Mariner 4: Our First Close Look at Mars
In the grand scheme of things, not so long ago, the body of knowledge of our solar system was built only with observations from the earth. Turning first their naked eyes to the sky, then a succession of increasingly complex and sensitive optical and radio telescopes capturing the light of all spectra, our astronomically curious ancestors did a commendable job in determining the broad outlines of this happening in the neighborhood.
But there is only a limited amount of information that can be gathered by instruments operating on the bottom of an ocean of choppy air, so when the opportunity to send instruments to our planetary neighbors began to crumble. be possible about 60 years ago, scientists began to plan how to accomplish it. The result was the Mariner program, a series of interplanetary probes launched between 1962 and 1973 that performed overflight missions of the inner planets.
The list of achievements of the Mariner program is long and the number of firsts achieved by its ten spacecraft is impressive. But it was Mariner 4, the first Mars flyover mission, that paved the way for much of the science being performed on and around Mars today, and the first mission where NASA wisely took an approach. photos or it did not happen ”. 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 towards the stars
Given that we were still climbing the earliest and steepest part of the learning curve of spaceflight in the late 1950s, it was quite daring to start thinking about reaching out to our neighboring planets. But by the turn of the decade, the broad outlines of what would become the Mariner program had already been defined. They were designed as limited-range missions – as opposed to the soon-to-be “Grand Tour” missions – of the inner planets.
While each of the ten Mariner missions had specific science goals in mind, and each spacecraft was custom-built for that purpose, there was one overarching theme: learning to explore outer space. By the time of Mariner’s design, Russia and the United States had performed few valuable missions, most of which were either suborbital or low earth orbit. Given the demands of the Cold War, many of these missions were devoted to outbidding and proving that one nation or another had captured the heights, and with relatively little expense paid to planetary science, it was known very little about how to get into 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. Paired with the second Agena stage, the stack was able to lift larger payloads and reach Earth’s escape velocity. The Atlas-Agena combination would launch the first five Mariner probes, with more success than failures – Mariner 1 and Mariner 3 were both lost due to guidance issues and payload fairing damage respectively.
All Mariner spacecraft have been designed around a more or less common, hexagonal or octagonal bus, 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 direction they were heading – two if they were heading the sun towards Venus or Mercury, or four for missions to Mars, in order to absorb more of the weaker sunlight. The solar panels were constructed from extremely light aluminum panels, corrugated for rigidity, and each was adorned with an incredibly expensive 7,056 solar cells. The four panels of Mariner 4 were stored folded under the pressure of a spring; they were deployed once in orbit with pyrotechnic firecrackers which removed a retaining pin.
And a star to guide her
The Mariner probes were also the first spacecraft designed to take advantage of the communications infrastructure offered by the growing Deep Space network, with large satellite dishes that could be pointed at Earth. This posed the problem of precise attitude control – constant contact with the DSN would require precision positioning in space to a degree that had not yet been achieved.
Before the Mariner program, the navigation and attitude control of spacecraft was a relatively straightforward task. Keeping a spacecraft orbiting Earth pointed in the right direction involved little more than sensing 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 attempted.
The Mariner spacecraft would all need a two-position cue 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 essential to maximize electricity production. Solar collectors on Mariner’s upper deck facing the sun provided inputs to the mainframe and sequencer (CC&S), the controller for the entire spacecraft, which activated the nitrogen jets around the spaceship to control its pitch and yaw.
Mariner’s second patch, which controlled the roll axis passing through the satellite dish waveguide, 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 choosing a particular star from a group of thousands is not trivial. The Canopus tracker consisted of an optical telescope with light deflectors to prevent stray light from the detection electronics, which consisted of a special vacuum tube called an image dissector. It was basically a scanning photomultiplier tube that could watch a small slice of space as the spacecraft slowly performed a search maneuver around its roll axis. Light from any star in sight produced a signal proportional to both its intensity and location, as determined by where in the scan field of the image dissector it was spotted. When a star matching Canopus’ intensity was found, the tracker sent a roll error signal to the reaction control jets, which then kept Canopus locked in sight.
The Canopus star tracker took a little while to function properly on Mariner 4. While it was able to lock onto Canopus, intermittent light signals appeared unexpected in its field of view, causing the tracker to lose lock. on Canopus and starting another cycle. of “star jump”. Flight controllers were puzzled about this for a while until they determined that a cloud of dust particles and paint stains streaking through space with Mariner 4 was likely the culprit – even some something tiny as close to the tracker could look as bright or brighter than Canopus. The issue was resolved by disabling a CC&S routine used to protect the star tracker from the Earth’s glow, as Mariner was then far enough away from home that the star tracker could ignore him.
The seven scientific instruments carried by Mariner 4 were primarily aimed at capturing data in the electromagnetic domain between Earth and Mars. This would be the first time that a spacecraft had the chance to sample the interplanetary medium, and the instrument suite therefore included a magnetometer, cosmic-ray telescope, radiation detectors, a plasma probe, and a solar collector. cosmic dust to measure the impacts of micrometeoroids. This science load would serve as a model for the science payloads of many subsequent interplanetary probes.
While all of these instruments were important, the designers at Mariner recognized that fostering the public interest was an important mission objective, and by the early 1960s there was no better way to win the hearts and l public mind than television. And so long before the mission, a special TV camera was designed to take photos of Mars as Mariner speeded past.
The design of the camera, centered around a special vidicon tube, was described in a film presented by “Mr. Wizard ”himself, Don Herbert – an even more calculated move to capitalize on the power of television.
The original plan for Mariner’s camera was to keep it covered just before the Mars flyby began. But with the experience of the dust cloud interfering with the Canopus tracker, the controllers decided to drop the camera lens cap earlier, to give the debris a chance to disperse. The camera performed flawlessly, capturing 22 narrow angle photos of the surface of approximately
ten 9,800 km altitude. The images revealed well-defined and numerous craters, putting an end to 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 much more alien and interesting place than our home planet. And Mariner 4 had proven that not only could interplanetary space be safely navigated, but it was possible to send instruments and do useful science into it, even through the yawning chasm of space. In very tangible ways, the Mariner probes set the standard for planetary science and paved the way for us to reach Mars and beyond.
[Main image: Mariner 4 Image of Mars via NASA]
[Thanks to Noel for the idea to dive into this topic.]