Published: December 23, 2003
ars passed closer to Earth this summer than it had in thousands of years, and now three emissaries from Earth are about to repay the neighborliness — and then some. They are set to descend on the planet, and stay.
The three visiting spacecraft, two of them carrying robotic roving vehicles, will be searching the Martian surface for signs of life or conditions conducive to life, at least in the distant past. Their quest is the latest scientific response to an abiding human fascination about the world next-door, a place cold and arid but sufficiently Earthlike to inspire visions of extraterrestrial life.
The Europeans are to arrive first, early Christmas morning (late Christmas Eve, in American time zones). That is when Beagle 2, a 73-pound lander named for Darwin's ship of discovery, is scheduled to parachute to the surface of a broad basin, Isidis Planitia, and begin looking for traces of life.
Two American spacecraft are close behind. They are to deliver to the Martian surface identical 400-pound roving vehicles about the size of golf carts. Their destinations are places where spacecraft orbiting the planet have observed evidence suggesting the presence of water, one of life's essentials, at some time in the past.
One of the robotic rovers, called Spirit, is on course to land on Jan. 3 near the center of Gusev Crater, which geologists suspect once held a lake. The other, Opportunity, is scheduled to reach Meridiani Planum on Jan. 24. The landing site, geologists say, appears to be rich in an iron oxide mineral that usually forms in association with liquid water.
If all three missions succeed, their investigations are expected to test the working hypothesis behind current Mars exploration. For a multitude of reasons, scientists think that Mars was once a much more dynamic planet, warmer and wetter. It could thus have harbored some living organisms, perhaps beginning at about the same time, more than 3.6 billion years ago, when life emerged on Earth.
But mission officials are only too aware that success cannot be taken for granted.
"The easy part is over," said Dr. Edward J. Weiler, NASA's associate administrator for space science. "Just getting to Mars is hard, but landing is even harder."
Two-thirds of all flights to Mars have failed. Only one of the six attempts by Russia to land there made it, and that craft ceased transmissions 20 seconds later. The United States has had three successful landings: the two Viking spacecraft in 1976 and Mars Pathfinder, with its robotic rover, Sojourner, in 1997. But two years later, the Mars Polar Lander disappeared in its descent toward the southern polar region.
The loss of the polar lander, combined with the failure of its companion orbiter, led to a management shake-up at the Jet Propulsion Laboratory in Pasadena, Calif., which developed the craft, and the cancellation of a landing mission that had been set for 2001. More money and more rigorous testing than before went into this year's two missions, formally known as the Mars Exploration Rover project, estimated by NASA to cost $820 million.
"We have done everything we know that could be humanly done to ensure success," Dr. Charles Elachi, director of the Jet Propulsion Laboratory, said in a recent statement. "We have conducted more testing and external reviews than for any previous interplanetary mission."
The plague of failures claimed another victim two weeks ago. Japanese space officials announced that they had abandoned efforts to steer their Mars-bound spacecraft back on course. It was supposed to go into orbit of the planet last week for geophysical studies and a close-up examination of the two small Martian moons. But malfunctions had driven the craft off its intended trajectory.
The two American spacecraft and the first Mars explorer of the European Space Agency have had virtually trouble-free flights. Beagle 2 successfully separated from its mother ship on Friday and is on course for a landing.
Looking on the bright side, David Lavery, NASA's program executive for the Mars rovers and coordination with the European mission, said: "Historically, there have been only three successful landings on Mars. In the span of only one month, we may double that number."
The spacecraft were launched early this summer, the two American ones from Cape Canaveral, Fla., and the European one from Russia's Baikonur Cosmodrome, in Kazakhstan. Their destinations were selected by geologists poring over maps and photographs of Martian equatorial regions.
Even though Viking landers 27 years ago failed to detect unambiguous signs of microbial life, orbiting spacecraft have tantalized scientists with numerous images of what appear to be dry lake beds and sinuous channels, ancient coastlines and other landscapes that could have been shaped by running water.
On the strength of this, and more evidence of water ice at the poles, scientists have revived speculation that Mars may have had enough liquid water for life to have gained a foothold there in ancient times, and that it has perhaps survived in warm springs and cavities deep underground.
Dr. Cathy Weitz, the chief NASA scientist for the rover program, said that the Gusev and Meridiani sites appeared to have "evidence for past and persistent water."
Gusev is a four-billion-year-old crater the size of Connecticut. A channel cutting through the crater, geologists say, probably brought water and sediments into what could have been a lake. Meridiani attracted attention because remote sensors on an orbiting spacecraft detected large deposits of gray hematite, an iron oxide mineral that is typically, though not invariably, formed on Earth in the presence of water.
Each of the rovers, Spirit and Opportunity, carries identical sets of geological instruments. Cameras mounted atop the rover's mast will survey the lay of the land in color, the first step in planning travels over the site. An infrared instrument will classify rock types from a distance.
Dr. Steven W. Squyres of Cornell, the principal scientist for rover operations, said it may be eight or nine days after touchdown before either rover is ready to roll.
Each is much more mobile, capable of traveling about 125 feet a day, than its predecessor, Sojourner, which managed little more than a daily three feet. In three months of planned travels, each rover could cover about 2,000 feet. But the rovers' pace will be deliberate and slow. They have "about the same mass and top speed of a Galápagos tortoise," Dr. Squyres remarked.
As planned, when the rovers approach an interesting rock, they are to analyze its chemical composition with spectrometers, examine its texture and mineralogy with a microscopic imager and scrape beneath its weathered surface with a tool at the end of the robotic arm, the equivalent of the geologist's handy field hammer. The objective is to reveal how and to what extent water was involved in the origin of Martian rocks.
For example, if Spirit finds sedimentary rocks at Gusev, the size, uniformity and shape of their grains could provide important evidence of the watery conditions when they formed. Was the water moving or still? Were there mudslides or tumbling currents? Similarly, scientists expect the texture and distribution of hematite deposits at Meridiani to reveal the mineral's origins. In a lake, from rain or snow, warm springs or only water vapor in the atmosphere?
Geologists may find that water had nothing to do with this hematite. It could have come from iron-rich volcanic eruptions.
Observations by the Mars Global Surveyor, a spacecraft orbiting the planet, have raised questions about the water hypothesis as an explanation for the ample evidence of erosion on arid Mars.
In a report in the Aug. 22 issue of the journal Science, a research team led by Dr. Philip R. Christensen of Arizona State University said that a six-year survey by the spacecraft found a scarcity on the planet's surface of limestone and other carbonates usually formed in the presence of liquid water. The amounts of carbonates, they concluded, appeared to be too small to have come from large bodies of surface water. If so, Mars may never have had a warmer, more humid past.
Dr. Squyres conceded that the findings placed limits on how warm and wet Mars could have been in earlier times. But he said, "The evidence we have for water at the two landing sites is utterly independent of the carbonates."
Mars Express, the main spacecraft of the European mission, is equipped with seven remote-sensing instruments to conduct observations of the planet's thin atmosphere and surface geology for a full Martian year — 687 Earth days. Its lander, Beagle 2, will be immobile, concentrating its search for signs of life in the soil where it comes to rest.
Beagle 2, developed by British scientists and engineers, is designed to operate for at least six months.
The lander's robotic arm and boring devices will collect soil samples for study in a gas analyzer with 12 ovens. A mass spectrometer is to examine the heated gases for any carbon signature of life, past or present. It is an ambitious undertaking for Europe's debut in Mars exploration.