By Dr. Cynthia B. Phillips Research Scientist, SETI Institute
The final mission of Space Shuttle Atlantis has spawned a whole series of perspective pieces on the history, state, and future of space exploration. Some, like the YouTube video "NASA's increase of awesome to continue," are unabashedly exuberant celebrations of the future in store for us in space; others, like this thoughtful piece in Technology Review entitled "Was the Space Shuttle a Mistake?," are depressingly and effectively critical of the cost both in dollars (more than $200 billion) and in human lives lost (14 astronauts plus at least 6 ground support staff) of the Shuttle program. Some authors have even posited the end of the space age altogether, as in a piece subtitled "Inner space is useful. Outer space is history" in The Economist.
While some of these articles make throwaway references to the number of robotic missions that could have been launched for the cost of one Shuttle flight (estimates range from $450 million per launch, one Discovery-class mission, up to $1.6 billion per launch, half a Flagship mission), there has been little analysis to date of the relative merits of human exploration vs. robotic exploration.
As a planetary scientist, I have very mixed feelings about the human spaceflight program. On a purely scientific level, there is very little in our current solar system exploration program that can't be done just as effectively by a robot as by an astronaut. Robots excel at the tedious work of taking similar pictures or analyzing similar rocks over and over and over again, without complaint (usually!) or a need for life support systems. Robots don't need food or water, they can withstand much more damaging radiation, and, perhaps most importantly, they don't need to come home at the end of the mission. Simply put, a one-way trip requires only half the fuel of a round trip voyage, and even though you'd likely get plenty of volunteer astronauts signing up for a one-way trip to Mars, it's unlikely that our current moral and ethical code would allow us to send such a mission.
And yet. While orbiting robotic probes have taken stunning vistas of the outer Solar System, it is clearly our rover missions on Mars that have won the hearts and minds of the general public. Perhaps it is the anthropomorphic, almost cute appearance of Spirit and Opportunity (and don't underestimate the cuteness factor), but I think it's also that rover tracks are the closest thing we currently have to astronaut footprints on the surface of another world.
If you think of an iconic image of space exploration, chances are you'll come up with an astronaut planting a flag on the Moon, or perhaps that classic image of a single bootprint from Apollo 11. Robots have flown past, orbited, and/or landed on all of the major bodies in our solar system and quite a few of the minor bodies -- we've landed spacecraft not only on Mars, but also on Venus, the Moon (of course), and Saturn's moon Titan; we sent an atmospheric probe into Jupiter; we're currently in orbit around Mercury and Saturn; and we've flown past Neptune and Uranus (and are on our way for a flyby of that pesky has-been, Pluto). Yet without that bootprint, that iconic image of "touching the surface," have we really explored those worlds?
Ironically, if the next target of human space exploration remains a trip to a near Earth asteroid, the "boots and flag" doctrine becomes extremely complicated in an irregular microgravity environment. If an astronaut visits an asteroid, she probably won't actually be able to stand on it because there won't be enough gravity. Landing would be extremely difficult, but perhaps a spacecraft could rendezvous with an asteroid by matching its speed and let an astronaut drift over to pay a visit to a large space rock. Rather than trying to make a bootprint on the surface, wouldn't a large pole be easier for her to poke the surface with? It could even collect a sample while making a boot-shaped impression. But then do we need the astronaut outside in her suit in the dangers of space after all? Couldn't she just operate the pole with a boot on the end from safely inside her spacecraft? What if she operated it remotely, from a larger spacecraft perched a safe distance away from the asteroid? What if she stayed home and operated it truly remotely, from the comfort of a safe NASA facility's virtual reality tank? What if the boot-and-pole contraption didn't need to wait for signals from a faraway human astronaut at all, but could be programmed to autonomously tap the surface on approach, make and photograph a nice boot-shaped mark, and perhaps even scoop up a sample or two of asteroid dust to send back to Earth in a small capsule? Would that allow us to check off "asteroid exploration" from our list? What if I told you that mission has already been accomplished -- the Japanese Hayabusa spacecraft successfully used a robotic probe to capture a small sample of dust from the asteroid Itokawa in 2005, placed it in a capsule, and sent it back to Earth, where it was successfully recovered in 2010 and is currently being studied.
But what about the science, you say, all that science that can only be done by trained astronauts? What about the amazing green moon rocks that were found by the Apollo astronauts on the lunar surface but surely would have been missed by a robot? To this, I would argue that while certainly human astronauts have the benefit of intelligence, quick thinking, problem solving, and ingenuity, robots have certain inexorable advantages in the realm of space exploration. Aside from not needing to eat, sleep, or return to Earth, robotic missions are significantly cheaper than human missions. For the cost of putting two astronauts on the surface of a planet like Mars for a few days or weeks, you could afford an army of robots that could comb the surface of the planet for years. While they might not spot that green rock right away, through sheer brute force chances are they'd eventually stumble across it and catalog it as an interesting sample to be sent back to Earth someday. And if you don't have humans to return to Earth (and keep alive and safe all the way home), you have room for a whole lot more rocks in your return capsule!
So why, then, do I have mixed feelings about the end of the Space Shuttle program? If everything humans can do in space, robots can do better, cheaper, and safer (in terms of loss of human life), then why send humans at all? The answer is simple: don't try to justify human spaceflight using science. NASA got it right when it split off the Science Mission Directorate (SMD) from the Exploration Systems Mission Directorate (ESMD). Keep exploration, dominated by human astronauts, separate from science, where robots rule. Sure, there may be some chances for science to be done along the way, as in the interesting geological observations made by the Apollo astronauts or the microgravity chemistry experiments performed by astronauts on the International Space Station, but if you try to justify a program like Apollo or the ISS purely on scientific terms, you'll fail. For the $25 billion spent on the Apollo program (a more inclusive estimate in 2009 dollars is $170 billion), or the $100 billion spent on the International Space Station (which includes construction costs plus the cost of 33 Space Shuttle missions to construct and supply the station), we could have had a small army of robotic lunar geology rovers or a fleet of microgravity satellites. Apollo and the ISS belong firmly in the Exploration side of NASA's portfolio, and that's as it should be.
Why, then, should we explore at all, if not for scientific gain? As a scientist, this might be a hard position to admit, but exploration has political and psychological benefits that go far beyond science in many respects. The Apollo program was a unique circumstance of the Cold War era -- instead of fighting a war on the ground, with potentially devastating consequences from the use of atomic or nuclear weapons, the United States and the Soviet Union chose astronauts and cosmonauts to fight in a modern-day arena: space became the gladiator ring of the past. In a post-Cold War era, the International Space Station became a sign of international cooperation and trust, as governments from many countries shared resources to build and staff a technological marvel in orbit.
Human spaceflight gives us a new perspective on our world, fragile and beautiful in an endless, empty sea of darkness. It is inspirational, a source of pride in our nation's accomplishments, a uniting subject at a time of great national divisions. And it is space travel that lights up the minds of children, of the next generation of explorers, sparking a seed of innovation and excitement and exploration that will carry this nation forward. NASA should be on the cutting edge of that exploration.
The Space Shuttle was an innovative technological marvel in 1981, when it was first launched, but it has kept flying far longer than originally intended, and the fleet of spacecraft has grown old and unreliable. Many things have changed since 1981, and it's time for a change in our human spaceflight policy, as well. I believe that the Obama administration made a tough but correct decision when it decided to encourage commercial development of spacecraft that can serve as the cargo ships and true "space shuttles" of the future, bringing supplies and astronauts to the International Space Station and other future destinations in Low Earth Orbit. Atlantis' final journey into space turned out to be both majestic and mundane, as the science fiction spacecraft of the future (at least the future as seen in 1981) ends its days ferrying a load of spare parts and bologna sandwiches into orbit and bringing a load of broken equipment and assorted trash back to Earth. Such resupply missions are clearly better suited to the shipping containers of the future, robotic resupply ships that are already automatically docking with the ISS which are unloaded, then filled up with trash and released to burn up in the Earth's atmosphere on the way down.
NASA's future of human spaceflight will focus on innovation and true exploration, not the day-to-day operations of the supply chain. As Space Shuttle flights became ordinary, they lost their place in the human imagination because they ceased to be exploration - while the scenery is stunning, there are only so many times that a trip around Earth or a visit to the same Space Station can be spun as new and exciting on the evening news. A focus on developing flexible, innovative, and realistic new spacecraft that can take astronauts to Near Earth asteroids (with or without a boot-on-a-pole), or back to the Moon, or even on to Mars, will give NASA's ESMD a true purpose and destination once more.
Robotic precursor missions to asteroids and other destinations can certainly do some great science along the way, but NASA needs to make sure that the budget for true scientific exploration, as done by the amazing spacecraft of SMD, remains insulated and protected from the almost boundless demands of human spaceflight. Humans have a place in space exploration, as ambassadors and proxies for the human race as a whole, but robotic missions will venture to the far reaches of our solar system to study environments too remote or dangerous for humans to visit any time soon. Fortunately, there's plenty of space for both! About the Author: Dr. Cynthia B. Phillips is a planetary scientist at the SETI Institute and author of over a dozen books, including Space Exploration for Dummies. Dr. Phillips acknowledges helpful discussions with Dr. David Morrison for this article.