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

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 use sextants and other naked-eye sighting methods, both routinely, and during emergencies. Buzz Aldrin resorted to a hand-held sextant and chart during the Gemini XII spaceflight in 1966, due to a problem with the radar. And during the near-disastrous Apollo 13 flight in 1970, Jim Lovell famously used the sight of the Earth on the last stretch to get home.

The 2018 experiment with the sextant on the ISS was also successful. The results were accurate enough to suggest that navigating by hand-held sextant could get the crew back home in an emergency. And the astronauts reported that they found the sextant comfortable to use in microgravity – more so than using it on Earth, in fact.

Speaking by video call from Houston, Holt holds up a black sextant – essentially, the same age-old tool used by seafarers, with some small modifications for space. Its edges have been rounded off, for example, to prevent it accidentally scraping something. And it has a hook-and-loop fastener to attach it to the wall of the spacecraft, and prevent it from floating around in microgravity.

Other than that, it's exactly the same device you'd find on board a normal ship on an Earthly ocean, Holt says.

On the Orion spacecraft, astronauts use a high-tech version of a sextant – an optical navigation system – as a first back-up if other navigation systems fail. A camera attached to the spacecraft takes images of the surrounding stars and planets, "doing all of the same things that a crew member would be doing with the sextant", Holt says. Using image-processing techniques, the camera takes measurements such as the angles between stars, the angles between stars and planets, and the diameter of a planet. The readings are then used to automatically calculate course-corrections, for example.

"On the Orion spacecraft, the Optical Navigation system serves as a back-up, providing onboard navigation capability if communication and ground-based navigation support from Earth is lost," says Holt. During the Artemis missions, the system is routinely activated about once per day. "This allows us to confirm the system remains working and healthy, [to] collect valuable imagery and performance data for our engineering design team," he says. It is also "a cross-check confirmation of the navigation solution from the Earth-based tracking".

But if this high-tech system fails, the hand-held sextant comes in.

During the Artemis II mission to the Moon in April, a hand-held emergency sextant like the one Holt demonstrated was kept ready during pre-launch, he says. If there had been a problem with the cameras of the high-tech system at that point, the sextant would have been put on board at the last minute. It is not routinely packed on Orion, as it's not necessary equipment: "Every gram of mass counts when flying space missions," he points out.

If a disastrous systems failure happens after the launch, in space, the astronauts therefore won't have that hand-held sextant on board. But they are trained to know other naked-eye methods, Holt explains. One is to look for timings of when specific stars disappear behind the Earth, or the Moon, as a rough indicator of one's own position.

Holt says familiarity with the original tool is useful for Nasa's engineers, too. When leading the teams that develop the high-tech systems, "I make sure they all go out and practice with a sextant so they understand basics where all this is coming from, the maths and geometry," he says.

There are also other timeless navigational tricks that help modern-day astronauts. For example, the bright, easily recognisable stars and constellations that have historically guided seafarers, are also useful navigational points for astronauts in space, says Holt. That's because they are easy to spot and recognise by humans – whether those humans are on Earth, or in a spacecraft.

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For Mars missions, those navigational points may include asteroids, Holt says. "We would say [to the astronaut], we expect that the asteroid is going to be half-way between the two head stars in Gemini, so look over there, find that asteroid between those two stars, take your readings, then compare that against the expectations and do the maths." If the readings are off, the astronauts correct the course to get to where they want to be.

"To me, part of the fascination of my job is seeing how all of these techniques are all interrelated, from the earliest forms of star sightings off ships, all the way to how we're looking to navigate off to Mars and even further places," says Holt.

Even the most far-flung missions still involve the same fundamental concepts, he says: looking at stars, looking at objects in comparison to the stars, and using geometric relationships to determine where we are. "Whether we are doing it manually with a stick, or a sextant, or a high-tech camera, it's fascinating to me that a lot of these same techniques are all linked and have those common heritages and mathematical foundations."

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