Mariners to the Red Planet
Interview with William Momsen
Joining Caltech’s Jet Propulsion Lab forty years ago, William Momsen was in charge of making sure Mars could be seen up close.
Momsen worked on the Mariner series, which were the first spacecrafts to leave Earth for the outer reaches of our solar system. The Mariners were conceived as small-scale, frequent explorers to the near planets in our solar system: Mars, Venus and Mercury.
|Mariner IV flyby across face of Mars, July 14, 1965. Click image for larger view|
In 1964, while the Mariner IV spacecraft beamed back its first images of Mars –and the first close views of any planet, Momsen had the responsibility to transition their martian flyby from a wide angle to a tight one degree field of view. The spacecraft followed on the coat tails of a mission that never reached Mars, Mariner III. "Actually," noted Momsen the tracking command "was sent twice, to make sure. The time came, the button was pushed, and then the eternity of the wait. If it didn’t work, there wouldn’t have been time to restart the platform and try again…We held our collective breaths."
"Since it would not be known exactly where the spacecraft would be as it passed Mars," within 6,118 miles, noted Momsen, "the TV camera was mounted on the scan platform. A few hours before it reached the planet, power was applied, and the platform bobbed up and down. When the planet came in view of the wide-angle (40 degree field of view) sensor, the platform-driving motor switched from scanning to tracking mode, which kept the scan sensor, and hence the TV camera, pointed directly at the planet. When the planet came in view of the TV camera with a narrow field of view (one degree), it started taking a series of pictures as the spacecraft swept across the planetary surface."
The Mariner missions greatly modified our scientific views about Mars. As late as the 1950′s, Mars was still thought a possibly habitable world of canals, since telescopes could not resolve much on the surface other than what some believed were connected waterways. When Mariner IV imaged frost on some craters and a cloud in the atmosphere, the mission team was startled and at first suspected a malfunction. "No one had been to Mars before, so we really had no idea what to expect." When Mariner IV revealed a crater-ridden portion of the red planet, Mars looked less hospitable. Momsen wrote: "Among many important discoveries, [Mariner] consigned the ‘canal’ theory to limbo".
The Mariner series showed that interplanetary exploration was possible. It was the "longest and most complex deep space mission" attempted. Some of the earliest Mariner technologies seem today to be scrappy. Coloring of Mars images necessarily included teletype numbers drawn in with various shades of gold and brown crayons. The microwave signal transmitted with less power than a modern cellphone, but from 100′s of millions of miles away. The total spacecraft power used could fire up two 100-watt light bulbs.
|Mariner mission control, JPL, 1965.|
Credit: NASA/Caltech JPL
The flyby mission took eight months to get to Mars, but lasted 24 minutes taking pictures. JPL issued about a dozen progress reports in 12 months. The magnetic tapes recording from radio dishes in Canberra, Australia, Johannesburg, South Africa, and Goldstone, California were limited to only parts of pictures because of an 8 bit per second transmission from Mars. These tapes were mailed to JPL via courier to be reassembled into pictures covering 150 mile areas on Mars.
As JPL described the process at the time, "After the pictures are finally processed, they will be released to the public as scientists continue their intensive investigation of what can be learned from man’s first close-up look at Mars."
But when Mariner IX later would photograph Mars’ great volcanoes, valleys, and erosion patterns that looked like dry riverbeds, that imagery eventually started the scientific debate that continues today. What happened to Mars, what is the martian surface history and the fate of liquid water?
Momsen shared his personal Mariner experiences with Astrobiology Magazine, to reveal how space pioneers got their jobs done on the first flight to Mars?
Astrobiology Magazine (AM): You joined Caltech’s Jet Propulsion Laboratory in 1962 and worked on the Mariner program to return the first close-up images of Mars. In your recollections of that time, you mentioned a grant to the laboratory of $250,000 for a reflecting pond, which the mission team wanted to divert to a quick and dirty flyby of Venus. Was it really possible financially to do a mission like that in 1962?
William Momsen (WM): Although there was a Mariner-Venus program, I was not affiliated with it, only the Mars mission. We begged them to give us the funds for a new mission, but the funds were allocated for landscaping. I believe it would have been possible to send a quick and dirty probe to Venus for that amount, using the many spares left over from other missions. That would not include the cost of the launch vehicle, however.
AM: Mariner IV returned the first close views of Mars [and was also the first spacecraft to leave Earth for the outer reaches of the Solar System]. But the spacecraft used less power than two 100-watt light bulbs. Sometimes it is recounted that the average digital wrist watch today has more computing power than the manned lunar landers did in 1969. In retrospect, would you see computing power as one of the big performance differences between then and now?
WM: Mariner IV didn’t require heavy computer power, as most of the functions were rather simple, such as timing events, multiplexing data, etc. There was no need for complex computations. Of course, current missions need the power to perform more complex functions.
|First of 21 images taken by Mariner IV. The middle series of teletype numbers was processed for transmission by the spacecraft electronics (Mariner IV scanbus, lower) to yield the top approach picture to Mars|
Credit: NASA/ JPL Caltech
AM: As the scan subsystem engineer, you had to get the tracking just right or there would be no Mars pictures. When a spacecraft is hurtling supersonic towards a flyby, with a 12-minute one-way transmission time, how does one gauge precisely when to flip the switch? A JPL press release from February 1965 announced "This time lag could seriously affect accomplishing mission objectives."
WM: We relied on two signals. The first was the scan platform position. Of course, with a transmission time of 12 minutes, we could see where it had been 12 minutes ago. By plotting the platform position on graph paper, we could obtain a plot of where it had been at any time, and predict where it would be any time in the future. The second signal was the output from the optical sensor. At first, it was zero, pointing in outer space.
When it started "seeing" the planet, we could correlate that with the scan position signal. From that we could calculate at what position in a future cycle the planet would be centered in the field of view, when the signal should arrive at the spacecraft. Then we subtracted 12 minutes (plus some other factors) to arrive at a time when the command should be sent.
AM: The European Beagle 2 lander was intended to send back a faint 5-watt signal from 200 million miles away on Mars. That was like picking up less than a mobile phone broadcasts terrestrially. Wouldn’t Mariner’s 10-watt radio transmitter therefore also have less broadcast power than an average cellphone today? Goldstone and other dishes in the Deep Space Network were used to receive that signal? [The faint signal was one-tenth of one-billion-billionth of a watt].
WM: Yes. To capture that microwatt signal, a 210 foot parabolic dish antenna at Goldstone, California, was employed. Mariner had two antennas, a low-gain omnidirectional, which could beam low data rates in the general direction of Earth when lock was lost. The other was a high-gain, narrow beam antenna, used when it was pointed at Earth.
AM: You were co-engineer on the scan sub-system, which had to transition the spacecraft from scanning mode (40 degree field of view) towards tracking (1 degree FOV). This made sure the TV camera could keep the planet in its pointing direction. When you first got tracking, did Mariner adjust itself duing flyby on power or with gyroscopes or both?
WM: The spacecraft attitude was fixed. The only part that moved was the scan platform, with the TV camera mounted on it. The spacecraft was on a fixed trajectory, which couldn’t be determined exactly, stabilized in roll on Canopus, and pitch and yaw on the Sun. If Canopus or Sun lock was lost, the guidance system was controlled by the gyros. For that reason, the TV camera was mounted on the scan platform, which nodded up and down as it passed the planet. As designed, when a sensor "saw" the planet, it waited until the Mars image was centered in its field of view. Then the scan platform motion was inhibited, and it stopped with the TV camera pointed at the center of the planet.
AM: What was special about locking Mariner navigation on the star Canopus when cruising from Earth to Mars? Would any star have worked with the proper guidance plans?
WM: I’m not an expert in guidance systems, but it was one of the brighter stars. I suppose another would have done as well, but that is the one they chose.
AM: You tell an amusing story that power fluctuations were large, and you noticed a white film on the components from a subcontractor. What was the cause of the power problem?
WM: The problem was, when the subcontractor had finished soldering on the components, they washed the board with green soap found in a 5-gallon can in the men’s room! This left a conductive film on the components, which prevented their proper operation.
|The moon Phobos eclipses the sun over Mars, as imaged by Mariner 9 |
Credit: NASA/ JPL
AM: All the overtime you put into Mariner allowed you to get a Jaguar eventually. What was the fate of your maroon Jaguar, which you were able to purchase from overtime 12 hour daily schedules? Still around?
WM: I wish I still had it! Sadly, it got too many speeding tickets and I was forced to put it up in the garage. Through disuse, the seals dried out. I sold it when I moved to Florida.
AM: There are scores of pyrotechnics on virtually all modern spacecrafts, including the current rovers. When engineers refer to how critical a pyrotechnic bolt or cable cutting is to the success of a mission, it seems to have the air of single-point failures. But even Mariner had lots of pyro. Is this the most reliable way to fasten and unfasten no matter what kind of harsh space environments?
WM: Pyrotechnic squibs are ultra-reliable, being in sealed containers. Often they were backed up by a second unit.
AM: Mariner launched on Nov. 28, 1964 without incident. Most of your workstations for subsystems had mainly TV monitoring and some telephone capabilities to send diagrams or equations. What was the main way for engineers to communicate with the spacecraft?
WM: The TV monitors and telephones were for the different specialists to communicate with each other. We had various printers and plotters to display data received from the spacecraft. Engineers could not send commands to the spacecraft; these were sent from the tracking stations. We specified the time and the event.
AM: During last year’s 40th anniversary of the Kennedy assassination, many historical accounts indicated that the events of November 1962–only two years before Mariner’s launch in 1964–were really the first strong imprint that live TV made on American consciousness. For instance, many people recounted they didn’t believe TV (like the internet today) until they could read the story in newsprint. In 1964-65, JPL released about a dozen press releases over the mission lifetime, compared to what at last count was four Williamion web hits on the current Mars-JPL website. Any thoughts on how Mariner was a TV event, from a controller point of view? And what was the role of teletype for command and control?
WM: I didn’t get much chance to watch TV, being in the control room most of the time. Teletypes (operating in receive mode only) were a "last ditch" source of raw data, in case our computers went down. Interpreting this data was quite daunting and painstaking – It was much easier displayed on a screen or printed out.
|In 1965, Mariner IV saw whitish crater rims, a scientific result since not only had craters not been seen close, but later orbital pictures (inset) show frost forming on crater embankments. Lower left shows frost, middle lower shows what may be seepage erosion, lower right shows a dust devil moving across dunes.|
Credit: NASA/ JPL/ MSSS MOC
It took some getting used to, picking up the telephone and saying "Bus Chief, this is Space Chief" without breaking into laughter. (No, we didn’t wear funny hats).
AM: Among the early discoveries of Mariner, one was the lack of a sensible magnetic field on Mars. There are current theories that this loss of core magnetism quickly dispersed what might have been a much thicker, warmer atmosphere once. The loss of the atmosphere then may have dispersed any liquid surface water under pressures about 1% of our Earth’s. What remains are erosion patterns of that 4 Williamion year trek in time. Was the lack of a strong magnetometer reading a big surprise then?
WM: I was elected to comment on the encounter to an audience of big wigs, including Dr. Van Allen. I predicted the time at which we should see the magnetic field in the data. Well, it didn’t, and I thought the magnetometer must be broken.
Surprise isn’t the word for it!
AM: The first images were 40,000 pixels total covering a 150 mile square area on Mars, which today would be considered a 200×200 pixel, gray-scale ‘thumbnail’. The received lines of teletype numbers were stapled, side by side, to a board, arbitrary colors were assigned to sets of numbers, and each number was colored with crayons by hand. Twenty images could be stored on a tape recorder. How long would this data transmission involve for JPL when awaiting the next image?
WM: Our data rate at Mars was 8 1/3 bits per second (bps–yes, bits!).
There were 5 million bits of data from the 20 pictures. They were stored on a tape recorder and played back over an 8-day period.
In comparison, the Spirit and Opportunity rovers have three modes of communications:
1) Low gain antenna from Rover to Earth (120 bps).
2) High gain antenna from Rover to Earth (12Kbs–12,000 bps).
3) UHF relay from Rover to an orbiting satellite (128Kbs–128,000 bps).
AM: Two things stood out early in those Mariner images, a cloud and lots of craters. The clouds were unexpected because of the thin atmosphere?
WM: Exactly. We assumed it was a flaw in the camera lens. (Oh no! Not another instrument "failure"!)
AM: And the martian craters were almost like a view of our own Moon. Was that interpreted to be evidence of a cold, dead place where no active erosion was weathering these impact remnants?
WM: True. Of course, it just happened that we were looking at a part of Mars that was featureless except for heavy cratering.
AM: The flyby photographed a 4000 mile long pass across the face of Mars: "When the first of up to 21 pictures is taken, Mariner’s camera will be pointing at the northern Martian desert Amazonis. The camera’s coverage will then sweep southeast below the Martian equator covering the Mare Sirenum, the southern desert Phaethontis, Aonius Sinus and into the terminator or shadow line". About how much of the surface could be viewed during the flyby?
|Clouds and frost cover on the north Martian pole from Mars Orbital Camera |
Credit: NASA/ JPL/ MSSS MOC
WM: Less than 1%, an atypical part of the planet. If a flyby from another civilization happened to take pictures of the Sahara desert, they would probably conclude that the rest of the planet was similar.
AM: Mariner 9 saw the first glimpse of dry riverbeds on Mars. Sometime around 1997, when the controversy first ignited over the fossil-like shapes in a Martian meteorite, it was said that twenty years earlier, the ‘Viking experiments had ruined Mars for biologists, but saved Mars for geologists’. Any thoughts on the ups and downs that the whole Mars exploration program has followed over four decades, and how our view of Mars is being shaped today?
WM: We certainly have revised our view of Mars over the last 30 years. From a barren, moon-like planet, to one with possible evidence of past surface water, to today’s ongoing analyses of the Martian rocks and soil, our views will most certainly be revised again.
AM: To bring us to the present, what are your activities today, and what are your initial impressions of this generation of Mars rovers? Do you have a personal hunch about the questions of water [or even microbial life] on Mars?
WM: Today I am retired, and am fascinated by Spirit and Opportunity. I watch everything I can regarding the rovers on TV and the internet.
There is certainly water in the polar caps. There may be more in the form of permafrost, or even in the form of liquid underground, warmed by vulcanism. If so, microbial life is possible. "Extremophiles" are organisms living in the most inhospitable conditions on Earth.
If, indeed, there are fossils on Mars, we will probably have to wait for human exploration, or be extremely lucky. It’s hard enough to find them on Earth. Who knows? There are many questions to be answered and some that haven’t even been asked yet.
We will continue to be fascinated as Mars’ story unfolds.