Besides Cassini, eight missions have passed the asteroid belt – and several are still broadcasting from the furthest solar system and beyond.
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
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
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
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
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 moon Europa has a subsurface liquid ocean.
Current status: Mission terminated with a plunge into Jupiter’s atmosphere on 21 September 2003
Launched: 6 October 1990
The prime objective of the Ulysses probe was to survey the sun, but it took a long gravitational slingshot around Jupiter, thus entering an orbit over the top of the solar system that enabled it to monitor the sun’s north and south poles.
Current status: Decommissioned 30 June 2009
Launched: 15 October 1997
Spending 13 years cruising Saturn’s moons, Cassini fulfilled the goal of sending a probe to the moon Titan.
Current status: Mission due to terminate in Saturn’s atmosphere, 15 September 2017
Launched: 19 January 2006
It is the fastest spacecraft ever launched, but by the time New Horizons reached Pluto on 14 July 2015, its destination had changed: Pluto had been controversially downgraded by the International Astronomical Union from “planet” to “dwarf planet” in August 2006. New Horizons took intriguing photos of this rocky world’s hazy atmosphere and surprisingly varied, craggy surface, as well as its moons. It is now on its way for a rendezvous with the snappily titled space rock (486958) 2014 MU69 in the Kuiper belt, where it is expected to arrive on 1 January 2019.
Current status: In the Kuiper belt, 5.7 billion kilometres from Earth
Launched: 5 August 2011
Unlike previous probes to the outer solar system, Juno doesn’t have a nuclear reactor at its heart: it is powered entirely by solar panels. Juno entered into a polar orbit around Jupiter on 5 July 2016, with the intention of measuring the composition and gravitational and magnetic fields of the solar system’s largest planet, as well as testing theories of how it formed. Its first results indicate some surprises: huge magnetic and atmospheric storms, and the revelation that Jupiter isn’t as uniform as had been assumed.
Current status: In orbit around Jupiter, currently 0.95 billion kilometres from Earth