Great robotic missions to explore space
The world watched in awe as the New Horizons spacecraft sent back the first detailed pictures of Pluto. And this week, scientists published results from the Philae lander which touched down on a comet last year. Such missions not only add to our scientific knowledge, they cause cultural shifts that expand our perception of ourselves and our place in the Universe. Here are five other robotic space missions that have also had that effect.
Hubble Space Telescope
For 25 years, the Hubble Space Telescope has captured the beauty and majesty of the cosmos. HST has remained in Earth's orbit - but through its powerful mirror has enabled us to travel to the very edge of the visible Universe. It has shown us that our galaxy is just one among 100 million.
The telescope has taken pictures of clouds of gas that come together to give birth to stars in stellar nurseries, just as our own once did. The images were confirmation that we are all the stuff of stardust. Hubble has also shown us the mortality of our Solar System, capturing images of other stars dying cataclysmically in spectacular supernovas. The images have made the telescope a beloved icon as well as a legendary scientific instrument.
Two Voyager spacecraft were launched in 1977 to embark on a tour of the planets. It was the start of a journey that would take them both to the outer reaches of the Solar System and in the case of Voyager 1, beyond. The spacecraft is now the first human-made object to have left the Solar System and entered a new region of space.
In the days before the internet, people packed into planetariums to watch Voyager images arrive. People gasped as they saw in detail the Great Red Spot of Jupiter and Saturn's shimmering rings. Just as the "Blue Marble" view of the Earth captured by Apollo 17 in 1972 showed the fragile magnificence of our planet, a look back from Voyager 1 captured in 1990 showed our home as a pale blue dot in the vastness of space, bringing home our smallness in the cosmos.
Voyager 1 journeyed out past the planets of our Solar System and, as it travelled, it was bombarded by particles from the Sun - something scientists call the solar wind. The further it went, the quieter the wind became. In 2012, it passed a point where it began to sense the particles that lie between the stars - the breeze of interstellar space.
Voyager's mission will continue as it becomes an emissary of human civilisation. As one of the Nasa team put it at the time, Voyager is now "through the door and on its way to eternity".
Saturn is one of the largest bodies in the Solar System. The gas giant planet and its rings are clearly visible through simple telescopes. But until the Cassini Huygens mission, which arrived in the Saturnian system in 2004, scientists had few details about the planet and its moons. A highlight was the landing of the European Space Agency's Huygens probe on Saturn's largest moon Titan in 2005.
The Cassini spacecraft swooped through a gap in Saturn's rings and toward the giant, orange moon. The first pictures showed that Titan was shrouded by clouds but infrared and ultraviolet cameras were able to peer through the haze to the surface. Then, Cassini released the Huygens probe which fell toward the mysterious moon below. An onboard microphone sent back the first audio from Titan: a sound that seemed like the whooshing of wind.
And then radar measurements picked up evidence of rain and an active weather system followed by startling pictures of what seemed to be a large lake or sea bed. They also showed white clouds and river-like channels that could have been carved out by liquid methane.
The first colour picture showed orange skies, a recently dried-up lake bed, a thick fog in the background. And in the foreground, cold, hard pebbles made of water-ice.
Viking, Spirit and Opportunity
There have been numerous missions to Mars - many of them spectacular in their own way. Even some of the failures, such as Britain's Beagle 2, have been memorable. Among the most vivid, though, was the first Viking lander, which set down on the Martian surface in 1976. The pictures it sent back included the first panoramic photograph from Mars.
Astonishing as these pictures were, the twin Nasa rovers, Spirit and Opportunity, which landed in 2004 provided many with a more profound experience. The comparative deluge of images from these robots made us feel that we were truly on the Martian surface. And as the rovers roamed, so did we - as if on a tour of the Red Planet. Through Spirit and Opportunity's cameras, we were able to view the planet's diverse terrain in exquisite detail.
The Giotto mission was the first to look inside a comet in 1986. This mission was made all the more special because it was taking a look inside Halley's, the most famous of them all. For centuries Halley's comet made regular visits toward Earth, inspiring awe. Many believed the comet to be a harbinger of doom or a fiery angel. Even as late as 1910, there were fears that Halley's arrival would lead to mass poisonings. Unscrupulous people sold "comet pills" and gas masks.
Finally, scientists were able to lift the veil on this feared denizen of the outer Solar System. The mission discovered that Halley had a dark, dusty peanut-shaped body. The spacecraft also detected jets of gas being pushed out from the comet's core. Later analysis showed that Halley had formed in the early days of the Solar System from ice and dust and remained little altered since.
Prof Andrew Coates, of the Mullard Space Science Laboratory, worked on the mission as a young scientist. He vividly recalls the days that Giotto sent back its extraordinary images.
"That was my first adventure in space, it was the first mission I worked on," he said.
"It was Europe's first venture into interplanetary space. Scientifically, it was unveiling comets for the first time. It was a huge triumph for Europe."
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