Sen—As well as helping us learn more about other planets in the Solar System, space probes have given humankind a new perspective on this world that we all inhabit.
Since the dawn of the space age, countless images of planet Earth have been returned from spacecraft at a range of distances.
Iconic examples include a view of our planet rising above the Moon as seen by the the first humans to travel so far, the crew of Apollo 8 at Christmas 1968.
But perhaps the most famous and thought-provoking of them all was one taken 25 years ago yesterday. It was the least detailed ever, but snapped from the depths of the Solar System by a robotic probe. Voyager 1 was racing away at 40,000 mph when it took the memorable shot that became known as the Pale Blue Dot.
The craft and a twin probe, Voyager 2, had completed their main tasks, taking advantage of a fortuitous alignment of the outer planets to make a grand tour of these distant worlds.
Earth is revealed as just a speck in a chance beam of light in Voyager 1's iconic photo. Image credit: NASA/JPL
Voyager 1, launched in September 1977, had given us spectacular close-ups in 1979 of the clouds and belts of Jupiter, looking rather like pop-art. At Saturn in 1980 we got our first view of the planet as a crescent, its dark shadow cutting across the brilliant rings. Its biggest moon Titan was revealed to have a thick orangy atmosphere.
The spacecraft began heading upwards and out of the general plane of the planetary orbits thanks to a gravitational boost from Saturn. Then in February 1990, from a distance of four billion miles, and beyond the orbits of Neptune and Pluto, it did something extraordinary that had not been part of its original flight plan. It turned round and took a look back at the family of worlds it was leaving.
A sequence of 60 images was taken and saved to tape. They took three months to be radioed home, during March, April and May, once the spacecraft had returned to its usual orientation.
Snapshots of six planets were captured—Venus, Earth, Jupiter, Saturn, Uranus and Neptune. (Mercury was lost in the Sun's glare and Mars and Pluto, which was then still regarded as a planet, were too small to show at such a distance.) Earth was revealed as just a tiny speck caught in a ray of light produced by sunlight reflecting within the camera’s optics.
This “family portrait” was the idea of two people, astronomy populariser Carl Sagan, famous for his TV series Cosmos, and independently Carolyn Porco, who is now well known for her work with Cassini, but was then a young member of the Voyager team from the University of Arizona.
Initially they came up against much opposition to the idea from mission managers at NASA’s Jet Propulsion Laboratory in Pasadena who failed to see the point or feared that Voyager 1 might be damaged.
Porco told me later: “Soon after I joined the Voyager project in October 1983—so somewhere between Saturn and Uranus—I started promoting the idea of having Voyager turn around and take a picture of the Earth and all the other planets. I thought it would be a magnificent ‘we are here’ look at our cosmic place.
“The mission managers at JPL looked at me like I was crazy because there was clearly no ‘science’ in doing so—all these objects would be a pixel across or less—and the engineers didn’t want to waste precious resources to make a useless observation.
“A couple years later I learned that Carl Sagan had ALSO proposed this ‘portrait of the solar system’ to the Voyager project a year or two earlier, for the same reasons, and he too was given the same response. When I found this out, I spoke to him about it, and we decided to join forces.”
The frames taken by Voyager 1 that captured six of the planets, including Earth. Image credit: NASA/JPL
Sagan went to the very top at NASA, and persuaded the then Administrator, Richard Truly, to agree to the picture while Carolyn worked with JPL’s Candy Hansen on the practicalities of making it happen. Years later, as head of Cassini’s imaging team, she captured Earth again from the orbit of Saturn.
In Voyager 1’s original image, Earth was close to invisible, a tiny crescent occupying less than a pixel across. But that was exactly what made the picture so powerful. It revealed the fragile nature of our small and vulnerable home, suspended in the void of space.
Sagan dubbed it the Pale Blue Dot, and later wrote: “Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives . . . every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.
“Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves . . . To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.”
They are words we might do well to remember today more than ever.