This week’s news that Voyager 1 has left the solar system is pretty cool.
Actually, not just cool. Totally awesome. JPL scientists were able to use Voyager’s ‘plasma wave’ instrument to measure oscillations in plasma around the probe, giving its density; and this week, the plasma science team at the University of Iowa analysed the data and confirmed that the probe had entered interstellar space.
The important part is what it means. For the first time in the 4.6 billion year history of our world, something from Earth has left the solar system.
It is the stuff of dreams.
Not bad for a space probe launched when trousers were flared and disco was in. The day was 5 September 1977. With its twin, Voyager 2, the 770kg probe was the low-budget reality of a ‘grand tour’ that fell victim to post-Apollo penury. And once their Titan-Centaur boosters had exhausted their fuel, the Voyagers were on a rail for the interstellar void. Both had gold records on board – not Bee Gees – recording human endeavour, should the probes ever be found some time.
But both Voyagers were still up for major course changes for free. In the late 1970s, the four major planets – Jupiter, Saturn, Uranus and Neptune – were aligned in such a way that a probe could be lobbed from one to the other, using the gravitational field of each planet to bend the trajectory. Some of the momentum of each world was transferred to the probe, slowing the planet infintesimally, and radically altering the course of the Voyagers.
Voyager 2 flew past all four giant planets. But Voyager 1 encountered only Jupiter and Saturn. By taking a different trajectory, Voyager 1 could get a close look at Titan, Saturn’s largest and most enigmatic moon.
That carried a cost. Voyager’s course was twisted out of the plane of the ecliptic, the narrow band in which the planets orbit the Sun. And now – 36 years and 18.6 billion kilometres after launch, Voyager 1 has passed through the heliopause – where the solar wind thrown off by the Sun meets the interstellar medium – and stops.
It’s unknown space. Although the Oort cloud and dwarf planets such as 90377 Sedna, have orbits that go beyond it, the heliopause is a practical line in the sand (well, in the vacuum) for scientific purposes.
Voyager 1 detected that the solar wind had dropped to zero in 2011, meaning it was approaching the border. It’s likely the probe crossed into the interstellar void in August 2012. Certainly it was well into the interstellar medium this year – some 125 times as far away from the Sun as Earth.
It’s worth remembering that these measurements involve detecting miniscule numbers of particles. Despite interesting artistic renderings like the one above, both heliopause and interstellar medium are hard vacuum, with a density of one atom per cubic centimetre.
The practical mission of both Voyager probes will finish in a decade or so. They are each powered by three radioactive thermal generators, and those are dwindling. By 2025-30 there won’t be enough power to run even individual instruments. But that won’t stop both hurtling onwards into deepest space until they hit something – or the universe ends, monuments to human endeavour that will last longer, perhaps than our own planet. In 40,000 years, Voyager 1 will come within 1.6 light years of the star Gliese 445 – an M-class dwarf star which, by then, will be a mere 3.45 light years from us. It’s coming towards us faster than Voyager 1’s heading for it.
The whole thing is pretty cool and amazing.
Copyright © Matthew Wright 2013