President Barack Obama set a lofty next goal this week for Americans in space: Visiting an asteroid by 2025. But reaching a space rock in a mere 15 years is a daunting mission, and one that might also carry the ultimate safety of the planet on its shoulders.
"It is probably the hardest thing we can do because the asteroid is not coming on a schedule," NASA chief Charles Bolden told reporters late Thursday after Obama announced his space vision.
And when a specific asteroid is eventually selected, the window to launch a spaceship toward it will be much less forgiving than the windows for NASA space shuttles bound for the International Space Station, Bolden said.
"The space station gives us five minutes," he explained. "I'm not sure what an asteroid gives us, but then it doesn't come again for a lifetime."
And there's another compelling reason for touching an asteroid: Saving the planet.
In a panel discussion that followed President Obama's Thursday space vision speech, astrophysicist John Grunsfeld -- a former NASA astronaut who flew on five shuttle missions -- suggested sending humans to purposely move an asteroid, to nudge the space rock to change its trajectory. Such a feat, he said, would show that humanity could deflect a space rock if one threatened to crash into the planet.
"By going to a near-Earth object, an asteroid, and perhaps even modifying its trajectory slightly, we would demonstrate a hallmark in human history," said Grunsfeld, who flew on three shuttle missions to fix the Hubble Space Telescope. "The first time humans showed that we can make better decisions than the dinosaurs made 65 million years ago."
Take the moon, Grunsfeld said. Tycho crater, a huge impact crater on the moon visible from Earth, was created when an asteroid crashed into it 95 million years ago, he said.
"The dinosaurs saw that," Grunsfeld told reporters. "Thirty million years later they're snuffed out when the same thing happens to the Earth." [Asteroids Up Close.]
If humanity doesn't develop a capability to meet space rocks head-on, and win, than it is almost a certainty that an asteroid will eventually threaten life on Earth, he added.
TV's Bill Nye the Science Guy, vice president of the Planetary Society, said the president's asteroid plan carries risk, since it sends astronauts so far from home. But it is a risk worth taking.
"You're saving all of humankind," Nye said. "That's worthy, isn't it?"
What's out there
Scientists estimate there are about 100,000 asteroids and comets near Earth, but only about 20,000 are expected to pose any risk of impact. NASA has found about 7,000 of those objects, 1,000 of them flying in orbits that could potentially threaten the Earth in the future, NASA scientists have said.
Astronomer Donald Yeomans, head of NASA's Near-Earth Object program office at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said there are about a dozen near-Earth asteroids that could be within reach of manned spacecraft, but most of those are relatively small. To make a crewed mission worth it, the target space rock would likely have to be at least 300 feet (100 meters) wide.
For comparison, the space rock that exploded in a magnificent fireball over Wisconsin this week was just 3 feet (1 meter) wide, Yeomans said.
"If you could study a few of them up-close, you get a better idea on how best to deflect them," Yeomans told SPACE.com."
And more asteroids are being found all the time. NASA's WISE infrared space telescope is discovering dozens of asteroids every day that were previously unknown. New surveys and spacecraft will add to that space rock bounty over the next 15 years to offer more candidates for a crewed asteroid mission, Yeomans said.
New firsts in space
The bold new mission for NASA unveiled by President Obama Thursday was ultimately aimed at sending humans to Mars in the mid-2030s. The asteroid mission is just the first step.
"By 2025, we expect new spacecraft designed for long journeys to allow us to begin the first-ever crewed missions beyond the moon into deep space," Obama said. "We'll start by sending astronauts to an asteroid for the first time in history."
Astronauts have been to the moon and it's time to do something new, Obama said. He pledged to revive the Orion spacecraft, initially canceled along with the rest of NASA's Constellation program building new rockets and spacecraft. Now it will be used as a space station escape ship and, later, play a role in deep space missions, Obama said.
A mission to an asteroid would likely take months. Astronauts would rendezvous with a space rock, not land on it because of its weak gravity, but NASA would not send humans to an asteroid to just look at it, Grunsfeld said.
"If you go up to this, you're going to want to crawl around on it and find out what makes it tick," Grunsfeld said. Tethers or pitons would be required to keep asteroid explorers from floating away, he added.
Astronauts on an asteroid mission would be flying outside the Earth's protective magnetosphere, which shields the planet from harsh space and solar radiation. Even the Apollo astronauts who landed and walked on the moon didn't face such a risk.
"It's every bit as exciting in a different way, we're going to deep space. You turn around and take a picture of the Earth, and it's going to be a dot. You're not even going to see the atmosphere," Nye said. "Going to an asteroid, man, it's tough and risky and dangerous, how cool is that?"
Space radiation and long-term isolation would be among the biggest challenges for deep space missions, said MIT professor Edward Crawley, who participated in the panel discussion with Grunsfeld and served on the White House committee that reviewed NASA's human spaceflight program.
Crawley recommended a tiered approach to training missions, with a series of ever-longer expeditions preparing astronauts to the long treks to asteroids and, eventually, Mars.
In general, asteroids are no strangers to the people of Earth. Astronomers have long-watched the space rocks from the ground while spacecraft have visited -- some even landed on -- asteroids in deep space.
Today, Japan's Hayabusa spacecraft is returning back from a huge asteroid called Itokawa, where it attempted to collect samples to send back to Earth. Hayabusa is due to return in June. Meanwhile, NASA's ion-powered Dawn spacecraft is headed out to the asteroid belt to rendezvous with Vesta and Ceres, the two biggest space rocks in the solar system.
But robots are only as good as their programming, and ultimately still rely on human operators.
"Robots have never discovered things," Grunsfeld said. "People have discovered things, using robots."
There are secrets locked away on asteroids that may hold the key to understanding the formation of the solar system. Asteroids are thought to be the leftover remnants of the solar system's buildings blocks. The organic molecules and compounds on them may offer clues on how life began on Earth, and if it's possible elsewhere in the universe, Nye and Grunsfeld said.
For Yeomans, who has studied asteroids for 40 years, hearing President Obama's commitment to send humans to visit them was uplifting, to say the least.
"It was pretty exciting to hear him say that," Yeomans told SPACE.com. "Of course, Congress still has to pass the budget, so all these things are up in the air a bit."