When the gods dance...

Saturday, January 5, 2013

Letter from space: Farewell to Voyager 1

A montage of images taken by the Voyager spacecraft of the solar system
A montage of images taken by the Voyager spacecraft of the solar system. Image: Nasa

After a journey of 11 billion miles, the Voyager 1 spacecraft could be forgiven for asking the question: “Are we there yet?” Un­fortunately for the probe, the answer is no. Launched in the summer of 1977, the spacecraft Voyager 1 and Voyager 2 have been travelling in space for more than 35 years. They hold the record for being the man-made objects that have travelled the furthest.

The primary mission was supposed to last just five years but it is a project that just keeps giving. Even now, data coming back from Voyager 1 is redefining our thoughts on what makes up the edge of the solar system. (Although Voyager 2 was launched first, it has been overtaken by its sister craft and is currently in a semi-dormant state.)

As Voyager 1 heads for outer space, part of the challenge is how to define what the “edge” of the solar system is (its limit is usually agreed to be where the sun’s influence ends). At a distance of 11 billion miles, Voyager 1 now feels the effect of the sun mainly through its magnetic field and the solar wind – a stream of energetic particles flowing through space that originated from the sun’s surface. These particles form a sort of envelope around the solar system, known as the “heliosphere”. In August 2012, Voyager entered a region where these solar winds increased in speed and where high-energy particles from outside the solar system are also entering the heliosphere via interstellar magnetic fields.

An artist's impression of Voyager 1. Image: Nasa

Edward Stone, the current Voyager project scientist, says: “Voyager 1 still is inside the sun’s environment. We can now taste what it is like on the outside, because the particles are zipping in and out on this magnetic highway.” Models had not predicted this intense magnetic region. Stone adds: “The new region isn’t what we expected but we’ve come to expect the unexpected from Voyager.” That sentiment seems to sum up the mission to date.

When the two spacecraft were launched, the hope was that they would follow in the footsteps of the Pioneer 10 and 11 missions, which were launched in the early 1970s. They were designed to investigate the solar system beyond Mars, visiting the gas giants and, where possible, their more interesting moons. Although the primary five-year mission was intended to focus on Jupiter, Saturn and its rings, the mission was extended to include Neptune and Uranus and 49 moons in orbit around the gas giants. It was then further extended to look at conditions beyond our solar system.

The challenge of flying past these multiple planets and moons, with limited on-board propulsion, was solved by a convenient cosmic alignment that occurs once every 170 years. The trajectories of the Voyagers, according to the science writers Kevin Orrman-Rossiter and Alice Gorman, enabled them to “receive a gravity-assisted boost to their speed and direction” as they flew past each planet. (The technique was first demonstrated on Nasa’s Mariner 10 Venus and Mercury mission in 1973-74.) “Without this,” write Orrman-Ros­siter and Gorman, “the trip to Neptune would have taken 30, rather than ten, years” – and Voyager 1 would not have made it to its current position at the cusp of the solar system.

Jupiter and its moons. Image: Nasa

Both spacecraft are still sending scientific information about their surroundings through Nasa’s Deep Space Network. It takes about 13 hours for a signal from earth, travelling at the speed of light, to reach Voyager 2 and 16 hours to reach Voyager 1.

The equipment they use for this is surprisingly low-tech: 23-watt radios. This is higher than the three watts that a typical mobile phone uses but it’s still quite modest, considering the distances the signals have to travel. Therefore, crucial to receiving the signals over such large distances is not just the power of the radio but also size of the antennae and the directional accuracy with which they point at each other.

How do you provide power for a mission that could potentially last decades and in which the journey takes you well beyond the radiation limits that can supply solar panels? For the Voyager spacecraft, the secret is in their radioisotope thermoelectric generators (RTGs). These are, in effect, miniature nuclear power plants on-board the spacecraft. The RTGs are fuelled by plutonium-238, which Nasa hoped would keep the ships going for 50 years when the mission was launched. However, to make the most of the remaining power, many of the on-board instruments are turned off.

When the two spacecraft are well beyond the solar system and get powered down in 2025, they will continue to serve the mission as earth’s envoys. Each spacecraft has a 12-inch, gold-plated copper disc attached to its outer shell. These contain etchings of pictures, music and greetings from earth, as well as scientific data and instructions of how to play them. The discs are phonographic records full of information. These may seem a little primitive to serve as the first introduction to earth and its people but they are very much a product of their time. In the 1970s, the disc was avant-garde.

The Voyager spacecraft were fitted with five main instruments; each instrument is identically duplicated on the two spacecraft. They have the ability to measure low-energy-charged particles and magnetic fields and have ultra­violet and infrared spectrometers for atmospheric analysis.

A wealth of scientific information has been gathered through each of Voyager’s close encounters with planetary bodies. However, while the scientific community has relished this mountain of data the mission has gathered, it is the Imaging Science Subsystem, made up of a wide-angle and a narrow-angle camera, that has caught the public’s imagination.

The pictures these cameras have sent back are some of the most awe-inspiring ever seen. On 18 September 1977, just two weeks into the mission, Voyager 1 took a picture of the crescent-shaped earth and the moon in a single shot – the first image of its kind ever taken by a spacecraft. By that time, the spacecraft had already travelled 7.25 million miles.

A long way from home: an image taken by Voyager 2 on 18 September 1977, when it was 7.25 million miles from Earth. Image: Nasa

Photography of Jupiter began in January 1979 and ended in August that year. Between them, the two Voyager spacecraft took more than 33,000 pictures of the great planet and its five major satellites and gathered a wealth of other scientific data. Voyager 1 and 2 captured a large volcanic eruption occurring on Jupiter’s moon Io – the first time volcanic activity had been seen on any planetary body other than earth. We have since learned that there is 100 times more volcanic activity on Io than on earth.

Perhaps the best-known photo taken by the Voyager mission is one of the least striking – at least, on first glance. In 1981, the spacecraft had finished its primary mission and was travelling beyond Saturn when Nasa suggested it should point back towards earth and take a photo. Concerns that using the camera equipment so close to the sun risked damaging it delayed the picture but, in 1990, the image known as Pale Blue Dot was finally taken: earth, a tiny speck, is barely visible from the distance of six billion kilometres away. Of it, the astronomer Carl Sagan said: “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 . . . on a mote of dust, suspended in a sunbeam . . . There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish that pale blue dot, the only home we’ve ever known.”

The pale blue dot. Image: Nasa

As the Voyager spacecraft crosses the threshold between our solar system and the rest of the universe, we should mark its departure with admiration. As a space scientist, I laud its amazing engineering and the discoveries it has made. The Voyager spacecraft have given us a sneak preview of what is out there in the outer solar system. The knowledge they have imparted has given rise to other missions undertaken, planned and to be implemented in the future. They have shown us where the interesting bits of the outer solar system are.

Will either probe ever rendezvous with intelligent life beyond the solar system? In about 40,000 years, Voyager 1 will pass within a few light years of the minor star AC+79 3888 but the probability of it being discovered is very low. If, by chance, it is found by some civilisation, what is the probability that we humans will still be around when they try to contact us?

Maggie Aderin-Pocock is a space scientist and science communicator


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