Space scientists are excitedly poring over electromagnetic postcards received from interstellar space: data sent back by the Voyager 1 spacecraft as it speeds past the edge of the solar system more than 12 billion miles away. Even at the speed of light, radio signals sent over such an immense distance takes 17 hours to reach Nasa’s deep space tracking antennas.
It’s been an amazing journey for Voyager 1 since it was launched from Florida in 1977, a couple of weeks after its sister probe, Voyager 2, lifted off. Travelling along different flight paths and at different speeds out of the solar system, these doughty twins have, between them, explored all the giant planets of the solar system — Jupiter, Saturn, Uranus, and Neptune. In 1979, the space probes relayed to Earth astonishing images and data of over 50 of the outer planets’ moons, their ring systems, and the magnetic fields swirling around them. Voyager 1 focused on Jupiter and Saturn, collecting information on the planets’ atmospheres, interiors, satellites and magnetospheres. With that part of its mission completed, Voyager 1 switched off its cameras for more than 10 years until, in early 1990, they were turned back on to take pictures of the Sun and six of the planets — the first shots ever taken from so far out.
The technology of the 1970s that powers Voyager 1 rivals today’s know-how for space exploration. And Voyager 1 has probably the fastest propulsion system ever devised — faster than spacecraft with superior technology launched years later (never mind if this is largely due to Voyager 1’s close encounters with the giant planets when it used their gravitational fields as ‘slingshots’ to propel itself faster into space).
Scientists looked for three signs to confirm Voyager 1’s exit from the solar system into interstellar space: a drop in the density of plasma (the bubble of charged particles the Sun blows out into space to form the heliosphere), a surge in the plasma density and a shift in Voyager 1’s magnetic orientation that would point to changes in plasma density. The spacecraft’s plasma sensor, however, stopped working soon after it left Saturn’s orbit in 1981. So scientists were forced to try and detect changes in the plasma by measuring fluctuations in the magnetic field. Then last April, their luck turned when a sudden outburst from the Sun caused the plasma around Voyager 1 to oscillate. The resulting measurement of plasma density showed that it was 40 times denser than the plasma inside the heliosphere. Which meant the spacecraft was now sailing in interstellar space.
The extremities of the solar system are not defined by the orbit of Pluto, or by the heliopause, where the solar ‘wind’ of electrically charged particles — blowing at a million miles an hour — no longer pushes outward against the interstellar ‘wind’. In fact, the Sun’s gravity is felt, albeit feebly, more than a thousand times farther out in the Oort’s Cloud where comets graze. This is where the final frontiers of the sun’s empire gradually melt into what’s truly outer space, a mysterious realm where no man-made craft has gone before. Voyager 1 is headed that way now.
Even at speeds touching 40,000 mph, it will be some 20,000 years before Voyager 1 wanders into the Oort’s Cloud, and another 40,000 years before they pass the edge of cometary space. By this time, it will have travelled a distance of two light-years — half way to Proxima Centauri, the nearest star. On each Voyager probe is attached a golden gramaphone record of greetings and music pieces, along with iconographic instructions to access them, in the hope that it may someday reach sentient beings in a faraway star system. If not, chances are that eons after humans have migrated to other parts of the galaxy, or self-destructed, Voyager 1 will still be travelling across the universe.
Prakash Chandra is a science writer
The views expressed by the author are personal