BOSTON — IT'S been 16 years since American metal touched the sands of Mars. Now a new United States spacecraft is ready to head for the Red Planet.
Unlike the Viking craft that sent robot laboratories to the Martian surface in 1976, the National Aeronautics and Space Administration (NASA) Mars Observer will merely circle the planet. But it will have a planet-probing capability as powerful as that of the best Earth-observing weather and remote-sensing satellites. Working from a 378-by-350-kilometer (235-by-218-mile) high near-polar orbit, the spacecraft will compile the first comprehensive database of Martian geophysics and climate.
"When we finish this mission, we'll know Mars," says project scientist Arden L. Albee at the California Institute of Technology's Jet Propulsion Laboratory (JPL). The Pasadena, Calif., lab manages the mission for NASA.
Dr. Albee explains that to "know Mars" means "we'll have a better [geophysical/environmental] database than we have for Earth." Referring to NASA's planned Earth Observing System (EOS) satellite-based study of our own planet, Albee says Mars Observer is going to do its survey "the way we'll do it for Earth." He adds that "EOS can learn from this mission."
A Titan 3 rocket now is nearly ready at the Cape Canaveral Air Force Station in Florida. It sits at the same Pad 40/41 complex that launched the Viking missions in 1975. If the rocket sends Mars Observer on its way during the Sept. 16 to Oct. 13 launch window, the 2,487-kilogram (5,483-pound) craft should begin its survey mission late next year. NASA would have to wait two years for Earth and Mars to again be in a favorable position if it misses this year's launch opportunity.
Kennedy Space Center technicians at the Cape are working on a tight schedule. They passed a major milestone Aug. 3 when they mated the spacecraft to the upper-stage rocket that will boost it to Mars from an initial parking orbit. JPL's project manager Glenn Cunningham notes that "our teams have been willing to do what was necessary to have this milestone happen as close to schedule as possible."
It will take 11 months for the spacecraft to cover the 740 million kilometers (450 million miles) to Mars. Assuming a Sept. 16 launch, it would arrive next Aug. 19. JPL controllers then will take several more months to ease the craft into its working "sun-synchronous" orbit. This will bring the craft across the equator on the sunlit side at 2 p.m. local Mars time on every pass. Mars Observer data will thus be gathered systematically at roughly the same time of day all over the planet.
Scientists are counting on this kind of coordination among the data, as well as on the precision of their instruments, to sharpen their Martian knowledge. "It's not certain any one instrument will make an astounding discovery," Albee says adding, "It's the synergy between all the observations that's important."
Mars Observer has a battery of instruments that, as a group, use most of the electromagnetic spectrum. At the lower frequency end of that spectrum, some experiments use radio waves to probe the atmosphere. Some instruments sense infrared (heat) radiation. At the high frequency end of the spectrum, gamma-ray detectors prospect for minerals.
Two visible-light cameras and a laser altimeter will map the Martian surface with a precision never attained before. The altimeter can fix the heights of areas 160 meters (525 feet) wide to within 2 meters (7 feet). A wide-angle general-science camera can resolve details as small as 240 meters (800 feet) across. Only about 15 percent of Mars has so far been mapped to that degree of detail. And a high-resolution camera can resolve detail as small as 3 meters (10 feet) across for selected targets.
THIS sharp-eyed camera has raised hopes that the spacecraft could glimpse one of the Viking landers or the so-called "face." The latter is a shadow-etched pattern that looks like a human face together with features that, with some imagination, can look like artificial structures. There's no scientific support for speculation that these features are the handiwork of intelligent beings. But scientists as well as the speculators would like to get high-resolution pictures of them.
This will be difficult. Controllers can't aim the high-resolution camera, which can only image what comes into its view as it orbits the planet. Moreover, controllers can't leave the camera turned on all the time. That would choke its data-handling capacity. To image a specific target, they have to know exactly where it is and when it will pass through the camera's rapidly moving view field. That's not easy.
Project scientist Albee calls the prospects for spotting specific targets "problematic." He explains that the wide-angle camera will map the whole planet, "but at high resolution, it's little postage stamps here and there."
One of the main effects for which scientists will be looking is the grand interplay of atmosphere and surface as Mars goes through its seasonal cycles. Large shifts of moisture and carbon dioxide occur across the equator as first one hemisphere goes into winter and then the other. Water and even some of the carbon dioxide that constitutes most of the tenuous atmosphere condense out at the winter pole. Although the atmosphere is very thin - with a surface pressure only 1/125 that on Earth - this seasonal circulation involves vast wind patterns and sometimes planet-enveloping dust storms.
Mars Observer's nominal mission will last one Martian year - 687 Earth days - to include all four seasons. It should return more data than all previous planetary spacecraft combined, except for the Magellan radar mapper now finishing its survey of Venus. Scientists should be able to use these data to gain a much deeper insight into the processes that have shaped all of the terrestrial planets - Mercury, Venus, Earth, as well as Mars.
IN this sense, Mars Observer marks the end of the era of initial planetary exploration. It also provides a transition to a new era in which Mars exploration will be a more cooperative international venture.
Mars Observer scientific teams include about a dozen Russian scientists as well as British, French, and German participants. The spacecraft also carries a special radio receiver to relay signals from a French-supplied instrumented balloon that is part of the Russian Mars96 mission. The balloon will collect atmospheric and surface data.
The two long-planned Russian Mars missions to launch, respectively, in 1994 and 1996 are rapidly becoming true international projects. Both missions are to place orbiters around the planet. Mars94 also will carry landers and penetrator probes to study the surface.
Russia's ability to fully fund these missions now is uncertain. France and Germany are providing financial aid that may amount to as much as 10 percent of the Mars94 mission cost to help stabilize its funding. NASA is considering buying one of the mission's three landers to carry American experiments to the surface.
Roger Bourke at JPL points out that the benefits of cooperation are already "even more rich" than these prospects may seem. The Russian Mars craft will carry the same French-inspired radio relay unit as does Mars Observer. These could help the Mars Environmental Survey (MESUR) program that NASA is considering.
Instead of using large costly spacecraft such as Mars Observer, MESUR would send 16 smaller and cheaper landers to set up a surface instrument network on Mars over a several-year period beginning in 1999.
Dr. Bourke, who is mission engineer for this project, notes that the European Space Agency (ESA) is considering a similar type of "Marsnet" program. He expects that, if either the US or ESA formally approves one of these projects, both parties would likely join forces with Russia and perhaps Japan to make Marsnet a fully international program. "This isn't the kind of competitive race the space program was in 20 years ago," he observes.