A team of U.S. and German astrophysicists have made the first-ever detection of X-rays coming from a comet. The discovery of a strong radiation signal -- some 100 times brighter than the most optimistic predictions -- was made March 27, during observations of Comet Hyakutake using Germany's orbiting ROSAT satellite.
"It was a thrilling moment when the X-rays from the comet appeared on our screen at the ROSAT ground station," said Konrad Dennerl of the Max Planck Institute for Extraterrestrial Physics. Following the initial detection, the team reported repeated X-ray emissions from the comet over the next 24 hours. The comet was near its closest approach to Earth at a distance of less than 10 million miles when it was first detected by ROSAT.
The strength and rapid changes in intensity of the comet's X-ray emission both surprised and puzzled astronomers. "We had no clear expectation that comets shine in X-rays," said Michael J. Mumma of Goddard Space Flight Center. "Now we have our work cut out for us in explaining these data, but that's the kind of problem you love to have."
The comet was examined repeatedly during March 26 and 27 as it swept across the sky. The German scientists were able to correct satellite attitude for the comet's motion during each observation, and produce accurate images with the aid of a computer.
X-rays have never been observed from a comet before, and scientists had optimistically predicted an intensity that turned out to be about 100 times weaker than the radiation actually detected by ROSAT. Strong changes in the brightness of the X-rays were another surprise. There were pronounced changes in the X-ray brightness from one ROSAT observation to another, typically over a time difference of a few hours.
Still another puzzle is what generates the X-rays. The ROSAT image may contain clues: In the image, the X-rays seem to come from a crescent-shaped region on the Sunward side of Hyakutake.
Explaining the unexpected bright X-ray emission is the next major task for the science team. One preliminary theory is that X-rays from the Sun are absorbed by a cloud of water vapor surrounding the comet's nucleus and then reemitted by the water molecules in a process called fluorescence. According to this idea, the cloud is so thick that its Sunward side absorbs nearly all the incoming solar X-rays so that none reach the remainder of the cloud, which might explain why the X-ray emission source is crescent- shaped, rather than spherical around the comet's nucleus.
A second possible explanation is that the X-rays are produced from the violent collision between the cometary material and the supersonic solar wind of plasma and particles streaming away from the Sun.
"We always learn something new when we study an object at different wavelengths," commented Carey M. Lisse of Goddard, the leader of the X-ray investigation. "Now we have to determine why the comet is so bright in X-rays and see what we can learn about its structure and composition from these unique images."
An advanced, lightweight scientific instrument designed to produce visible and short-wave infrared images of Earth's land surfaces has been selected for the first NASA New Millennium Program mission dedicated to Mission to Planet Earth. The Advanced Land Imager will serve multiple purposes, according to Dr. Charles Kennel, NASA Associate Administrator for Mission to Planet Earth.
The new instrument will demonstrate remote-sensing measurements of the Earth consistent with Landsat data collected since 1972 for resource monitoring and assessment. In addition, it will acquire data with finer spectral resolution, long sought by Earth observation data users, and it will lay technological groundwork for less costly, more compact land imaging instruments in the future.
"We looked at nearly a dozen different mission concepts in some detail, and a land surface imaging mission clearly was at the top of this year's priority list," Kennel said. "It will ultimately enable first-class science by validating breakthrough technology with clear potential capabilities, both commercially and to the future of NASA's Earth Observing System."
The Advanced Land Imager represents about a sevenfold decrease in mass and electrical power demands compared to the current Landsat 5 multispectral instrument. In addition, it extends existing measurement capabilities by incorporating an advanced high-resolution hyperspectral imaging spectrometer-on-a-chip. This novel, wide-field observing system requires no scan mirror. It is built around a lightweight, integrated silicon carbide structure and optical system with an innovative in-flight calibration system.
Under project management by Goddard, the Advanced Land Imager will be developed by a team of industry partners led by MIT's Lincoln Laboratory, which will provide open access to design and performance information to expedite transfer of this technology into the U.S. commercial sector.
The instrument will feature 10-meter ground resolution in the panchromatic band and 30-meter ground resolution in its other spectral bands, using a four-chip, multispectral focal plane array that covers seven of the eight bands of the current Landsat. Hyperspectral capabilities, which further split these bands into highly differentiated images, will be tested to show that they can be combined into traditional Landsat- equivalent datasets.
The spacecraft support structure, including advanced electrical power and data-handling subsystems, will be provided by Swales & Associates, Beltsville, Maryland, and Litton Industries, College Park, Maryland. Additional advanced spacecraft technologies will be made available through the New Millennium Integrated Product Development Teams.
The power and data subsystems will be provided through a Space Act cost-sharing agreement that calls for Litton to develop the hardware and integrate it into the New Millennium spacecraft, while providing the company with a two-year license to commercialize the technology. "This innovative arrangement, which includes a major commitment from Litton to integrate and deliver the hardware, represents an exciting new way of doing business for Goddard," said Center Director Joseph Rothenberg.
Further industry partnerships in the mission will be solicited in a workshop to be held during upcoming advanced definition studies. The total NASA cost of the first New Millennium Earth science mission, including its Small Expendable Launch Vehicle, has been capped at $90 million. Launch is planned for late 1998.
The current mission concept for the flight has the spacecraft flying autonomously several minutes ahead of the ground track flown by the planned Landsat 7 satellite to provide accurate paired-scene comparisons between new and traditional observing technologies. Evolutionary versions of the Advanced Land Imager would be candidates for flight on future generations of Earth Observing System missions, beginning with the AM-2 spacecraft.
Astronomers have discovered an object toward the center of the Milky Way galaxy that exhibits a combination of behaviors never before seen in the 35-year history of gamma-ray astronomy. During the first day it was observed, the source produced over 140 powerful bursts of gamma-rays; since then, it has settled down to a daily rate of about 20 bursts, and it is currently the brightest source of hard X- rays and gamma-rays in the sky.
The discovery was described in a paper published on February 29 in Nature by scientists from Marshall Space Flight Center, the University of Alabama in Huntsville, Massachusetts Institute of Technology, and the University of Amsterdam. The unusual object in the southern sky was discovered in early December 1995 by researchers using the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. Since December 2, the new burster has produced more than 1000 hard X-ray bursts. "We're particularly excited about the discovery of a new X-ray source," said Marshall astrophysicist Gerald Fishman. "The object's strange behavior is one of the major discoveries in X-ray astronomy in the past decade."
The skies had more surprises in store. In mid December, the observers discovered an additional source of steady radiation from the same position as the burster. This new object further surprised scientists when it was observed to continuously emit pulses about twice per second _ a pulsar. The question the observers faced was "What was the relation, if any, between the two objects?" said Chryssa Kouveliotou of the Universities Space Research Association at Marshall.
The answer was soon apparent -- the burster and the pulsar had one and the same source.
"The properties of this X-ray source are unlike those of any we know," explained Kouveliotou. "The burst repetition rate makes this phenomenon very different from gamma-ray bursts that we have observed several thousand times from throughout the universe. Also, the longer duration and persistent bursting makes the object very different from so-called Soft Gamma Ray Repeaters, which have been observed to burst in short, isolated episodes separated by several years."
"What's unique about this object is that it does so many different things all at once," said Fred Lamb, an astrophysicist at the University of Illinois at Urbana-Champaign. "We've seen some sources that play the drums, some that crash cymbals, and a few that play the trumpet, but this source is a one-man band."
This bursting pulsar was later found by Mark Finger, Universities Space Research Association at Marshall, to be part of a binary system, performing one full revolution around its low-mass companion every 12 days. "The most likely explanation at this time is that the bursts of X-ray energy may result when the lighter of the pair of stars loses its material by gravitational or magnetic forces to the neutron star," said Kouveliotou. A neutron star is an exotic star with a mass greater than the Sun and a diameter of only about 10 miles. "The discovery of the new X-ray source may lead to a better understanding of how neutron stars form and evolve," Kouveliotou said.
The source was also observed by NASA's Rossi X-ray Timing Explorer (RXTE) spacecraft, which carries the largest collecting area of X-ray detectors ever flown in space. The two large instruments on the spacecraft, provided by teams led by Jean Swank, Goddard Space Flight Center, and Richard Rothschild, University of California at San Diego, quickly found the source to be very bright across the X-ray band from 2 to 60 keV, with strong persistent emission as well as numerous bursts.
"First, matter is accelerated to half the speed of light because of the neutron star's enormous gravitational force. Then, it crashes into the surface of the neutron star and is heated to nearly one billion degrees," Lamb explained. "Because it is so hot, it radiates almost entirely in X-rays rather than visible light, in this case with a power comparable to 1 million times the power of the Sun originating from an area about the size of the National Mall in Washington, D.C."
RXTE made repeated scans across the source to determine its position accurately, allowing observers to identify a radio source and a very faint visible star in the direction of the X-ray source. Since the bursting pulsar is a transient X-ray star that is expected to die out within a few months, scientists are working feverishly to try to unravel its mysteries while it still shines.