In the decade between mid-1964 and late 1975, the United States' National Aeronautics and Space Administration (NASA) launched nine robotic spacecraft -- seven Mariners and two Vikings -- to explore Mars. All but two reached the planet and sent the first close-up pictures and scientific observations back to scientists on Earth. Mariner 9 and the Vikings went into orbit around the planet and the Vikings placed two landing craft on the surface of Mars.
During roughly the same time frame, the former Soviet Union launched eight spacecraft to Mars. Unfortunately, few of them returned useful scientific data about the planet, although a number of spacecraft achieved orbit or delivered surface capsules.
The Mariner spacecraft were relatively small scientific explorers, launched by the Atlas rocket with either the Agena or Centaur upper stages. They weighed less than half a ton (without onboard rocket propellant). The Viking Orbiter spacecraft were scaled-up Mariners, each carrying a lander sealed in a sterile atmospheric-entry capsule plus enough rocket propellant to place the compound vehicle in orbit and then send the capsule into the atmosphere and then to the ground.
The design of these missions was based on the developing communications capability of the Deep Space Instrumentation Facility -- later renamed the Deep Space Network -- and the concept of an attitude-stabilized, solar-powered spacecraft that could orient solar panels at the Sun, a communications dish antenna at the Earth and a camera or other sensors at the target planet. The Mariners and the Viking Orbiters were developed by NASA's Jet Propulsion Laboratory. The Viking project and its lander development were managed by NASA's Langley Research Center in Hampton, Va.
Mariners 4, 6 and 7 flew by the planet, spending less than an hour in close proximity. Mariner 4, equipped to measure the interplanetary environment as well as Mars, continued to send useful scientific data for three years. Mariner 9 spent 11 months mapping the planet from orbit. The two Viking orbiters lasted two and four years, respectively, in orbit. The pair of landers operated for almost two years and more than four years, respectively, on the surface of Mars.
The Mariner series actually began with a 203-kilogram (446 pound) spacecraft developed in 11 months for a flight to Venus. Mariner 2 carried no camera, but made important measurements at the planet and in interplanetary space. Mariner 4 then was launched in November 1964 on an eight-month odyssey to Mars, where in July 1965 it collected the first close-up photographs of another planet.
The pictures, played back from Mariner 4's small tape recorder over a long period of time, showed impact craters like those on the Moon, some of them sprinkled with frost in the chill of a Martian evening. After its encounter with Mars, Mariner 4 flew behind the planet from Earth, letting scientists use its deep-space radio link as a probe to measure the density of the Martian atmosphere right down to the surface.
In 1969, Mariner 6 and Mariner 7 completed the first dual mission to Mars, flying by the equator and south polar regions and analyzing the atmosphere and surface with remote sensors, as well as recording and relaying hundreds of pictures. By chance, both flew over cratered regions and missed both the giant northern volcanoes and the equatorial grand canyon. Their approach pictures did, however, show the dark features long seen from Earth (and named "seas" and "bays"), but no canals. Indeed, the three early Mars Mariners changed the perception of the planet from one that could possibly harbor life to a frozen, cratered, moon-like world -- a perception that soon changed again.
The first spacecraft to orbit Mars was Mariner 9, launched in May 1971. Achieving orbit that November, Mariner 9 observed that a great dust storm had obscured the whole globe from view. Since 1969, Mariner spacecraft operations such as science sequencing and pointing had been programmable, using simple flight computers with limited memory, and the spacecraft used a digital tape-recorder rather than film to store images and other science data. Two Soviet Mars orbiters, arriving right around the same time, were controlled by fixed clocks and recorded pictures on photographic film. They helplessly shot all their pictures of dust clouds.
Mariner 9 was able to wait until the storm abated, the dust settled and the surface was clearly visible to compile its global mosaic of high-quality images of the Martian surface. For the first time the giant shield volcanoes of the Tharsis region, the 4,000-kilometer (2,500-mile) grand canyon named for Mariner and many river-like channels became visible. Mariner 9 also transmitted the first close-up photographs of Mars' two small, irregular moons, Phobos and Deimos. A detailed Mars globe was created from more than 1,500 images of Mariner 9's 7,329 images.
The Viking Orbiters deepened the global view presented by Mariner 9, especially in the case of full-color images or high resolution studies of selected surface features. The Viking landers sent back horizontal landscape panoramas from the landing gear to Mars' horizon during every season of the Martian year, as well as long-term climate records, surface chemical analyses, studies of physical properties and searches for Martian life at the two landing sites, Chryse Planitia and Utopia Planitia.
From orbit, Mars displayed a greater variety of terrains than was found on the Moon or Mercury and appeared to still be evolving and changing, with winds driving the erosion process and playing a powerful role in surface changes. Many areas revealed the effects of flooding and volcanic activity that had occurred in Mars' distant past, at least 50 million years ago and, perhaps, more like several billion years ago. Where the floodwaters came from and what happened to them remained a puzzle. No liquid water and almost no atmospheric water were found; the deeply layered north polar cap, with dust and permanent ice layers under seasonal carbon-dioxide dry ice deposits, was almost the only Martian water observed.
The local weather at the landing sites was uniformly cold and dry. Viking 1 measured high and low temperatures of between minus 30 degrees and minus 86 degrees Celsius (minus 22 degrees and minus 123 degrees Fahrenheit) the day after landing. Winds reached 29 kilometers per hour (18 miles per hour) with gusts up to 52 kilometers per hour (32 miles per hour). Soil analysis found almost 100 times as much sulfur present in the Martian terrain as was the case in either terrestrial or lunar soil, with iron and magnesium found in abundance among other expected chemical elements. The biology and organic chemistry experiments discovered chemical activity in surface samples and some other surprising results. But the mission uncovered no evidence of organic chemicals and or other clear evidence that life had ever existed at the Viking landing sites.
The next logical step in the exploration of Mars was a mission that could provide scientists with a global profile of the planet's surface and atmosphere over the course of a full Martian year, the equivalent of two Earth years. Mars Observer was to take that next scientific step. The spacecraft was launched on September 25, 1992, atop a Titan III launch vehicle and spent 11 months cruising to Mars. But after a 71l-million-kilometer (442million-mile) journey, and just three days short of entering orbit around Mars, the spacecraft fell silent. One black-and-white approach picture of Mars had been taken before the spacecraft was lost, showing clear skies over the planet.
To recover from this loss, NASA quickly set in place a decade-long program of Mars exploration that would fulfill the science objectives of the Mars Observer mission and take scientists much further along in their knowledge and studies of this planetary neighbor. In 1996, NASA plans to launch two spacecraft to Mars: Mars Global Surveyor, a scaled down version of Mars Observer, and Mars Pathfinder, a lander and rover designed to demonstrate new ways of landing and exploring the surface. Every two years after that, another pair of landers, orbiters or combinations of the two will be launched to Mars. NASA's new 10year-long program is called the Mars Surveyor program.
US AND USSR (*) MARS EXPLORATION MISSIONS LAUNCHED
Title, Launch Date, Mission, Description (LOS - Loss of Signal)
*Mars 1, 11/1/62, Mars probe, LOS at 65.9 million miles
Mariner 3, 11/5/64, Mars probe, shroud failed
Mariner 4, 11/28/64, Mars flyby 7/14/65 w/photos, LOS 12/20/67
*Zond 2, 11/30/64, Mars probe, failed to return planetary data
Mariner 6, 2/24/69, Mars flyby 7/31/69 w/photos, lasted to 12/70
Mariner 7, 3/27/69, Mars flyby 8/5/69 w/photos, lasted to 12/70
Mariner 8, 5/8/71, Mars orbiter; launch vehicle malfunctioned shortly after liftoff
*Mars 2, 5/19/71, Mars orbiter/lander arrived 11/27/71, no useful data returned
*Mars 3, 5/28/71, Mars orbiter/lander, arrived 12/3/71, data & few photos
Mariner 9, 5/30/71, Mars orbiter, operated in orbit 11/13/71 to 10/27/72
*Mars 4, 7/21/73, failed Mars orbiter, flyby 2/10/74
*Mars 5, 7/25/73, Mars orbiter, arrived 2/12/74, few days
*Mars 6, 8/5/73, Mars orbiter/lander, arrived 3/12/74, little data return
*Mars 7, 8/9/73, Mars orbiter/lander, arrived 3/9/74, little data return
Viking 1, 8/20/75, Mars orbiter/lander, orbited 6/19/76-1980, landed 7/20/76-1982
Viking 2, 9/9/75, Mars orbiter/lander, orbit 8/7/76-1987, landed 9/3/76-1980
*Phobos 1, 7/7/88, Mars/Phobos orbiter/lander, LOS 8/89 en route to Mars
*Phobos 2, 7/12/88, Mars/Phobos orbiter/lander, LOS 3/89 near Phobos
Mars Observer, 10/25/92, LOS at Mars arrival 8/21/93
Sources: TRW Space Log, On Mars (NASA SP-4212).
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