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Saturday, June 11, 2016

Reading List: Humans to Mars

Portree, David S. F. Humans to Mars. Washington: National Aeronautics and Space Administration, 2001. NASA SP-2001-4521.
Ever since, in the years following World War II, people began to think seriously about the prospects for space travel, visionaries have looked beyond the near-term prospects for flights into Earth orbit, space stations, and even journeys to the Moon, toward the red planet: Mars. Unlike Venus, eternally shrouded by clouds, or the other planets which were too hot or cold to sustain life as we know it, Mars, about half the size of the Earth, had an atmosphere, a day just a little longer than the Earth's, seasons, and polar caps which grew and shrank with the seasons. There were no oceans, but water from the polar caps might sustain life on the surface, and there were dark markings which appeared to change during the Martian year, which some interpreted as plant life that flourished as polar caps melted in the spring and receded as they grew in the fall.

In an age where we have high-resolution imagery of the entire planet, obtained from orbiting spacecraft, telescopes orbiting Earth, and ground-based telescopes with advanced electronic instrumentation, it is often difficult to remember just how little was known about Mars in the 1950s, when people first started to think about how we might go there. Mars is the next planet outward from the Sun, so its distance and apparent size vary substantially depending upon its relative position to Earth in their respective orbits. About every two years, Earth “laps” Mars and it is closest (“at opposition”) and most easily observed. But because the orbit of Mars is elliptic, its distance varies from one opposition to the next, and it is only every 15 to 17 years that a near-simultaneous opposition and perihelion render Mars most accessible to Earth-based observation.

But even at a close opposition, Mars is a challenging telescopic target. At a close encounter, such as the one which will occur in the summer of 2018, Mars has an apparent diameter of only around 25 arc seconds. By comparison, the full Moon is about half a degree, or 1800 arc seconds: 72 times larger than Mars. To visual observers, even at a favourable opposition, Mars is a difficult object. Before the advent of electronic sensors in the 1980s, it was even more trying to photograph. Existing photographic film and plates were sufficiently insensitive that long exposures, measured in seconds, were required, and even from the best observing sites, the turbulence in the Earth's atmosphere smeared out details, leaving only the largest features recognisable. Visual observers were able to glimpse more detail in transient moments of still air, but had to rely upon their memory to sketch them. And the human eye is subject to optical illusions, seeing patterns where none exist. Were the extended linear features called “canals” real? Some observers saw and sketched them in great detail, while others saw nothing. Photography could not resolve the question.

Further, the physical properties of the planet were largely unknown. If you're contemplating a mission to land on Mars, it's essential to know the composition and density of its atmosphere, the temperatures expected at potential landing sites, and the terrain which a lander would encounter. None of these were known much beyond the level of educated guesses, which turned out to be grossly wrong once spacecraft probe data started to come in.

But ignorance of the destination didn't stop people from planning, or at least dreaming. In 1947–48, Wernher von Braun, then working with the U.S. Army at the White Sands Missile Range in New Mexico, wrote a novel called The Mars Project based upon a hypothetical Mars mission. A technical appendix presented detailed designs of the spacecraft and mission. While von Braun's talent as an engineer was legendary, his prowess as a novelist was less formidable, and the book never saw print, but in 1952 the appendix was published by itself.

One thing of which von Braun was never accused was thinking small, and in this first serious attempt to plan a Mars mission, he envisioned something more like an armada than the lightweight spacecraft we design today. At a time when the largest operational rocket, the V-2, had a payload of just one tonne, which it could throw no further than 320 km on a suborbital trajectory, von Braun's Mars fleet would consist of ten ships, each with a mass of 4,000 tons, and a total crew of seventy. The Mars ships would be assembled in orbit from parts launched on 950 flights of reusable three-stage ferry rockets. To launch all of the components of the Mars fleet and the fuel they would require would burn a total of 5.32 million tons of propellant in the ferry ships. Note that when von Braun proposed this, nobody had ever flown even a two stage rocket, and it would be ten years before the first unmanned Earth satellite was launched.

Von Braun later fleshed out his mission plans for an illustrated article in Collier's magazine as part of their series on the future of space flight. Now he envisioned assembling the Mars ships at the toroidal space station in Earth orbit which had figured in earlier installments of the series. In 1956, he published a book co-authored with Willy Ley, The Exploration of Mars, in which he envisioned a lean and mean expedition with just two ships and a crew of twelve, which would require “only” four hundred launches from Earth to assemble, provision, and fuel.

Not only was little understood about the properties of the destination, nothing at all was known about what human crews would experience in space, either in Earth orbit or en route to Mars and back. Could they even function in weightlessness? Would be they be zapped by cosmic rays or solar flares? Were meteors a threat to their craft and, if so, how serious a one? With the dawn of the space age after the launch of Sputnik in October, 1957, these data started to trickle in, and they began to inform plans for Mars missions at NASA and elsewhere.

Radiation was much more of a problem than had been anticipated. The discovery of the Van Allen radiation belts around the Earth and measurement of radiation from solar flares and galactic cosmic rays indicated that short voyages were preferable to long ones, and that crews would need shielding from routine radiation and a “storm shelter” during large solar flares. This motivated research into nuclear thermal and ion propulsion systems, which would not only reduce the transit time to and from Mars, but also, being much more fuel efficient than chemical rockets, dramatically reduce the mass of the ships compared to von Braun's flotilla.

Ernst Stuhlinger had been studying electric (ion) propulsion since 1953, and developed a design for constant-thrust, ion powered ships. These were featured in Walt Disney's 1957 program, “Mars and Beyond”, which aired just two months after the launch of Sputnik. This design was further developed by NASA in a 1962 mission design which envisioned five ships with nuclear-electric propulsion, departing for Mars in the early 1980s with a crew of fifteen and cargo and crew landers permitting a one month stay on the red planet. The ships would rotate to provide artificial gravity for the crew on the trip to and from Mars.

In 1965, the arrival of the Mariner 4 spacecraft seemingly drove a stake through the heart of the romantic view of Mars which had persisted since Percival Lowell. Flying by the southern hemisphere of the planet as close as 9600 km, it returned 21 fuzzy pictures which seemed to show Mars as a dead, cratered world resembling the Moon far more than the Earth. There was no evidence of water, nor of life. The atmosphere was determined to be only 1% as dense as that of Earth, not the 10% estimated previously, and composed mostly of carbon dioxide, not nitrogen. With such a thin and hostile atmosphere, there seemed no prospects for advanced life (anything more complicated than bacteria), and all of the ideas for winged Mars landers went away: the martian atmosphere proved just dense enough to pose a problem when slowing down on arrival, but not enough to allow a soft landing with wings or a parachute. The probe had detected more radiation than expected on its way to Mars, indicating crews would need more protection than anticipated, and it showed that robotic probes could do science at Mars without the need to put a crew at risk. I remember staying up and watching these pictures come in (the local television station didn't carry the broadcast, so I watched even more static-filled pictures than the original from a distant station). I can recall thinking, “Well, that's it then. Mars is dead. We'll probably never go there.”

Mars mission planning went on the back burner as the Apollo Moon program went into high gear in the 1960s. Apollo was conceived not as a single-destination project to land on the Moon, but to create the infrastructure for human expansion from the Earth into the solar system, including development of nuclear propulsion and investigation of planetary missions using Apollo derived hardware, mostly for flyby missions. In January of 1968, Boeing completed a study of a Mars landing mission, which would have required six launches of an uprated Saturn V, sending a crew of six to Mars in a 140 ton ship for a landing and a brief “flags and footprints” stay on Mars. By then, Apollo funding (even before the first lunar orbit and landing) was winding down, and it was clear there was no budget nor political support for such grandiose plans.

After the success of Apollo 11, NASA retrenched, reducing its ambition to a Space Shuttle. An ambitious Space Task Group plan for using the Shuttle to launch a Mars mission in the early 1980s was developed, but in an era of shrinking budgets and additional fly-by missions returning images of a Moon-like Mars, went nowhere. The Saturn V and the nuclear rocket which could have taken crews to Mars had been cancelled. It appeared the U.S. would remain stuck going around in circles in low Earth orbit. And so it remains today.

While planning for manned Mars missions stagnated, the 1970s dramatically changed the view of Mars. In 1971, Mariner 9 went into orbit around Mars and returned 7329 sharp images which showed the planet to be a complex world, with very different northern and southern hemispheres, a grand canyon almost as long as the United States, and features which suggested the existence, at least in the past, of liquid water. In 1976, two Viking orbiters and landers arrived at Mars, providing detailed imagery of the planet and ground truth. The landers were equipped with instruments intended to detect evidence of life, and they reported positive results, but later analyses attributed this to unusual soil chemistry. This conclusion is still disputed, including by the principal investigator for the experiment, but in any case the Viking results revealed a much more complicated and interesting planet than had been imagined from earlier missions. I had been working as a consultant at the Jet Propulsion Laboratory during the first Viking landing, helping to keep mission critical mainframe computers running, and I had the privilege of watching the first images from the surface of Mars arrive. I revised my view from 1965: now Mars was a place which didn't look much different from the high desert of California, where you could imagine going to explore and live some day. More importantly, detailed information about the atmosphere and surface of Mars was now in hand, so future missions could be planned accordingly.

And then…nothing. It was a time of malaise and retreat. After the last Viking landing in September of 1975, it would be more than twenty-one years until Mars Global Surveyor would orbit Mars and Mars Pathfinder would land there in 1996. And yet, with detailed information about Mars in hand, the intervening years were a time of great ferment in manned Mars mission planning, when the foundation of what may be the next great expansion of the human presence into the solar system was laid down.

President George H. W. Bush announced the Space Exploration Initiative on July 20th, 1989, the 20th anniversary of the Apollo 11 landing on the Moon. This was, in retrospect, the last gasp of the “Battlestar” concepts of missions to Mars. It became a bucket into which every NASA centre and national laboratory could throw their wish list: new heavy launchers, a Moon base, nuclear propulsion, space habitats: for a total price tag on the order of half a trillion dollars. It died, quietly, in congress.

But the focus was moving from leviathan bureaucracies of the coercive state to innovators in the private sector. In the 1990s, spurred by work of members of the “Mars Underground”, including Robert Zubrin and David Baker, the “Mars Direct” mission concept emerged. Earlier Mars missions assumed that all resources needed for the mission would have to be launched from Earth. But Zubrin and Baker realised that the martian atmosphere, based upon what we had learned from the Viking missions, contained everything needed to provide breathable air for the stay on Mars and rocket fuel for the return mission (with the addition of lightweight hydrogen brought from Earth). This turned the weight budget of a Mars mission upside-down. Now, an Earth return vehicle could be launched to Mars with empty propellant tanks. Upon arrival, it would produce fuel for the return mission and oxygen for the crew. After it was confirmed to have produced the necessary consumables, the crew of four would be sent in the next launch window (around 26 months later) and land near the return vehicle. They would use its oxygen while on the planet, and its fuel to return to Earth at the end of its mission. There would be no need for a space station in Earth orbit, nor orbital assembly, nor for nuclear propulsion: the whole mission could be done with hardware derived from that already in existence.

This would get humans to Mars, but it ran into institutional barriers at NASA, since many of its pet projects, including the International Space Station and Space Shuttle proved utterly unnecessary to getting to Mars. NASA responded with the Mars Design Reference Mission, published in various revisions between 1993 and 2014, which was largely based upon Mars Direct, but up-sized to a larger crew of six, and incorporating a new Earth Return Vehicle to bring the crew back to Earth in less austere circumstances than envisioned in Mars Direct.

NASA claim they are on a #JourneyToMars. They must be: there's a Twitter hashtag! But of course to anybody who reads this sad chronicle of government planning for planetary exploration over half a century, it's obvious they're on no such thing. If they were truly on a journey to Mars, they would be studying and building the infrastructure to get there using technologies such as propellant depots and in-orbit assembly which would get the missions done economically using resources already at hand. Instead, it's all about building a huge rocket which will cost so much it will fly every other year, at best, employing a standing army which will not only be costly but so infrequently used in launch operations they won't have the experience to operate the system safely, and whose costs will vacuum out the funds which might have been used to build payloads which would extend the human presence into space.

The lesson of this is that when the first humans set foot upon Mars, they will not be civil servants funded by taxes paid by cab drivers and hairdressers, but employees (and/or shareholders) of a private venture that sees Mars as a profit centre which, as its potential is developed, can enrich them beyond the dreams of avarice and provide a backup for human civilisation. I trust that when the history of that great event is written, it will not be as exasperating to read as this chronicle of the dead-end of government space programs making futile efforts to get to Mars.

This is an excellent history of the first half century of manned Mars mission planning. Although many proposed missions are omitted or discussed only briefly, the evolution of mission plans with knowledge of the destination and development of spaceflight hardware is described in detail, culminating with current NASA thinking about how best to accomplish such a mission. This book was published in 2001, but since existing NASA concepts for manned Mars missions are still largely based upon the Design Reference Mission described here, little has changed in the intervening fifteen years. In September of 2016, SpaceX plans to reveal its concepts for manned Mars missions, so we'll have to wait for the details to see how they envision doing it.

As a NASA publication, this book is in the public domain. The book can be downloaded for free as a PDF file from the NASA History Division. There is a paperback republication of this book available at Amazon, but at an outrageous price for such a short public domain work. If you require a paper copy, it's probably cheaper to download the PDF and print your own.

Posted at June 11, 2016 18:54