Five robotic space missions that transformed our understanding of the universe: Voyager, Hubble, Cassini-Huygens, Kepler and the James Webb Space Telescope, with what each found and why it mattered.
Human spaceflight gets the headlines, but much of what we know about the solar system and beyond comes from robotic missions. Here are five that changed science, and what made each one special.
1. Voyager 1 and 2 (launched 1977)
NASA's twin Voyager probes launched in 1977 to take advantage of a rare alignment of the outer planets. Voyager 2 is still the only spacecraft to have visited Uranus and Neptune. Together they revealed active volcanoes on Jupiter's moon Io, the intricate structure of Saturn's rings, and the strange tilted magnetic fields of the ice giants.
Voyager 1 crossed into interstellar space in 2012, and Voyager 2 followed in 2018, making them the first human-made objects to do so. Both still send data, though NASA has been switching off instruments to conserve dwindling power. Each carries a Golden Record of sounds and images from Earth.
2. The Hubble Space Telescope (launched 1990)
Hubble had a famously rocky start: its main mirror was ground to the wrong shape, blurring early images. A Space Shuttle servicing mission in 1993 fitted corrective optics, and four further servicing missions upgraded it over the years.
Hubble helped pin down the expansion rate of the universe, and observations of distant supernovae by teams using Hubble and ground-based telescopes contributed to the discovery that the expansion is accelerating, work recognised by the 2011 Nobel Prize in Physics. Its Deep Field images showed thousands of galaxies in a tiny patch of apparently empty sky. More than three decades on, it is still operating.
3. Cassini-Huygens (launched 1997)
A joint mission between NASA, the European Space Agency (ESA) and the Italian Space Agency, Cassini arrived at Saturn in 2004. Its European-built Huygens probe landed on Saturn's largest moon, Titan, in January 2005, the most distant landing yet achieved. It revealed a world with rivers and lakes of liquid methane and ethane.
Cassini also discovered plumes of water ice spraying from the small moon Enceladus, pointing to a subsurface ocean that makes it a target in the search for life. In 2017, low on fuel, Cassini was deliberately plunged into Saturn to avoid any risk of contaminating those moons.
4. Kepler (2009 to 2018)
NASA's Kepler space telescope stared at one patch of sky, measuring tiny dips in starlight as planets passed in front of their stars. It found thousands of exoplanets, and its data showed that planets are common: most stars are thought to have them, and many are similar in size to Earth.
After a mechanical failure in 2013, engineers devised a clever workaround using sunlight pressure to stabilise the spacecraft, extending the mission as "K2". Kepler ran out of fuel in 2018. Its successor, TESS, continues the search across the whole sky.
5. The James Webb Space Telescope (launched 2021)
A partnership between NASA, ESA and the Canadian Space Agency, Webb launched on an Ariane 5 rocket on 25 December 2021. It orbits about 1.5 million kilometres from Earth, near a point called L2, and observes mainly in infrared light, which lets it see through dust and detect the faint, stretched light of very early galaxies.
Webb has found galaxies from within a few hundred million years of the Big Bang, analysed the atmospheres of exoplanets, and produced detailed images of star-forming regions. Its huge gold-coated mirror, made of 18 segments, had to unfold in space after launch, one of the most complex deployments ever attempted.
What these missions have in common?
- Longevity: many outlived their planned lifetimes by years or decades.
- International collaboration: several were joint projects between agencies.
- Open data: much of their data is publicly available, and amateurs and citizen scientists have made real discoveries in it.
Follow along
NASA and ESA publish mission news and free image archives. Citizen science projects on Zooniverse let you help classify real data, from galaxies to exoplanet signals.
Sources and further reading
- NASA: Voyager
- NASA: Hubble
- NASA: Cassini
- NASA: Kepler
- NASA: Webb
- ESA: Webb
- Nobel Prize in Physics 2011
- Zooniverse
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.
Frequently asked questions
What should you know about 1. Voyager 1 and 2 (launched 1977)?
NASA's twin Voyager probes launched in 1977 to take advantage of a rare alignment of the outer planets. Voyager 2 is still the only spacecraft to have visited Uranus and Neptune.
What should you know about 2. The Hubble Space Telescope (launched 1990)?
Hubble had a famously rocky start: its main mirror was ground to the wrong shape, blurring early images. A Space Shuttle servicing mission in 1993 fitted corrective optics, and four further servicing missions upgraded it over the years.
What should you know about 3. Cassini-Huygens (launched 1997)?
A joint mission between NASA, the European Space Agency (ESA) and the Italian Space Agency, Cassini arrived at Saturn in 2004. Its European-built Huygens probe landed on Saturn's largest moon, Titan, in January 2005, the most distant landing yet achieved.
What should you know about 4. Kepler (2009 to 2018)?
NASA's Kepler space telescope stared at one patch of sky, measuring tiny dips in starlight as planets passed in front of their stars. It found thousands of exoplanets, and its data showed that planets are common: most stars are thought to have them, and many are similar in size to Earth.