While people around the world celebrate Valentine’s Day today, a famous astronomical photo turns thirty years old. In 1990, the Voyager 1 spacecraft—then at a distance of 6.4 billion kilometres from Earth—turned around and took pictures of all the planets. When the images were collected and placed together, they created a family portrait of the solar system. On one photo was a tiny blue speck, almost lost in the Sun’s glare: the planet Earth. The dot was so insignificant that astronomers initially thought it was a dust particle on the equipment. The picture has come to be known as the ‘Pale Blue Dot’.
The Voyager Missions
Image from NASA
In the mid-1960s, when NASA started to consider sending probes beyond Mars, they made a tantalising discovery: every major planet was on the same side of the Sun at the same time. The planets’ orbits aligned, almost perfectly, between the late 1970s and early 1980s. This event only occurs every 176 years. During this time, the planets could be used as gravity assists: as a spacecraft heads towards a planet, it’s pulled in by gravity. If the craft passes the planet at the right angle, it’s hurtled off in the direction of its next target. Through this method, NASA could visit every planet in a single mission.
Launched in 1977, the twin Voyager spacecrafts visited the gas and ice giant planets. Voyager 1 flew by Jupiter in March 1979, Saturn in November 1980 and then headed towards deep space. Voyager 2 travelled to Jupiter and Saturn but also Uranus in January 1986 and Neptune in August 1989, making it the first and only spacecraft to reach the two furthest planets.
Over 67,000 photos were taken between both spacecrafts as well as many other types of scientific measurements. They discovered many new moons and witnessed other firsts, such as working volcanos on Jupiter’s moon Io (the first known geologically active planetary body after the Earth) and a thick atmosphere covering Saturn’s moon Titan. Scientists were surprised because they expected giant planets’ moons to be desolate, with lots of craters, like our own.
The Portrait of the Planets and the Pale Blue Dot
Image from Planetary Society
Scientist Carl Sagan had campaigned for a ‘Portrait of the Planets’ since the early 1980s. However, NASA and the Jet Propulsion Laboratory had rejected the idea because they didn’t want to risk the Voyager spacecrafts before completion of their initial missions. By the end of the decade, the two spacecrafts had completed their flybys of all the major planets. Sagan got his wish and his portrait was allowed before the Voyagers began their interstellar missions. Voyager 1 was picked for the portrait because it was more expendable if its scientific instruments were damaged when it turned and faced the Sun.
Sagan is best known as the host of the original Cosmos TV series, which came out in 1980. He was also a NASA advisor and played a big part in a number of missions, including early attempts to flypast and land on Mars. He was the first scientist to predict Venus’s hellish climate.
Image from The New Yorker
Voyager 1 took 60 photos all together, including three colour-filtered ones. Every planet was captured in the images except two: Mercury was lost in the Sun’s light and Mars was too small to spot. Still regarded as a major planet at the time, Pluto was too far away and was missed as well. This mission was the last time Voyager 1’s cameras were used.
Earth appears less than a pixel in diameter in the Pale Blue Dot picture. The image was special because it reminds us how small our world really is against the endless vastness of the universe. Sagan presented the media event announcing the photos, but he best summed up the general feeling in his 1994 book of the same name:
‘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. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every “superstar,” every “supreme leader,” every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.’
The Voyager Interstellar Missions
Sagan had a hand in the beginnings of the Voyager mission. With the help of fellow scientists, he designed a ‘hello’ plaque in case the crafts encountered alien life. The plaque showed what humans looked like and where Earth was located in the galaxy. The spacecrafts also came with two gold-plated vinyl records. They contained analogue photos of our home, greetings in 55 languages and over 90 minutes of popular music. The songs were quite diverse and included works from Beethoven to Chuck Berry. Sagan picked them himself.
Since 1990, the two Voyager spacecrafts have been on their interstellar missions. They take readings of the Sun’s influence as the head towards deep space. Voyager 1 officially left the Solar System in August 2012 and Voyager 2 followed in November 2018.
The spacecrafts have been in flight for over 40 years, well beyond the single-decade lifetime they were designed for. No other human-made object in history has travelled as far from Earth as they have. Voyager 1 and 2 are getting more distant with each passing day. NASA are also expecting to get at least another few years of science out of the two Voyager spacecrafts before power levels fail completely.