Space probes to the planets and beyond

Voyager 1, launched in 1977, will be one light-day from Earth, about 26 billion km, in November 2026.

Robots that travel for decades

People have been to the Moon, but every other world we know up close was visited by robots. A space probe carries cameras and instruments, a radio to talk to Earth and enough power to keep working for years, sometimes decades. Sunlight weakens with distance, so many outer-planet probes use radioisotope generators that turn the heat of decaying plutonium into electricity. Some, such as Juno at Jupiter, use large solar arrays instead. Voyager 1 left with three radioisotope generators, giving about 158 watts each at launch.

Gravity assists

Rockets alone cannot give a probe enough speed for the outer planets, so mission designers borrow it. When a probe flies close behind a moving planet, the planet's gravity pulls it along and flings it onward faster, taking a tiny, unnoticeable amount of the planet's orbital energy. This gravity assist is how the Voyagers reached the outer planets in years rather than decades. Danuri, in the last unit, used a gentler version of the same idea to save fuel on its way to the Moon.

Voyager 1 and 2

The two Voyagers launched in 1977, Voyager 2 first on 20 August and Voyager 1 on 5 September, but Voyager 1 was on a faster route and overtook its twin in December. NASA lists Voyager 1's closest approach to Jupiter on 5 March 1979 and to Saturn on 12 November 1980. Voyager 2 went on to Uranus and Neptune, taking 12 years to reach Neptune, and is still the only spacecraft to have flown past those two planets. In August 2012 Voyager 1 crossed the heliopause, the edge of the bubble of particles and magnetic field blown out by the Sun, and became the first human-made object in interstellar space.

Voyager 1 is still talking to Earth. NASA expects it to reach one light-day from Earth in November 2026: the distance light covers in 24 hours, about 26 billion km. A radio command sent from Earth then takes a full day to arrive, and the answer another day to come back. Its power is running down, so engineers switch off instruments one by one to keep the rest working.

Distance makes every conversation slow. Pluto was about 32 times as far from Earth as the Sun when New Horizons flew past, so its radio signals, travelling at the speed of light, needed about four and a half hours to arrive: 32 × 149.6 million km ÷ 299,792 km/s ≈ 16,000 s. No one on Earth could steer the flyby live. The probe ran the whole encounter from commands loaded in advance and sent its pictures home slowly over the following months.

Pluto and the Sun

NASA's New Horizons made the first visit to Pluto. After nearly a decade in flight it made its closest approach on 14 July 2015, about 7,750 miles (12,500 km) above the surface, roughly the distance from New York to Mumbai, then continued into the Kuiper Belt. Going the other way, NASA's Parker Solar Probe was built to touch the Sun. On 24 December 2024 it flew about 3.8 million miles (6.1 million km) from the solar surface, the closest any spacecraft has come, at about 430,000 miles per hour, roughly 192 km every second, the fastest any human-made object has travelled.