The 300-year-old tool helping stranded astronauts get home

An old seafaring instrument could help astronauts on ever more far-flung and perilous missions.

Imagine you're an astronaut flying to Mars. Suddenly, disaster strikes: all your electronic navigation systems fail. How do you find your way back home?

For Nasa astronauts, the answer could be a wayfinding tool invented around 300 years ago: the sextant. Used by seafarers on Earth, and astronauts on early Nasa missions, sextants are a last-resort navigation option for missions to the Moon, and potentially, beyond – in case the high-tech ones fail.

Traditionally, sextants are used to measure the angle between celestial bodies such as stars and planets, to calculate one's own position. Apart from emergency use, these ancient star-sighting skills are also attracting increased interest from Nasa engineers as humans plan to venture to Mars and beyond.

"The further and further we go out, the less and less we can rely on some of the Earth-based navigation technology," such as satellites, says Greg Holt, the navigation system manager for the Orion Spacecraft and its Artemis missions to the Moon. "And so you now have to fall back more and more on what I would call the classical navigation techniques. And some of those now harken back to the way navigation was done way back in the 16th, 17th, 18th Centuries," he explains. 

In that age of classical navigation, "you actually had to look at the stars to figure out where you were, and figure out your orientation relative to those celestial bodies," he says. These are skills that can be crucial in deep space "because those are some of the only references you have once you get far away from Earth".

In addition, knowing how to navigate by the stars can save lives, he points out: "We want to make sure that [the astronauts] have a way to safely navigate their way back home, if for some reason they lose communication with the Earth".

In 2018, Holt and his team asked two astronauts on the International Space Station (ISS), Serena Auñón-Chancellor and Alexander Gerst, to test a hand-held sextant, taking measurements with the naked eye and a steady hand.

"We had crew members demonstrate the use of a manual sextant, as an emergency means of back-up navigation to get the crew home from the Moon, if they had in fact lost communication," says Holt. "Because if you lose communication, you also lose that radio navigation link with the Earth, and you're on your own at that point."

The experiment tapped deep into navigational history. In the 1960s, Nasa adapted ancient star-sighting methods to space flight, though initially astronauts were not entirely sure this would work.

Neil Armstrong, the first man on the Moon, recalled that "one of the things that I was concerned with at the time was whether our navigation was sufficiently accurate [...] if we lost communication with Earth, for whatever reason, could we navigate by ourselves using celestial navigation? We thought we could, but these were undemonstrated skills."

The answer was yes – he and other astronauts on those early flights did successfully........

© BBC