Retro-Rockets to a Red Planet

A NASA robotic geologist named Spirit began its seven-month journey to Mars June 10th, at 1:58:47 p.m. Eastern Daylight Time (10:58:47 a.m. Pacific Daylight Time), when its Delta II launch vehicle thundered aloft from Cape Canaveral Air Force Station, Florida.

The spacecraft, first of a twin pair in NASA’s Mars Exploration Rover project, separated successfully from the Delta’s third stage about 36 minutes after launch, while over the Indian Ocean. Flight controllers at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., received a signal from the spacecraft at 2:48 p.m. Eastern Daylight Time (11:48 a.m. Pacific Daylight Time) via the Canberra, Australia, antenna complex of NASA’s Deep Space Network.

mars_rover
Delta launch rocket for MER mission.
Credit: NASA

All systems are operating as expected.

Spirit will roam a landing area on Mars that bears evidence of a wet history. The rover will examine rocks and soil for clues to whether the site may have been a hospitable place for life. Spirit’s twin, Opportunity, which is being prepared for launch as early as 12:38 a.m. Eastern Daylight Time June 25 (9:38 p.m. Pacific Daylight Time on June 24), will be targeted to a separate site with different signs of a watery past.

"We have plenty of challenges ahead, but this launch went so well, we’re delighted," said JPL’s Pete Theisinger, project manager for the Mars Exploration Rover missions.

The spacecraft’s cruise-phase schedule before arriving at Mars next January. 4, Universal Time (Jan. 3 in Eastern and Pacific time zones), includes a series of tests and calibrations, plus six opportunities for maneuvers to adjust its trajectory.

The milestone of Spirit’s launch provides the impetus to revisit the remarkable journey of the earliest martian missions.

Excerpts from the lively debates that took place prior to the 1976 Viking missions give immediacy and perspective on both the rewards and challenges that the Red Planet offers. In forthcoming issues, Astrobiology Magazine is pleased to commemorate the descriptions offered in the words of then mission contemporaries. NASA historians have compiled these notes in their five-hundred page edition of: On Mars: Exploration of the Red Planet. 1958-1978 (NASA HQ SP-4212).


Touchdown

For many members of the Viking flight team, the early morning hours of 20 July 1976 were the culmination of 8 years of intense activity.

Several of the scientists had more than 15 years invested in preparations for the investigations that would begin once Viking safely landed on the surface of Mars. The focus of everyone’s attention on this day was the Viking I spacecraft in orbit around Mars. Across 348 million kilometers, the team maintained contact with the 3250-kilogram craft from the Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL this night stood jewellike, its brightly lit buiIdings contrasting sharply with the darkened silhouette of the San Gabriel Mountains.

Frost and Viking 2 Lander
White patches of frost on the ground are visible behind the Viking 2 Lander. Click to enlarge.Credit: NASA.

At 5:12:07.1 a.m. PDT July 20, a voice in mission control called out, "Touchdown, we have touchdown!"

A chorus of cheers rose for the event completed 19 minutes earlier on Mars. "We have several indications of touchdown. " Mars local time was 4:13:12 p.m. when Viking I landed on the surface.

Success Not Without Struggles

Mars, because it is reasonably close to Earth, has been the subject of much scientific examination. The Viking project was begun by NASA in the winter of 1968 to make landed scientific investigation of biological, physical, and related phenomena in the atmosphere and on the surface of Mars. The desire to explore for possible life forms on the Red Planet was one of the earliest goals of scientists who became part of the United States space science program, stretching Viking’s roots back to the early 1960s. While NASA’s first attempts to land craft on Mars were successful, that success did not come without a struggle.

The Viking Project Science Steering Group began to consider the interplay between landing sites and Viking lander science during its first meeting in February 1969. Mariner 4 had raised as many questions as it had answered, and data from Mariner 6 (Mariner 6 and 7), soon to be launched, would not be available until next year.

Before collection of scientific information could begin, landing sites for the craft had to be chosen. Data obtained from the 1971 Mariner orbiter assisted the specialists in this task but there was considerable debate over the best places to land, given both scientific interests and engineering constraints. Despite the time and energy given to site selection, Mars held some surprises for the Viking team. The first orbiter photographs, which the team hoped would certify the suitability of the preselected landing sites, showed extremely hazardous terrain.

Hazardous Workplaces

Carl Sagan led off the brainstorming session considering the problem in terms of three primary areas of investigation-biology, geology, and meteorology. Comments on biology centered on the availability of water, atmospheric and surface temperatures, and ultraviolet radiation. Each of these three variables could affect the possibility of finding life forms.

Project Viking’s goal, after making a soft landing on Mars, was to execute a set of scientific investigations that would not only provide data on the physical nature of the planet but also make a first attempt at determining if detectable life forms were present.

Close-up of frost on Martian surface
Close-up of water ice frost, as seen by the Viking 2 Lander. Though it colors the red soil white, the layer of frost is extremely thin perhaps no thicker than a thousandth of an inch. Click to enlarge.Credit: NASA.

The best guess at the time was that Martian life, "or at least that subset of Martian life which the Viking biology package is likely to detect," would be found where there was water near the surface. But there was still considerable debate about the nature and amount of water that might be found. Low atmospheric pressures and temperatures always below 0°C did not augur well for the presence of liquid water. Still, Sagan and others believed that it was possible to have life-sustaining water present in other forms.

One of Viking’s characteristics is its high-risk, high-gain mode of focusing on a search for life. Negative results on all the biologic experiments is not unlikely; the seismometer may never see a quake. To run a billion dollar mission and obtain largely negative results would be embarrassing politically for the projects as well as for NASA as an agency. Whether negative results reflect the lack of life, or the wrong kinds of experiments or the wrong landing locations might be difficult to see….

Thus, the high-imaging system may be considered as the "meat and potatoes" low-risk but guaranteed-significant-gain experiment in the mission. It was excellent insurance against critics who might say that Viking had been too narrowly focused.

Scientists Greet Engineers

In considering landing sites for the two Vikings, some factors would be certain to change. But those that would likely remain unaltered fell into two categories, engineering and scientific.

Under the engineering heading, the 30° south to 30° north latitude range for landing sites was dictated by the angle at which the spacecraft would have to enter the Martian atmosphere to obtain optimum aerodynamic deceleration and proper thermal conditions.

Second, nearly all of the working group members agreed that the lander should sit down where atmospheric pressures were the highest. As on Earth, high pressure corresponds with lower elevation, but whereas sea level pressure on Earth averages about 1013 millibars, surface pressures on Mars are 100 times lower. Pressure at the lowest elevation was believed to be close to 10 millibars and at the top of mountains less than l millibar but the uncertainty in these values was 20 or 30 percent at the time. The Viking scientists hoped that Mariner photographs and ground-based radar studies would give them more exact information on atmospheric pressure relative to topographical features.

A third engineering concern was the effect that Martian surface winds would have on the spacecraft. The Mars engineering model with which the team was working predicted winds of less than 90 meters per second, but Sagan noted that newer calculations indicated the possibility of winds up to 140 to 200 meters per second.

If such winds are encountered during landing maneuvers, the survivability of the spacecraft is very much in question; and such winds, even after a safe landing, might provide various engineering embarrassments.

Other technical factors affecting the choice of a landing spot included the time of day on Mars at touchdown, the size of the landing target, and a pair strategy calling for one very safe (but perhaps less interesting) site and one of greater scientific potential.

viking1orbiter
The Viking 1 orbiter, one of two early providers of Martian data.
Credit: NASA

Sagan thought that it might be desirable to land in the late afternoon to ensure that some lander images of the planet would be transmitted to the orbiter before it passed out of view of the lander, giving the team at the Jet Propulsion Lab maximum assurance of obtaining at least some initial pictures of the surface.

Turning to the target, or landing ellipse, Sagan indicated that it was currently 400 by 840 kilometers, which would eliminate areas appreciably smaller than this zone. The pair strategy had been devised for reasons of "survivability." One landing site would be selected with "safety considerations weighed very highly"; One landing site would be selected with "safety considerations weighed very highly”; if the first mission failed on entry, the team would want to have a preselected, extremely safe site for the second lander.

"It is therefore necessary to consider some sites almost exclusively on engineering grounds." Sagan hoped planners could "back off from this requirement a little bit and seek out safe contingency sites with at least acceptable science.

Alan Binder had made this same point earlier but somewhat more bluntly: "The engineering criteria must reign since it hardly need be mentioned that a crashed lander is not very useful even if it did crash in the most interesting part of the planet."

After considerable freewheeling debate of the kind that characterized many of the working group’s meetings, the group recommended three sites for each lander. It wanted to find water and it wanted to land one craft in the north and one in the south. The mission planners indicated that it would be best to land the first Viking in the northern latitude, or during the Martian summer.

A Picture Worth a Thousand Rocks

Besides searching for landing sites, the experts hoped an orbiter imaging system would return data on the activity of the Martian atmosphere, provide a much better understanding of the geological processes, and perhaps even yield clues to the existence or nonexistence of life. And there was the future to look to, they suggested. "The Viking landers will not be the last spacecraft to land on Mars. Others will surely follow and sites will have to be selected. Our whole lunar experience has been that the prime consideration in selecting any landing site is the availability of imagery."

No judgment could be made about the relative merits of different sites for engineering or scientific purposes without adequate images. "In the past, a [lunar] site without imagery has been rejected immediately. There is little reason to believe that for Mars the decision making process is going to be significantly different."

It was "imperative to collect as much imagery as possible to provide a decision making base for future mission."

Extreme Explorers' Hall of Fame
This composite of MOC daily global images, acquired in early May 2002, shows what the planet looked like in early northern spring. Click for global animation . Credit: NASA/JPL/Malin Space Science Systems, Caption by: K. S. Edgett and M. C. Malin, MSSS

Mariner 8′s launch from Kennedy Space Center on 8 May 1971 ended in failure. Anomalies began to appear in the Centaur stage main engine after ignition. It shut down early, and the Centaur stage and spacecraft fell into the ocean.

After a journey of 167 days, Mariner 9 went into Mars orbit on 13 November 1971, becoming the first spacecraft to orbit another planet.

Orbital parameters were close to those planned, and the spacecraft circled Mars twice a day (11.98 hours per revolution) at an inclination of 65°. Technicians referred to Mariner 9′s path as 17/35 – after 17 Martian days and 35 revolutions of the spacecraft the ground track would begin to repeat itself, giving the specialists the same images under essentially the same solar illumination.

The Great Dust Storm of 1971

The NASA team sent Mariner 9 to the Red Planet at a time when the southern polar cap was shrinking and the southern hemisphere was undergoing its seasonal darkening, and the spacecraft instruments were designed to observe these phenomena. But Mars gave the Mariner scientists more than they had bargained for.

On 22 September 1971, as the spacecraft made its way to its destination, ground-based astronomers noticed a brilliant, whitish cloud, which in a few hours covered the whole Noachis region of Mars. What they saw was the beginning of the greatest, most widespread Martian dust storm ever recorded.

The progress of the storm was amazing. It spread from an initial streaklike core, some 2400 kilometers in length. On 24 September, the dust cloud began to expand more rapidly to the west, blanketing a large area from the east edge of Hellas (a proposed Viking landing site), west across Noachis in three days, a distance two-thirds of the way around the planet. To the north, Syrtis Major was beginning to disappear beneath the haze. On 28 September, a new cloud developed in Eos, a region later found to be part of the canyon lands of Mars.

Peter Boyce, of the Lowell Observatory in Flagstaff, Arizona, reported that his observations taken in the blue-light spectrum had shown a reduction in contrast for several prominent features days before the dust cloud was visible to astronomers. This indicated that Martian dust had been drawn up into the atmosphere some time before the actual cloud could be seen. By the end of the first week in October, clouds or storms had engulfed nearly the entire planet. A zone about 12000 kilometers long had been obscured in only 16 days. Prospects were dim for a successful mapping of the planet when Mariner 9 reached Mars on 13 November. At Mariner mission control, there were some worried people, and the Viking team worried along with them.

On 8 November, the first pictures of Mars came back from the spacecraft. While these were essentially calibration shots designed to check out the television system, they were large enough to give a reasonably good view of the planet. But the dust was all-pervasive; no detail could be discerned. One scientist, in a bit of gallows humor, suggested that they must have visited Venus by mistake, since that planet is perennially blanketed by clouds. His remark was not well received.


JPL, a division of the California Institute of Technology, Pasadena, manages the Mars Exploration Rover project for the NASA Office of Space Science, Washington, D.C.

Related Web Pages

Evidence for Snow on Mars – and Perhaps an Abode for Life?
Mars Odyssey web site (with new images)
MARIE instrument
Valles Marineris
Olivine
Mars by Stories
Impact Crater Landing Sites for the 2003 Mars Exploration Rovers
Mars Exploration Rover Homepage
2003 Mars Exploration Rover Mission