As members of an elite band of cosmic explorers, they are among the few to have gone beyond the final frontier and looked down on the Earth from space.
Now, inspired by the unique perspective they gained of their home planet - and armed with startling new data about the scale of the threat it faces from asteroid strikes - a group of former Nasa astronauts is on an extraordinary mission to save the world.
Fourteen months after a meteor exploded over Chelyabinsk, Russia, on a scale equivalent to 30 Hiroshima bombs, the B612 Foundation, a non-profit group founded by the Apollo 9 astronaut Rusty Schweickart and the space shuttle astronaut Dr Ed Lu, is warning that only "blind luck" has so far saved us from worse.
"It's a giant game of chance we're playing. It's cosmic roulette," said Lu, whose group is working towards building and launching Sentinel, a US$250 million ($292 million) telescope that would spot space rocks on a collision course with Earth, giving years or even decades of notice to deflect a disaster.
"There's a saying in Vegas that 'the house never loses'. It's true; you can't just keep playing a game of chance and expect to keep winning," added Lu, the group's chief executive officer.
"Data obtained since the Chelyabinsk incident by Dr Peter Brown, a planetary scientist and asteroid expert at the University of Western Ontario, in Canada, reveals that since 2001 the Earth has been rocked by atomic bomb-scale asteroid impacts 26 times; 10 times more frequently than previously thought."
Today, which is Earth Day, the B612 Foundation will hold a press conference to unveil more critical details, including a video presentation that will reveal the locations and sizes of the multi-kiloton impacts.
"We are literally in a shooting gallery," said Schweickart. "That's the message we want people to understand. It's happening, it's ongoing, and the big ones will come. It's just a matter of when."
The Chelyabinsk asteroid ripped through the Earth's atmosphere as a 67,600km/h fireball, exploding nearly 30km above the ground. It damaged 7200 properties in six cities and injured 1500 people across a 40km radius.
Astronomers' attention was to have been focused that day on another asteroid - a 45m-wide rock called DA14, which had been identified through ground-based tele-scopes one year previously as being on a "near miss" trajectory.
Just 16 hours before DA14 made its closest approach, passing by the planet at a distance of 27,680km, came Chelyabinsk's unexpected visitor, a 19m wide rock weighing more than the Eiffel Tower.
It had gone undetected for years because it came from the same direction as the Sun's glare, making it impossible for ground-based optical telescopes to see it.
Sentinel, due to launch in 2018, will be positioned up to 273 million km from Earth, near Venus, from where its lenses would point away from the Sun. In the first month of operation alone, it is expected to detect and track more than 20,000 near-Earth asteroids, exceeding the discoveries made by all other telescopes combined over the past 30 years.
In 6years, it will make an inventory of 98 per cent of near-Earth asteroids; the current detection level is only 1 per cent.
Schweickart, who as an astronaut on Nasa's Apollo 9 mission in 1969 played a role in paving the way for man's first landing on the Moon, co-founded the B612 Foundation and now serves as chairman emeritus.
The group worked on designing technologies to deflect asteroids from collisions with Earth, before launching the Sentinel early warning project. The failure by the US government to do the job itself irks Schweickart.
"Scientific projects such as understanding that there's an ocean under the ice on Europa is a really wonderful thing, but it shouldn't compete in terms of government funding priorities with ensuring the safety and security of people here on Earth," he said.