If you’re lost somewhere on the earth, you can use GPS or public landmarks to find your way back. Navigation in the vastness of space isn’t so easy, however — especially if you’re on a spacecraft racing billions of kilometres away from the earth. There is no GPS in space and where a landmark is depends on where you’re looking from.
But recently, astronomers found that a deceptively simple trick could help the New Horizons spacecraft find its way through space using only two stars.
‘Navigation is essential’
NASA launched New Horizons in 2006 to explore Pluto, its moons, and, later, objects in the Kuiper Belt, a disc of icy rocks and dust in the outer edge of the solar system. New Horizons flew by Pluto in 2015, beaming back stunning images of the dwarf planet and its moons. In 2024, the spacecraft went beyond 60x the sun-earth distance, a new record.
“Navigation is essential, just as it is on the earth,” Max Planck Institute for Astronomy staff member Coryn Bailer-Jones said. “Spacecraft have to know where they are and where they are heading for many reasons” — from planning their fuel use to pointing their antennae to send data back home.
Interplanetary missions like New Horizons are tracked by NASA’s Deep Space Network (DSN), a global array of giant radio antennae that send and receive signals from spacecraft. This system is precise but always earth-centred, i.e. the spacecraft’s location is measured relative to our planet. As spacecraft travel farther, signals between ground stations and the spacecraft drops, increasing communication time and decreasing signal strength. Therefore spacecraft that can find their way without asking for directions from the earth will be at an advantage.
The new study proposes a simple way to do this using stellar parallax, the same method early astronomers used to measure stars’ distances from the earth.
The findings were published in The Astronomical Journal in June.
One eye at a time
As the earth orbits the sun, a star’s position relative to other stars might seem to shift. This is called stellar parallax.
This is because every six months, the earth is on opposite sides of the sun, providing two different viewpoints. You can achieve the same effect by looking at a candle first with only your left right and then only your right eye: the candle will change position against the background.
The amount of shift is called the parallax angle and it’s directly related to the distance to the star.
In the study, the astronomers observed two stars from the earth and using New Horizons’ instruments. They’re like two eyes that were 7 billion km apart on April 23, 2020, when the team made the observations.
The two stars were Proxima Centauri and Wolf 359, located 4.2 light-years and 7.9 light-years away from the earth, respectively.
“We wanted large parallaxes, which means using the closest stars,” U.S. National Science Foundation National Optical-Infrared Astronomy Research Laboratory astronomer and the study’s lead author Tod Lauer said.
“Proxima Centauri is the closest star besides the sun. Wolf 359 is in the top five. We also needed stars that were not too bright for the camera. Wolf 359 was … optimal for navigation as it’s close to 90º away from Proxima.”
Self-sufficient
For the study’s purposes, the European Space Agency’s Gaia spacecraft played a vital role: it provided the 3D positions of stars in a fixed, universal reference frame.
With these reference positions, the team measured how much the stars’ positions changed against the background by comparing their positions from New Horizons’ vantage point with those from the earth.
The parallax was 32.4 arcseconds for Proxima and 15.7 arcseconds for Wolf 359 at the time of measurement. (An arcsecond is equal to 1/3600th of a degree.)
This calculation indicated that New Horizons was positioned at 46.89-times the earth-sun distance — close to the 47.11 AU estimated by radio-tracking measurements from the earth.
The technique could be the simplest to navigate spacecraft because it doesn’t require special equipment, Lauer said.
“All it needs is a camera, a standard spacecraft computer, and a catalogue for reference positions on any spacecraft.”
The team wrote in its paper that its demonstration was “educational”, not yet ready for practical use.
“We can’t improve [the accuracy] that much with New Horizons, but a better camera on another spacecraft would help,” Lauer said.
Stars as compass
The “study is a good one and a nice demonstration,” Bailer-Jones, who wasn’t involved in the study, said. “As they themselves note, it is not of practical use to navigate New Horizons, because we can still use earth-based beacons for this [distance].”
“But for much deeper space missions, such as interstellar travel, where beacons are no longer usable, an extension of this method becomes very useful.”
He also said an existing method called stellar astrometric navigation also uses stars as well as accounts for the effects of special relativity to estimate a spacecraft’s 3D position and velocity by measuring the angular separation between two stars in the sky. The new study has proposed a simpler version of this method, he added.
Bailer-Jones also said researchers are investigating another method called pulsar navigation, which uses rapidly spinning neutron stars like lamps in space to show the way.
“It’s potentially much more accurate than stellar astrometric navigation, but it needs reasonable knowledge of the initial position, which the stellar astrometric navigation could supply,” he said.
Lauer said he grew up watching Star Trek and that in one scene, he noticed the stars appeared to move as a spacecraft found its way through deep space. Did the show’s producers know they were being accurate or had they simply missed a detail?
The new method has rendered this question irrelevant.
Shreejaya Karantha is a freelance science writer.
Published – September 08, 2025 05:30 am IST
