In the grand scheme of things, it wasn’t all that long ago that the entire body of knowledge of our solar system was built solely with Earth-based observations. Turning first their naked eyes to the heavens, and then a succession of increasingly complex and sensitive optical and radio telescopes gathering light from all across the spectrum, our astronomically curious forbears did a commendable job working out the broad strokes of what’s going on in the neighborhood.
But there’s only so much information that can be gathered by instruments operating at the bottom of a roiling ocean of air, so when the opportunity to send instruments to our planetary neighbors began to be possible some 60 years ago, scientists started planning how to accomplish it. What resulted was the Mariner program, a series of interplanetary probes launched between 1962 and 1973 that performed flyby missions of the inner planets.
The list of accomplishments of the Mariner program is long indeed, and the number of firsts achieved by its ten spacecraft is impressive. But it is Mariner 4, the first flyby mission of Mars, which set the stage for a lot of the science being done on and around Mars today, and the first mission where NASA wisely took a “pics or it didn’t happen” approach to planetary science. It was the first time a TV camera had traveled to another world, and it was anything but a sure bet that it would pay dividends.
First Steps to the Stars
Given that we were still climbing the earliest and steepest part of the spaceflight learning curve in the late 1950s, it was pretty audacious to start thinking about reaching out to our neighboring planets. But by the change of the decade, the broad outlines of what would come to be known as the Mariner program were already worked out. They were designed as limited-scope missions — as opposed to the “Grand Tour” missions that were soon to come — of the inner planets.
While each of the ten Mariner missions had specific scientific goals in mind, and each spacecraft was custom built for the purpose, there was an overarching theme: learn how to explore outer space. At the time of Mariner’s conception, Russia and the United States had flown precious few missions, most of which were either suborbital or into low Earth orbit. Given the exigencies of the Cold War, a lot of those missions were devoted to one-upmanship and proving that one nation or the other had captured the high ground, and with relatively short-shrift paid to planetary science, very little was known about how to get to outer space and how to operate there.
Mariner was also designed to take advantage of the new Atlas rockets, the first intercontinental ballistic missile in the US fleet. When paired with the Agena second stage, the stack was capable of lifting larger payloads and achieving Earth escape velocity. The Atlas-Agena combination would launch the first five Mariner probes, with more successes than failures — Mariner 1 and Mariner 3 were both lost due to guidance issues and payload fairing damage respectively.
All the Mariner spacecraft were designed around a more-or-less common bus, either hexagonal or octagonal, which was designed to maximize the use of space inside the Agena payload fairing. The probes were all solar-powered, with the number of panels depending on which way they were going — two if heading sunward to Venus or Mercury, or four for the Mars-bound missions, to soak up more of the weaker sunlight. The solar panels were built from extremely light aluminum panels, corrugated for rigidity, and each was adorned with 7,056 fantastically expensive solar cells. The four panels on Mariner 4 were stored folded under spring pressure; they were deployed once in orbit with pyrotechnic squibs that pulled out a retaining pin.
And a Star to Steer Her By
The Mariner probes were also the first spacecraft designed to take advantage of communications infrastructure offered by the growing Deep Space Network, with large dish antennas that could be pointed toward Earth. This raised the problem of accurate attitude control — constant contact with the DSN would require precision positioning in space to a degree that had yet to be achieved.
Prior to the Mariner program, navigation and attitude control of spacecraft was a relatively simple task. Keeping an Earth-orbiting spacecraft pointed in the right direction involved little more than detecting the biggest, brightest thing in the sky — the Earth itself. There were subtleties, of course, but they were nothing compared to the challenges of interplanetary travel, especially since the task had yet to be tried.
The Mariner spacecraft would all require a two-position fix in space to maintain the correct attitude. The sun was an easy target, especially since keeping the solar panels more or less oblique to the sun was critical to maximize power production. Sun sensors on the sun-facing upper deck of Mariner provided inputs to the central computer and sequencer (CC&S), the controller for the entire spacecraft, that activated nitrogen jets located around the spacecraft to control its pitch and yaw.
Mariner’s second fix, which would control the roll axis passing through the waveguide of the dish antenna, was based on the star Canopus. Located in the southern constellation of Carina, Canopus is the second-brightest star in the night sky after Sirius. It has been an aid to navigation since antiquity, and its characteristic brightness, as well as its position well below the plane of the ecliptic, make it perfect for interplanetary navigation.
Mariner 3 and 4 were the first spacecraft to include Canopus star trackers. The task of picking one particular star out among a field of thousands isn’t trivial. The Canopus tracker consisted of an optical telescope with light baffles to keep stray light from the detection electronics, which consisted of a special vacuum tube called the image dissector. This was essentially a scanning photomultiplier tube that could look at a small slice of space as the spacecraft slowly performed a search maneuver around its roll axis. The light from any star coming into view produced a signal proportional to both its intensity and its location, as determined by where in the image dissector’s scanning field it was spotted. When a star matching the intensity of Canopus was found, the tracker sent a roll-error signal to the reaction control jets, which then kept Canopus locked in view.
The Canopus star tracker took a bit of time to get working properly on Mariner 4. While it was able to lock onto Canopus, intermittent bright signals would appear unexpected in its field of view, causing the tracker to lose lock on Canopus and start another cycle of “star hopping”. Flight controllers puzzled over this for a while until determining that a cloud of dust particles and paint flecks whizzing along through space with Mariner 4 was the likely culprit — even something tiny that close to the tracker could appear to be as bright or brighter than Canopus. The problem was fixed by turning off a CC&S routine used to protect the star tracker from Earth-shine, as Mariner was by then far enough away from home that the star tracker could ignore it.
The seven scientific instruments Mariner 4 carried were mainly concerned with capturing data about the electromagnetic realm between Earth and Mars. It would be the first time a spacecraft had a chance to sample the interplanetary medium, and so the instrument suite included a magnetometer, a cosmic ray telescope, radiation detectors, a plasma probe, and a cosmic dust collector to measure micrometeoroid impacts. This science loadout would serve as a model for the scientific payloads of many subsequent interplanetary probes.
While all these instruments were important, Mariner’s designers recognized that stimulating the public interest was an important soft goal of the mission, and in the early 1960s, there was no better way to win the public’s hearts and minds than television. And so well in advance of the mission, a special television camera was designed to take pictures of Mars as Mariner sped past.
The design of the camera, which centered around a special vidicon tube, was outlined in a film hosted by “Mr. Wizard” himself, Don Herbert — a move further calculated to capitalize on the power of television.
The original plan for Mariner’s camera was to keep it covered until just before the Mars flyby started. But with the experience of the dust cloud interfering with the Canopus tracker, controllers decided to jettison the camera’s lens cap early, to give any debris a chance to disperse. The camera worked flawlessly, capturing 22 narrow-angle photos of the surface from about
10 9,800 km altitude. The pictures revealed well-defined and numerous craters, putting to rest any remaining hope that Mars still had a thick atmosphere and liquid water.
While Mariner 4’s up-close look at Mars was disappointing to some, especially those who still harbored visions of lush vegetation crisscrossed with ancient canals, it was proof that Mars was a far more foreign and interesting place than our home planet. And Mariner 4 had proved that not only could the interplanetary space be navigated safely, but that it was possible to send instruments there and do useful science, even across the yawning gulf of space. In a very real way, the Mariner probes set the standard for planetary science, and paved the way for us to reach out to Mars and beyond.
[Thanks to Noel for the idea to dive into this topic.]