by Gideon Marcus

Into the Wild Black Yonder

Ten years ago, when we started our planetary series of articles, none of other worlds in our solar system had been explored.  Since then, five intrepid spacecraft have toured two planets.  Mariner 2 and Mariner 5 probed Venus, returning the revelation that the shrouded world is a seething cauldron.  Mariner 4 returned the first pictures of the Red Planet, shocking humanity with images of Moonlike craters and reports of a vanishingly thin atmosphere, dashing forever the vivid, science fictional conception of Mars as an inhabitable world.

Now, twin Mariners 6 and 7 have flown by Mars, dramatically increasing what we know about the fourth planet.  While we'll never get back that fantasy so elegantly woven by Edgar Rice Burroughs and Leigh Brackett, the new Mars is also not a blasted husk either.

Distant views of Mars, as seen by Mariner 7 as it approached the planet

The Next Generation

Despite the blow to morale given by Mariner 4, Mars still seemed like the most hospitable place for life to have arisen apart from Earth.  After all, are there not microorganisms living in the harsh environments of Antarctica and at the bottom of the sea?  Even though the Martian atmosphere is just 1% as thick as that of Earth, this is still plenty dense compared to, say, the Moon.  Moreover, Earth's atmosphere is just 1% that of Venus.  Who's to say when an atmosphere is "thick enough"?

So, just a few months after Mariner 4 flew past Mars, Mariners 6 and 7 were authorized.  At first glance, they look a lot like their predecessor, but the differences are profound—both internally and externally.

From top to bottom: Mariners 2, 4, 5, and 6/7 (note the family resemblance of the last three—Mariner 5 was actually a modified Mariner 3/4 backup!)

First, the insides: the new Mariners are the first spacecraft made only to examine their target planet.  All of the prior Mariners had experiments for monitoring the interplanetary environment—solar wind, magnetic fields, that sort of thing.  Mariners 6 and 7 carry two TV cameras (one narrow, one wide-angle), an infrared radiometer (to measure the temperature of Mars), and ultraviolet and infrared spectrometers (to determine the chemical makeup of the Martian atmosphere and surface).  That's it.

As for the outsides, since 1965, when Mariner 4 passed by the Red Planet, there has been a revolution in communications technology.  Not only do the new Mariners carry more powerful transmitters and antennas, but with the construction of the new 210 foot antenna at Goldstone, supplementing the old 85 footers, data can be transferred between the spacecraft and Earth at a rate more than 2000 times the 8.33 bits per second speed of Mariner 4.  It also helps that Mars is closer to Earth this time around, and that the rocket carrying Mariners 6 and 7 is the beefy new Atlas Centaur, which can loft more weight than the old Atlas Agena so the onboard electronics can be heftier and thus more capable.

The 210' "Mars Dish" at Goldstone, California

What this means for us on the ground is that instead of sending back just 22 images of Mars, the new Mariners could transmit hundreds of pictures, all while returning real-time spectrographic and radiometer data.  All of this aided by the installation of the first computer equipped on an interstellar probe, capable of remembering 128 "words" some 22 characters in length.  And that computer can be reprogrammed on the fly from Earth!

On their Way

Jet Propulsion Laboratory, the same folks who built the other Mariners, assembled four spacecraft for the mission.  The first was a stay-at-home test model, the second a source of spare parts.  The other two were redundant probes—an understandable precaution given the loss of Mariners 1 and 3.  However, the twin ships weren't entirely redundant; Mariner 6 was targeted to fly over the Martian North Pole while #7 was aimed over the South Pole.

Problems with the spacecraft began before liftoff.  Mariner 6's Atlas rocket, which maintains its structure through internal fuel pressure, sprung a leak and began to deflate like a balloon.  It had to be replaced with Mariner 7's rocket, and a new one ordered from Convair.  This did not delay the launch, however (which had to go at a set time to reach Mars with a minimum of fuel use), and Mariner 6 blasted off on February 24th.  Mariner 7 took off on March 27, but because of its course, was set to reach Mars just five days after its sister.

The launch of Mariner 7

Both rockets performed beautifully, requiring only minor mid-course corrections early in the flight to ensure they zoomed close by the fourth planet.  There were some minor technical problems: The radio on Mariner 6, used to determine range from Earth, kept locking on its own signal rather than Earth's, making it useless.  It fixed itself later in the flight, however.

Similarly, the star tracker designed to keep Mariner positioned properly lost sight of Canopus.  After weeks of engineers scrambling to find an alternative guiding star (they even tried the Large Magellanic Cloud, but the galaxy was too diffuse to be useful), that system fixed itself, too.  Finally, the onboard solar sensors that told how much sunlight was hitting Mariner 6's power panels, began reading too low.  Was the Sun going out?  No.  The sensors had just drifted out of calibration.

Mariner 7's only issue was a radio receiver that dropped to about 20% of its sensitivity, apparently due to cold.  Ground controllers switched it to high power, which warmed the thing up and fixed it.

Thus, its vexing teething pains dealt with, NASA now had, for the first time, two fully operating probes with which to explore Mars.

The Great Galactic Ghoul

On July 29, even as Mariner 6 was finishing the transmission of 33 low-resolution approach images, Mariner 7 suddenly began spinning wildly, all of its scientific data telemetry channels scrambled.  Had an asteroid hit the spacecraft?  Had there been an instrument explosion or some kind of short circuit?  Was there some kind of Great Galactic Ghoul guarding the Red Planet?  No answer was quickly forthcoming.

Collage of Mariner 6 images as it approached Mars

Nevertheless, engineers raced to salvage the mission—with Mariner 7 arriving shortly after its sister, and from a more favorable angle, the JPL science team wanted the spacecraft's experiments all in working order. 

Cautiously, computer engineers went over every bit of code and methodically tested all of Mariner 7's instruments.  They were in working order, but because of the accident, uncalibrated and useless.  How to get real data points to use to base the radiometer and spectrometer data against?

Engineers at the Mariner control center at JPL

As it turned out, the two TV cameras on board were in good order and unaffected.  By pointing them at Martian targets and using the data they returned, it was possible to calibrate the other experiments.  And so, just in the nick of time, Mariner 7 was ready, come close encounter time, to do some real science.

Exploring Barsoom

So what did the two probes find as they whizzed past Mars, almost grazing it from a scant few thousand miles away?

Well, at first they seemed to confirm Mariner 4's findings.  There were all the craters in stark detail.  There was no evidence that there had ever been widespread water—absent was the erosion one would expect from oceans or even rivers.

A lunar landscape, courtesy of Mariner 7

On the other hand, if Mars wasn't Earth's twin, neither was it sister to the Moon.  As each Mariner went behind the planet, beaming radio signals through the Martian atmosphere, it was confirmed that surface pressure was around 7 millibar—a refinement rather than a revelation.  But they did determine that carbon dioxide makes up a greater percentage of the air than even on Venus.  Nitrogen was completely absent, which was a surprise.  So was ozone, which means that the surface is fairly baked by ultraviolet—again, a strike against life on the planet.

The Red Planet is not quite geologically moribund, however.  The vast Hellas region, smooth of craters, and a region of convoluted terrain akin to the American Badlands, suggests some kind of volcanic activity in comparatively recent times.

Unlike the Moon, clouds scud across the Martian sky, mostly composed of dry ice.  While it may not rain on the planet, it does frost, and maybe even snow ice and carbon dioxide.  The climate changes with the seasons, with polar (dry?) ice caps spreading and receding.  The tropical highs soar to a balmy 60 degrees, but the polar lows plunge to 240 degrees below zero.

A view of the Martian North Pole, snapped by Mariner 6—note the ice cap

Thus, Mars is an inhospitable place…but it if it lacks biological life, it is nevertheless an interesting living, breathing planet in its own right.

What's next?

Mariners 6 and 7 are still functioning, and their onboard systems should work until at least 1971.  Not only might they return pictures of any asteroids or comets that drift by, they will also constitute an experiment in and of themselves.  As they drift through the solar system, terrestrial scientists will measure variations in the timing of their telemetry signals and use them to prove General Relativity—something that requires great distances to detect subtle theoretical variations.

As for successors, a Martian orbiter is already in the works for the 1971 alignment, and in 1973, a probe will use the gravity of Venus to enable a probe to fly by and then visit, for the first time, Mercury, the closest planet to the Sun.

And also in 1973, the Viking orbiter/lander combo, successor to the overlarge Voyager project, will give Mars a real look.

The 1971 Mars Orbiter

If the 1960s were the dawn of interplanetary science, the 1970s will see its maturity.  I find this as momentous an achievement as footprints on the Moon.

I can't wait to rewrite all of the articles in our solar system series!

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