Nine probes reached the outer solar system: Where are they now?


Besides Cassini, eight missions have passed the asteroid belt – and several are still broadcasting from the furthest solar system and beyond.

Pioneer 10

Launched: 3 March 1972

Pioneer 10 was the first probe to cross the asteroid belt, traversing it between July 1972 and February 1973. Arriving at Jupiter in December 1973, it passed some 132,000 kilometres from its cloud tops, and obtained fuzzy images of the four large “Galilean” moons, Ganymede, Europa, Callisto and Io. Now out of contact, this true space pioneer was last spotted coasting towards the constellation Taurus and the red star Aldebaran, which it should reach some 2 million years from now.

Current status: Last contact 23 January 2003, now estimated to be 16 billion kilometres from Earth

 

Pioneer 11

Launched: 6 April 1973

Visiting Jupiter a year after Pioneer 10, Pioneer 11 continued to Saturn, testing the dangers of navigating the planet’s rings and flying within 21,000 kilometres of its surface on 1 September 1979. It almost collided with a small Saturnian moon and photographed Titan, the largest moon. An anomalous slowing of both the Pioneer probes brought long-lasting speculation that the established laws of gravity didn’t work in space. The “Pioneer anomaly” is now thought to be down to heat loss from the probes’ thermoelectric generators.

Current status: Last contact 30 September 1995, now estimated to be 14 billion kilometres from Earth, heading towards the constellation Scutum

 

Voyager 2

Launched: 20 August 1977

In the 1960s, space scientists realised that a happy configuration of the outer solar system would allow one probe to visit four planets. Voyager 2 remains the only probe to have visited the two furthermost ice giants: Uranus in January 1986 and Neptune in August 1989. Its primary radio receiver failed in 1978, but 40 years on it is still sending back data as it crosses the edge of the solar system, called the heliosheath, and enters interstellar space.

Current status: 17 billion kilometres from Earth, heading towards the constellation Telescopium

 

Voyager 1

Launched: 5 September 1977

Voyager 1 launched after Voyager 2, but took a faster trajectory to Jupiter and Saturn, arriving at both first. Its route was optimised to bring it within 6500 kilometres of Titan, confirming Pioneer 11’s observation that the moon possessed a thick atmosphere. On 14 February 1990, Voyager 1 turned its camera to take the first family portrait of Earth and other solar system planets. Still transmitting from interstellar space, Voyager 1 is now the furthest human-made object from Earth. Both Voyager probes carry “golden records” of sounds and images of Earth for any alien intercepter.

Current status: 21 billion kilometres from Earth, heading towards the constellation Ophiuchus

 

Galileo

Launched: 18 October 1989

Galileo was the first mission to spend years orbiting a planetary system, rather than simply passing through on its way elsewhere. On its six-year journey to Jupiter, it turned its instruments on Earth, picking up signs of life such as the absorption of red light by chlorophyll. Inserted into Jupiter orbit on 7 December 1995, Galileo’s activities included sending a probe into the giant planet’s atmosphere . It also collected data supporting the theory that Jupiter’s