NASA is developing a new Mars exploration strategy in fiscal year 1995. The Mars Surveyor program calls for start of development of a small orbiter that will be launched in November 1996 to study the surface of the red planet. The Mars Surveyor orbiter will lay the foundation for a series of missions to Mars in a decade-long program of Mars exploration. The missions will take advantage of launch opportunities about every two years when Mars comes into alignment with Earth. NASA requested $77 million in development costs in FY 1995 for the new Mars orbiter. The 1995 fiscal year runs from October 1, 1994, to September 30, 1995.
The Mars Surveyor program will be conducted within the constraints of a cost ceiling of approximately $100 million per year. The orbiter will be small enough to be launched on a Delta expendable launch vehicle and will carry roughly half of the science payload that flew on Mars Observer, which was lost on August 21, 1993. The specific instruments will be selected later.
NASA's Jet Propulsion Laboratory issued a request for proposals to industry in March to solicit potential spacecraft designs. Selection of a contractor to build the spacecraft will be made by July 1. NASA envisions an orbiter/lander pair of spacecraft as the next in this series of robotic missions to Mars. The orbiter planned for launch in 1998 would be even smaller than the initial Mars Surveyor orbiter and would carry the remainder of the Mars Observer science instruments. It would act as a communications relay satellite for a companion lander, launched the same year, and other landers in the future, such as the Russian Mars '96 lander. The U.S. Pathfinder lander, set to land on Mars in 1997, will operate independently of the Mars orbiter. The 1998 orbiter/lander spacecraft would be small enough to be launched on an expendable launch vehicle about half the size and cost of the Delta launch vehicle. JPL will manage mission design and spacecraft operations of the Mars Surveyor for NASA's Office of Space Science, Washington, DC.
Two NASA-sponsored scientists have produced the first-ever detailed, three-dimensional reconstruction of one of the thousands of near-Earth asteroids, those whose orbits bring them extremely close to Earth. Scott Hudson of Washington State University and collaborator Steven Ostro of the Jet Propulsion Laboratory created the computer model of the double-lobed asteroid 4769 Castalia from radar data obtained in 1989 by Ostro and others, using the Arecibo Observatory in Puerto Rico. The asteroid was discovered by Eleanor Helin of JPL at the Palomar Observatory in 1989.
"This computer model of Castalia represents the first detailed, three-dimensional reconstruction of a solar system body from radar data," Hudson said. The effective resolution in the reconstruction is about 100 meters. At just under 2 kilometers across, Castalia is smaller than any solar system object that has been imaged by spacecraft, including the two asteroids, Gaspra and Ida, recently imaged by Galileo.
Ostro said that previously it was very difficult to interpret radar images of small, irregularly shaped bodies. But with the development of this new reconstruction technique, the scientific value of radar observations has been dramatically increased. "I hope that the Castalia model will enhance interest in a program of exploration of these small bodies, including both Earth-based observations and spacecraft missions," he said. "A radar- derived model of a target asteroid would make close maneuvering easier, and the mission easier and cheaper."
Ostro also noted that the Castalia model verifies the suspicion of many astronomers that the near-Earth asteroids would prove to be the most irregularly shaped worlds in the solar system. "Understanding the origins of those shapes, especially the detailed role of collisions, is an important theoretical challenge," he said. The scientists believe that the double-lobed shape of Castalia shown by the model resulted from a gentle collision between two separate asteroids some time in the past.
Nearly 300 near-Earth asteroids are currently known. It is estimated that more than 1000 as large as Castalia, plus 100 million as large as a house, remain to be discovered. Most of them are thought to have been thrown into the inner solar system from the main asteroid belt, between Mars and Jupiter, by long periods of gravitational interaction with the planets. With unstable orbits, they eventually might be thrown out of the solar system by the same forces or possibly collide with planets. The scientists believe that continuing improvements in radar telescopes, expanded optical programs to search for near-Earth asteroids, and modeling techniques like this one will provide greatly increased knowledge of the properties and histories of these strange, nearby worlds.
Two astronomers have discovered that our Milky Way galaxy and most of its neighboring galaxies, contained within a volume of the universe one billion light-years in diameter, are drifting with respect to the more distant universe. This startling result may imply that the universe is "lumpier" on a much larger scale than can be readily explained by any current theory. "The new observations thus strongly challenge our understanding of how the universe evolved," says Dr. Tod Lauer of the National Optical Astronomy Observatories (NOAO).
This surprising conclusion comes from the deepest survey of galaxy distances to date, conducted by Dr. Lauer and Dr. Marc Postman of the Space Telescope Science Institute. The two astronomers used NOAO telescopes at Kitt Peak National Observatory, Arizona, and Cerro Tololo Inter-American Observatory, Chile, to study galaxy motions over the entire sky out to distances of over 500 million light years. They explored a volume of space about 30 times larger than had been surveyed previously.
The expansion of the universe causes all the galaxies in the volume surveyed to be moving away from us. Galaxies at the edge of the volume are receding from us at 5% of the speed of light. The large "flow" of neighboring galaxies that Postman and Lauer discovered comes from looking at residual galaxy motions after the expansion of the universe has been accounted for. The flow means that the nearby universe, as well as expanding, appears to be drifting with respect to the more distant universe.
Astronomers generally assume that the diffuse glow of microwave radiation left over from the Big Bang provides the backdrop or rest frame of the universe. In the mid 1970s, astronomers found that temperature of this radiation is slightly hotter toward the direction of the constellation of Leo. This effect has been interpreted to mean that the Milky Way is drifting with respect to the rest of the universe at about 380 miles per second in this direction. It has also been assumed that most of this motion is caused by the gravitational attraction of more distant galaxies; however, these galaxies have never been positively identified.
In the mid 1980s a group of astronomers surveyed the motions of galaxies out to about one-third the distance studied by Lauer and Postman, finding the galaxies to be flowing as a group with respect to the more distant universe. This team postulated that this flow was due to the gravitational pull of a large concentration of galaxies dubbed "The Great Attractor." However, these galaxies are located deep within the volume surveyed by Postman and Lauer and are not massive enough to cause the observed rate of drift.
In fact, the new result implies that the Milky Way and its neighbors are affected by much larger concentrations of mass at much larger distances than can be easily explained by popular theories of how the universe is organized. Lauer and Postman started their project in 1989 to measure the drift of the Milky Way with respect to 119 clusters of galaxies located all over the sky at distances as far as 500 million light years. If the motion of the Milky Way was caused by galaxies closer to us than the distant clusters, as was then presumed to be the case, then its motion with respect to the clusters should have been essentially identical to that with respect to the microwave background radiation.
Because the galaxy clusters are at a variety of distances from us, galaxies in the more distant clusters appear dimmer than the ones more nearby. However, once the various distances are accounted for, the brightest galaxy in each cluster is always found to give off roughly the same amount of light. Astronomers refer to such objects as "standard candles." The distances to the clusters are estimated from how fast they are moving away from us as the universe expands.
If the Milky Way Galaxy is drifting, however, its motion makes measurement of the expansion speeds dependent on which direction we are looking. If the drift is not corrected for, then the cluster galaxies will appear to vary slightly in brightness in a smooth pattern across the sky. Postman and Lauer used images of the cluster galaxies to detect this pattern and determine the motion of our own galaxy.
The motion of the Milky Way that Postman and Lauer measured from the distant clusters is in a completely different direction from that inferred from the microwave background. The most likely solution to this dilemma is that the clusters themselves are moving with an average velocity of 425 miles per second toward the constellation of Virgo. Because of the enormous size of the volume containing the clusters, however, this implies the existence of even more distant and massive concentrations of matter.
Most theories explaining the structure of the universe predict that the universe should be nearly uniform on the scale of the Lauer and Postman cluster sample. The motion of the Milky Way and its neighbors would then be due to concentrations of mass relatively close by.
If, instead, the portions of the universe as big as a billion light years in diameter are still drifting with respect to the larger universe, then the universe has structure or "lumps" of matter on much larger scales than predicted by most theories. The detection of galaxy flows across large volumes of space should improve our understanding of how the universe came to be organized the way we see it today.
A more provocative but probably less likely interpretation of the Postman and Lauer result is that the large volume of clusters really is at rest, with the temperature variation of the microwave background around the sky being a relic of the conditions of the Big Bang, rather then being caused by the motion of our galaxy. In this case, the microwave temperature variation would tell about the properties of the very early universe rather than about large-scale motions of galaxies.