used the NASA Infrared Telescope Facility, on Mauna Kea, Hawaii, to make five observing runs of the comet in 1996 and 1997. (A sixth run is scheduled for April 1997.) The scientists are studying the composition of the icy nucleus with infrared spectroscopy. Mumma reports that the team has detected many chemicals which provide insights into the origin of the cometary material. The saturated hydrocarbons, methane and ethane, carbon monoxide, hydrogen cyande, (Methyl) alcohol and water found in the comet are of particular importance. A search for deuterated (heavy) water was conducted to determine whether the Earth\'s oceans could have been created by comets. Additionally, Goddard scientists will be using the Kitt Peak National Observatory\'s new Phoenix infrared spectrometer and the 2.1 meter telescope in March-April 1997 to search for isotopes, isomers, and trace chemicals. Deployment of a new Goddard infrared spectrometer to the NASA 60-inch telescope on Mt. Lemmon, Ariz. in May is planned and will be used for high resolution spectroscopy and imaging of Hale-Bopp.
Extreme Ultraviolet Explorer (EUVE) - Goddard scientists using the EUVE spacecraft detected X-rays in Hale-Bopp in mid-September 1996, and have requested follow-up observations this summer. The scientists searched for soft X-ray emission from Comet Hale-Bopp, to further understand X-ray emission mechanisms in comets. Both images and spectra were acquired. The scientists hope to distinguish between two different models for excitation of X-rays: in the first model, X-rays are produced when higly ionized atoms in the solar wind impact the cometary coma and are converted to the next lower charge state consequently emitting an X-ray. In the second mechanism, solar X-rays are scattered from a population of heretofore unseen tiny dust grains each containing only a few tens of thousands of atoms. Another key objective is to determine the abundance of noble gases, neon and helium, in the comet.
These can be used as thermometers to test the temperature at which pre-cometary ices formed and to constrain the maximum temperatures to which they were later exposed.
Solar and Heliospheric Observatory (SOHO) -The Solar Wind Anisotropy (SWAN) on board the SOHO spacecraft, a joint project of the European Space Agency and NASA, has been recording the large hydrogen could that surrounds Comet Hale-Bopp for months. This cloud is invisible to the human eye, but is detected by its glow in the Lyman-Alpha line of hydrogen, an emission in the ultraviolet region of the spectrum.
Hubble Space Telescope - NASA\'s Hubble Space Telescope made a series of observations of Comet Hale-Bopp, particularly the nucleus, since September 1995. The last observation was made on Oct. 18, 1996. Hubble can not observe the comet during the next few months because the comet is too close to the Sun and Hubble\'s sensitive detectors could be damaged if pointed in that direction. The next possible opportunity will be in August 1997. Scientists are currently studying their research data.
|
