Recent posts about NASA and the space programm have prompted me to finish up and post my thoughts on manned Mars missions and manned space exploration in general.
The modern focus of the United States' space program centers on extending a past history of short- and long-duration manned spaceflight initiatives in low Earth orbit and repeating its 1960s and 1970s successes with landing crews on the Moon as a prelude to manned flights to Mars. Secondarily, the US maintains a program of sending unmanned instrumented probes to Mars, Jupiter, Saturn and its moons, and other destinations within the Solar System as well as fielding astronomy and other science instruments of the sort represented by the Hubble Space Telescope.
It needs to be understood that manned spaceflight conducted at the level pursued by the US, the former Soviet Union, and more recently China is many times more expensive than unmanned flights to the same destinations. The most general overall differences between the two types of missions are that a manned mission must be far more complex and reliable and the hardware has to be able to lift and land a lot more weight.
In the beginnings of the US manned space exploration program in the late 1950s and early 1960s, there was a cultural bias against risking astronauts' lives gratuitously; the administrators and mission planers knew that what they were doing was dangerous, but they took steps to thread paths through the danger. Establishing those paths placed challenges on every aspect of the design and construction of spacecraft, support facilities, and training, and those challenges added greatly to the complexity and cost of the mission. President John F. Kennedy's mandate to land Man on the Moon by the end of the 1960s explicitly included an inseparable mandate to "return him safely to the Earth." The loss of astronauts Ed White, Roger Chaffee, and Gus Grissom during a ground test of the Saturn spacecraft was taken not just as an unfortunate and perhaps inevitable loss but as a colossal failure in imagination, organization, administration, and engineering that shook the entire program to its roots.
It is a fallacy to assume that with modern equipment, materials, and techniques the US could mount a manned mission to Mars with a success rate equivalent to that of the Apollo lunar exploration program. Mars isn't just a bit further away than the Moon; it ranges from roughly 150 to 1000 times further away and has to be chased down in a different orbit around the Sun. And while manned Lunar missions were short enough that the danger from radiation could be confidently mitigated (a coronal mass ejection from the Sun aimed at the Earth-Moon system would have likely killed Apollo astronauts enroute but NASA had solar astronomers who would have recommended scrubbing a mission if conditions for such were likely), a Mars mission would last for many months, far outside any current capacity to predict a coronal mass ejection. While engineers could try to shield a crew compartment against cosmic rays (lead would actually make the danger from this particular sort of radiation worse; engineers would prefer to surround the astronauts with several feet of plain water), doing so would only add to the size and mass of the spacecraft, making it that much more difficult and expensive to fly.
Radiation is just one danger Mars-bound astronauts would face; disease, psychological pressures, and dependence on spacecraft propulsion systems and other components that would have to operate flawlessly after months of disuse are also factors that weigh gravely against success. Furthermore, Mars poses inherent challenges that the Moon did not; it has enough of an atmosphere to require a heatshield for entry but not enough of one to make parachutes terribly effective for a spacecraft that would have to be as heavy as one would have to be to land people and carry enough fuel to lift off again, especially given that Mars has about twice the Moon's gravity.
Perhaps someday, one of the nations of the Earth will try it. They may also lose entire crews in multiple attempts, or decide that the only practical mission would be a one-way trip. They may decide that their best chance at success lies with sending only one human being – one who actually thrives on long periods of time alone in enclosed spaces but can still carry out tasks and make independent sound judgments, as some high-functioning autistics are. But Americans will have no stomach for such, and will never consent to funding such missions. To insist on an almost certain chance of successful return even if the mission would have to be aborted as Apollo 13 was, a Mars mission would have to be astonishingly expensive, perhaps even Treasury-busting, to plan and accomplish. And even at that, there still may be risks that cannot be mitigated, suggesting that we could reasonably expect to mount a series of missions knowing that the first one, or two or three, may fail utterly – a scenario that was nowhere near being part of the calculus behind the scope and design of the Apollo program, in which each mission of the series was intended to build upon the knowledge gained from and leverage the experience of the missions before it. By contrast, a pragmatic Mars mission series would stage multiple missions hoping that they won't all result in a dead crew.
Faced with the alternatives of either a high-risk manned Mars program or a low-risk manned Mars program that is bankruptingly expensive, it is useful to take a step back and ask why the United States would pursue either course. The 1970s argument that the Apollo program was costing too much money when there was poverty domestically and war being fought in southeast Asia is no easier to wave aside today; to the contrary, the recent collapse of our country's central financial institutions, the teetering US auto industry, and two simultaneous protracted wars with uncertain futures make the case even more loudly. If one asserts that the primary returned value of the Apollo missions was the selection and return of Lunar materials for study, it would defy all reason to contend that returned Martian materials would be so vastly more valuable than Lunar materials that the vastly greater cost of a Mars mission is equally justifiable.
There is still a brass ring left in space exploration that, if achieved, would dwarf all of the Apollo and subsequent space accomplishments: the discovery of "Instance Two" – proof of the existence of past or current life in an environment other than the Earth. Whereas a lot of the instrumentation sent to Mars either onboard a lander or an orbiter has had much to do with detecting life, after more than 40 years of exploration no conclusive evidence of past or present life on Mars has ever been found. Meanwhile, scientific work in several disciplines suggest that our ability to search for life, whether on planets orbiting other stars or within our own Solar system, is undergoing dramatic improvements.
But those improvements are occurring all over the world and by no means is the United States uniquely positioned to discover Instance Two. For the United States to compete in that race, it must be able to field exploratory missions to places where life is probable and exercise technological prowess in Earth-based and orbital observatories. Liquid water, which we know was a vital part of the formation of life on Earth, is thought to be highly likely underneath the icy shells of Saturn's moon Enceladus and Jupiter's moon Europa, making them both tantalizing targets for further exploration and study. The ability to detect and perform remote sensing on extrasolar planets is sharpening by the month, opening the possibility that a planet may be identified with conditions similar enough to Earth to justify building and sending a very fast probe to perform reconnaissance for biology. Those programs would be expensive but not compared to current manned space programs.
Without a viable purpose to send people into space to places we haven't already been without writing off their lives, it's time to wind manned spaceflight down and move space science onward.