MARS TO RECEIVE NASA'S STRANGE VISITOR FROM ANOTHER PLANET.

Byline: Tony Knight Daily News Staff Writer

Earth invades Mars!

Space probe inflates air bags, lands with a bounce. Six-wheeled invader emerges, seems to be tasting rocks.

If there were a Martian press, those would be the headlines in less than two weeks as the unmanned NASA probe Mars Pathfinder marks the return of an Earth presence on the Red Planet after a 21-year absence.

In sharp contrast to last Independence Day, when swarms of aliens invaded Earth in the movie of the same name, this Fourth of July a lone spacecraft will streak across the Martian heavens to a landing on a sandy plain more than 100 million miles away.

Under a pink Martian sky, the tetrahedron-shaped Pathfinder will deflate its air bags, open like a blossom and deploy a tiny rover named Sojourner, whose mission is to ``taste'' Mars rocks to see what they're made of.

It will be the first time the public gets a chance to see pictures from the surface of another planet since two NASA Viking landers arrived on Mars in 1976.

And it promises to be one of the most spectacular NASA missions for the earthbound public, with real-time pictures from the Martian surface broadcast on public TV stations July 4 and available on a string of mirror-image Web sites capable of accommodating 56 million hits per day.

The Pasadena-based Planetary Society will hold a three-day Planetfest at the Pasadena Civic Center on July 4-6, giving the public a front-row seat for the mission with a 25-foot television screen broadcasting images fresh from Mars and NASA TV mission programming.

The lander and rover are equipped with dual cameras whose images will be processed by engineers at Pasadena's Jet Propulsion Laboratory and put up on the World Wide Web as soon as they are available to mission scientists.

``It fits in with what (NASA Administrator) Dan Goldin called our virtual presence on Mars,'' said David Dubov, the JPL's Mars Pathfinder Webmaster. ``He had this vision of people sitting at home, either watching TV or on their computer, and feeling like they're with us at mission control or with us on the surface of Mars.''

This is one of the first of NASA's new discovery missions, the ``faster, better, cheaper'' approach to space exploration in the 1990s. Unlike the multibillion-dollar ``Battlestar Galacticas'' of the past, the new missions are to cost about $150 million in planning and development.

Each mission accomplishes less, but there are more of them, and it avoids catastrophes such as the failure of the $1 billion-plus Mars Observer orbiter in 1992.

``We really view this as the start of a long-term campaign at Mars,'' said NASA spokesman Doug Isbel. ``If one of these fails or falls short of its goals, we've got another in the pipeline. The next Mars lander is already being built, and it's scheduled for launch Jan. 3, 1999.''

An Earthlike planet

Why Mars? It's the most Earthlike of the terrestrial planets of the inner solar system - a whole world with no oceans, but a land area roughly equivalent to that covered by Earth's seven continents.

``It has atmosphere. It rotates on its axis. We have everything necessary to support life on Mars,'' said Matthew Golombek, chief mission scientist. ``So Mars is our future, in a sense, if we're going to live on other planets.''

A Martian year is 687 Earth days long, and a Martian day is 24 hours and 37 minutes. But the atmosphere of Mars is very thin, mostly carbon dioxide, and unbreathable by humans. The planet is much colder than Earth, being so much farther from the sun. And it's also much smaller. A 150-pound Earthling would weigh only 57 pounds on Mars.

Mars has polar ice caps, and images of the Martian terrain taken by previous orbiters show signs that the planet once had plenty of water flowing on the surface. Scientists say it was probably much wetter and warmer in the early solar system.

Many scientists believe life could have arisen on Mars during this period.

Those views were bolstered last year with the discovery of what appears to be indications of some form of carbon-based life in an ancient meteorite found in Antarctica that is thought to have been blasted from the Red Planet by a collision with some primordial asteroid.

Pathfinder is headed toward a relatively flat region in Mars' northern hemisphere called Ares Vallis. Images taken by previous Viking orbiters indicate that a massive flood once washed over this area.

``The equivalent of about all the water in the Great Lakes carved this channel in about two weeks,'' Golombek said. ``It flooded out in a big fan onto this plain.''

The landing site was picked for its flatness, which makes for a safer landing, and because scientists think many types of Mars rocks will be found here, all washed down by the great flood.

The rover is equipped with a sensor called an alpha proton X-ray spectrometer that will tell scientists the composition of every rock sampled. But it won't have any instruments that could detect life.

The Mars team was laughing about a recently published cartoon that showed two Martians peering over a crater rim at the little rover, its arm dutifully extended to a rock.

``I threw it a rock,'' says one Martian. ``I think it likes it.''

Understanding the rocks will tell scientists how the planet was formed and under what conditions, and from that they might be able to extrapolate answers to where all the Martian water has gone and whether life could have existed there.

Parachute and air bags

JPL engineers and scientists are on pins and needles as the spacecraft streaks toward Mars at more than 48,000 mph because of the recent jinx that's plagued Mars missions.

Mars Observer was lost just as it arrived at the Red Planet in 1992, probably due to a fuel explosion. And the Russian Mars '96 mission, launched last fall about the same time as Pathfinder, met disaster when the fourth stage of its booster failed and the orbiter fell back to Earth.

``Getting to the surface of Mars is my responsibility,'' said mission chief engineer Rob Manning, whose team designed and built the novel air-bag landing system.

``I'll tell you, the rover team, I really hope they're successful, but even if they're not and we get down safely, I'm still going to have the champagne,'' said the 32-year-old Pasadena resident.

Mars Pathfinder is aiming to land somewhere in a 100-mile-wide circle. Hitting that spot is about the equivalent of hitting a bottle cap with a guided missile at a range of 16 miles.

``It's extremely hard to hit a target that small because we're coming straight in,'' said Richard Cook, mission manager.

There is no plan to orbit. Pathfinder, which is slowing as it is captured by Mars' gravity, will hit the thin Martian atmosphere at 17,000 mph, its heat shield turning red-hot from friction.

The atmosphere impact slows the craft to about 1,000 mph, when a parachute deploys, slowing it to 120 mph. Now the air-bag cocoon inflates, and that's when the fun starts.

Three retrorockets fire, completely stopping the lander at about 50 feet above the surface for two seconds. The tether to the parachute is cut, and Pathfinder plunges to the surface, bouncing as many as 10 times before coming to rest.

CAPTION(S):

2 Photos

Photo: (1) Rob Manning, chief engineer of the Pathfinder mission to Mars, stands next to a scale model of the lander at the JPL in Pasadena.

(2) Pathfinder's rover heads for the lander in the JPL's Mars Room, a mock-up of the Martian surface.

David R. Crane/Daily News