On July 20, 1976, NASA’s Viking 1 lander parachuted safely to the surface of
Mars, revealing an alien world that continues to puzzle scientists and tempt

July 20, 2001: Twenty-five years ago NASA’s Viking 1 lander made history by
parachuting from orbit to the surface of Mars. It was the first probe from
Earth to land intact on the Red Planet, and the first American spacecraft to
land on any world since the Apollo program.

Before Viking 1 touched down many people thought Mars might harbor abundant
plant life and microbes living among the rust-colored rocks. Scientists
guessed the skies might be tinged deep purple like Earth’s stratosphere,
which is about as tenuous as the Martian atmosphere. But Viking 1 and its
sister ship Viking 2, which arrived on Mars a few months later, quickly
dispelled those notions. The landers revealed an alien world with sterile
soil and eerie salmon-pink skies. No plants swayed in the breeze. No animals
scurried from rock to rock.

On the bright side, there were no hostile aliens either. If there was to be
a “War of the Worlds” — like the one popularized by Orsen Well’s famous
1938 radio broadcast — it wouldn’t likely begin on Mars!

The Viking 1 lander set down on Chryse Planitia, a flood plain at 23 degrees
north latitude.

“Chryse Planitia is an interesting place,” says Jim Garvin, Mars program
scientist at NASA Headquarters. “Long ago — perhaps billions of years — it
was the dumping ground for five wide outflow channels apparently carved by
flowing water.” Scientists were attracted to it because of its watery
history. Viking mission planners liked it because it was flat and seemed to
offer a safe landing spot.

“[Before Viking], Mars had been examined from orbit by the Mariner
spacecraft … but the images were on the scale of a football field,”
explains former Viking project manager James Martin of NASA’s Langley
Research Center. “That was the smallest thing we could see and that’s not
very distinct when you consider the landers are only six or eight feet
across. We didn’t have the slightest idea what was on the surface [at such
small scales].” Landing anywhere was risky!

Nevertheless, Viking 1 touched down safely and transmitted its first image
25 seconds later. The lander’s seismometer failed to uncage, and a sampler
arm locking pin was stuck and took 5 days to shake out. Otherwise, all the
experiments functioned as planned. The lander went about its business for
more than six years, from July 20, 1976 until November 1982, substantially
exceeding its design lifetime of 90 days!

On Sept. 3, 1976, Viking 2 set down at Utopia Planitia — a gently-sloping
plain half a world away from the Viking 1 site (6725 km, to be exact).
“Viking 2 landed at 48 degrees north latitude near the Mie crater,” says
Garvin. “It was a very different environment from the flood plains of
Chryse.” The weather at Utopia Planitia was different — a result of its
more northern latitude, there were curious pedestals that scientists thought
might be small volcanoes, and the terrain was littered with the ejecta of
the nearby impact crater.

“It was a bold move,” says Garvin. Indeed, Viking 2 settled with one leg on
a rock tilted at 8 degrees. Nevertheless, the lander performed well for
nearly four Earth-years. Mission controllers shut it down on April 11, 1980,
after its batteries failed.

Like its sister ship, Viking 2 was powered by long-lasting radioisotope
thermoelectric generators, or RTG’s for short. RTG’s create electricity from
heat given off by the natural decay of plutonium. Such a power source
allowed long-term science investigations that otherwise would not have been
possible. (RTG’s also power the distant Pioneer 10 spacecraft, which is
still alive after almost 30 years in space.)

Working steadily for years, the two landers accumulated 4,500 up-close
images of the Martian surface. They also collected more than three million
weather-related measurements — including the first in situ observations of
a global Martian dust storm. Meanwhile, two Viking mission orbiters circling
high above the planet snapped 52,000 images covering 97 percent of the
Martian globe.

The Viking landers were successful in so many ways, but they may be
remembered best as the first space probes to conduct on-the-spot biological
tests for life on another planet. The experiments discovered puzzling
chemical activity in scooped-up samples of Martian soil — but there was no
clear evidence of living microorganisms.

Perhaps that’s not surprising. Garvin explains: “Mars has a thin atmosphere
and no global magnetic field, so its surface is constantly bombarded by
cosmic rays, solar flares, and harsh ultraviolet (UV) light from the Sun.”
UV radiation produces oxygen ions at the Martian surface that destroy
organic molecules. “The sampling arm on the Viking landers dug down only
about 22 cm,” says Garvin. That might not have been deep enough to reach the
“safe zone” for tiny Martian life.

Both of the Viking lander sites were extremely dry desert environments.
“Other sites on Mars, such as nearer the polar caps or other places where
liquid water may be found, are perhaps more likely places to look for signs
of present or past life,” speculates Garvin. “Our long-term plans call for
missions to find liquid water on or under the surface, which will be the
best places to begin looking for signs of life.”

Since the Viking program, NASA’s missions to Mars have included the
ill-fated Mars Observer, the successful Mars Pathfinder lander and Sojourner
rover, the prolific Mars Global Surveyor (still operating in orbit around
Mars), and the Mars Climate Orbiter and Mars Polar Lander — both of which
failed as they neared the Red Planet. The 2001 Mars Odyssey spacecraft is
now more than halfway to Mars and is due to arrive there on October 23rd.
Operating from orbit, the Odyssey spacecraft will not only search for signs
of underground water and interesting surface minerals, but also it will test
the radiation environment to learn if Mars is safe for humans.

The next pair of landers to visit Mars are scheduled to leave Earth in
mid-2003 and to arrive on Mars early the following year. Unlike the Viking
landers, however, the 2003 Mars Exploration Rovers won’t be confined to
their landing sites. The mobile geology laboratories — each the size of a
desk and capable of traveling up to 110 meters a day — will roam the
Martian terrain analyzing rocks and soil.

Like most Mars missions, the rovers will begin their interplanetary journey
when Earth and Mars are close together. Every 26 months the distance between
Earth and Mars shrinks as their orbits bring them together. Astronomers call
the time around closest approach “opposition.” NASA plans to take advantage
of as many Mars oppositions as possible to send probes to the Red Planet
roughly every two years. Future missions on the drawing board include
longer-lasting and more capable rovers, reconnaissance orbiters — maybe
even Mars-exploring airplanes and balloons.

Perhaps one day we humans will go ourselves!

And when we arrive we’ll find a pair of dusty pioneers already there — the
long-lasting, prolific, but now-silent Viking landers. They blazed the trail
for all who followed.