BEYOND THE SOLAR SYSTEM

Five of NASA's spacecraft are heading beyond our solar neighbourhood. What will they find?

- Andrew May

When NASA's Cassini mission to Saturn ended in 2017, the spacecraft was deliberately destroyed by crashing it into the planet. The next year, at the end of the Dawn probe's exploration of the asteroid belt, it was placed in a graveyard orbit around the dwarf planet Ceres. These are the two most common fates of interplanetary missions, but there's a third possibility. If a probe has sufficient speed to carry it out of the Solar System and into the space between stars, it can keep travelling forever.

Five probes have ended up on interstellar trajectories so far. Although their designers knew this would happen, it wasn't their main purpose. The first four - Pioneer 10 and 11 and Voyager 1 and 2 - were launched in the 1970s to study the Solar System's outer planets. More recently they've been joined by New Horizons, launched in 2006 en route to the Kuiper Belt, where it flew past Pluto in 2015 and Arrokoth in 2019.

Pinning down the edge of the Solar System isn't easy. By some definitions it might include the Oort Cloud, which surrounds the Sun at a great distance. By common convention, however, 'interstellar space' starts at a point called the heliopause. This is where the Sun's non-gravitational effects - its magnetic field and the solar wind - cease to be discernible against the background of the interstellar medium.

Voyager 1 passed this point in 2012, followed by Voyager 2 in 2018. The other three probes will follow over the next few decades, although we don't know exactly when, as the heliopause tends to drift about in an unpredictable way.

VOYAGER 1 & 2 THE GRAND TOUR