Explainer: Out-of-this-world destinations
We are headed to Mars ... eventually. But first we need the rocket technology and human spaceflight savvy to get us there safely and efficiently. And the best way to do that is to visit places such as asteroids, our moon, a Martian moon and even no man's lands in space called "Lagrange points," NASA administrator Charles Bolden explained during the unveiling of the agency's revised vision for space exploration.
The vision shifts focus away from a return to the moon as part of a steppingstone to Mars in favor of what experts call a "flexible path" to space exploration, pushing humans ever deeper into the cosmos.
Click the "Next" label to check out six other potential destinations astronauts may visit in the years and decades to come en route to Mars.
— John Roach, msnbc.com contributor
Lessons to learn on the space station
The cooperation required to build and maintain the International Space Station will be a key to propelling humans on to Mars, according to Louis Friedman, co-founder of The Planetary Society. The society is a space advocacy organization that supports the flexible path to space exploration. In fact, the space station itself could be a training ground for Mars-bound astronauts.
Astronauts can spend ever longer blocks of time on the station to gain experience in long-duration flights, for example. They could also practice extravehicular activities akin to those expected on a Mars mission, Friedman noted.
Lunar orbit, a test of new technology
Lunar orbit, too, is a familiar destination for human spaceflight, but a return to the familiar with new technology would allow astronauts to test the engineering of systems designed to go deeper into space, according to Friedman.
A return to the moon is still in the cards on the flexible path, but going to lunar orbit first defers the cost of developing the landing and surface systems needed to get in and out of the lunar gravity well, according to experts.
The famous "Earthrise" image shown here was made in 1968 during Apollo 8, the first human voyage to orbit the moon.
Stable no man's lands in space
There are places in space where the gravitational pulls of Earth and the moon, or Earth and the sun, have a balancing effect on a third body in orbit. Those five locations, known as Lagrange points, could offer relatively stable parking spots for astronomical facilities such as space telescopes or satellites. Human spaceflights to these points would allow astronauts to service these instruments.
In addition, space experts believe a trip to a Lagrange point could serve as a training mission for astronauts headed to points deeper in space, such as an asteroid. Nevertheless, reaching a Lagrange point would be more of a technical achievement than a scientific achievement, according to Friedman. "It is an empty spot in space," he said.
Visit an asteroid near you?
The first stop astronauts may make in interplanetary space is one of the asteroids that cross near Earth's orbit. Scientists have a keen interest in the space rocks because of the threat that one of them could strike Earth with devastating consequences. An asteroid mission would allow scientists to better understand what makes the rocks tick, and thus how to best divert one that threatens to smack our planet.
Humans have also never been to an asteroid, which would make such a visit an exciting first, noted Friedman. "Imagine how interesting it will be to see an astronaut step out of a spacecraft and down onto an asteroid and perform scientific experiments," he said. What's more, since asteroids have almost no gravity, an asteroid encounter would be like docking with the space station, which doesn't require a heavy-lift rocket for the return. That makes an asteroid a potentially less expensive destination than the surface of the moon.
Back to the moon?
The moon-Mars path of human space exploration originally envisioned the moon as a training ground for a mission to the Red Planet. While the flexible-path strategy broadens the training field, the moon remains a candidate destination, according to NASA.
Several other nations also have the moon's surface in their sights, including Japan, India and China. Some experts fear the dedicated lunar programs of these nations will eventually leave the United States in the dust as it focuses on an ambiguous flexible path.
Friedman, of The Planetary Society, said NASA should support the lunar programs of Japan, India and China as part of team building for an international Mars mission, but sees no reason for NASA to focus on the moon. "We've done that already and that was Apollo," he said.
Martian moon a final pit stop?
Before astronauts go all the way to Mars, there's reason to make a final stop at one of its moons, Phobos or Deimos. The two moons are less than 20 miles across at their widest, which means landing on them would be less expensive than the Red Planet itself.
Friedman used to consider a mission to a Martian moon nonsensical - akin to going to the base camp of Mount Everest instead of going to the top of the mountain. "I've now turned myself around on that, because you do go to the base camp and you do actually conduct training activities there before you attempt the summit," he said.
"By all means go there," he added. "Test out your rendezvous and docking at Mars, conduct your three-year, round-trip mission, maybe tele-operate some rovers of the surface (of Mars). That will all be interesting and then the next mission will finally go down to the surface."