Probes & rovers

Humans have so far travelled no further than the Moon, and the last astronauts of the Apollo missions stood on its surface back in 1972. Our robots, on the other hand, have visited every planet of the solar system, landed on a comet and are rolling across Mars right now. Space probes need no air, no food and no return ticket. That is exactly why they are our boldest explorers.

Voyager: mail between the stars

The two Voyager probes launched in 1977: both flew past Jupiter and Saturn, and Voyager 2 then went on as the only probe to visit Uranus and Neptune. Then they simply kept going. In 2012 Voyager 1 became the first human made object to reach interstellar space, and today it is around 25 billion kilometres away, so far that its radio signal takes almost a full day to reach Earth. After nearly 50 years both probes are still transmitting, with a transmitter weaker than a light bulb. Each of them carries a gold plated record with music, sounds of nature and greetings in many languages, a message in a bottle in case somebody ever finds it.

Rovers: geologists on wheels

Rovers are on the move on Mars: driving laboratories with cameras, drills and instruments. Curiosity has been exploring an ancient crater lake since 2012, Perseverance has been collecting rock samples since 2021 for a later return to Earth, and even carried Ingenuity, a small helicopter that became the first aircraft to fly on another planet. Rovers drive at a snail's pace, often only around 100 metres per day. Caution comes first: on Mars, no tow truck will ever come.

Why rovers have to think for themselves

You cannot drive a Mars rover with a gamepad. Radio signals travel at the speed of light, but still need 3 to 22 minutes to reach Mars depending on where the planets are, so a reply takes twice that. By the time the camera image arrives and your command is sent back, the rover would long since have rolled into the rock. So the team sends a plan for the day in the morning, and the rover works through it on its own: it spots obstacles with its cameras and picks its own path.

Borrowing momentum from a planet

For the outer planets, a rocket's fuel alone is often not enough. The trick is the flyby manoeuvre: the probe passes close to a planet and lets its gravity drag it along. In doing so it steals a tiny bit of the planet's momentum and leaves faster than it arrived. The planet is so huge that it never notices. Voyager 2 swung its way from Jupiter past Saturn and Uranus all the way to Neptune, and almost every probe to distant targets uses such gravity slingshots today.