Read more: Could "solar sailing" take us to Alpha Centauri?
It’s not easy to get very excited about NASA’s Juno spacecraft mission to Jupiter, which entered orbit successfully around the giant planet on 4th July after a five- year voyage. Unlike the New Horizons mission unveiling the eye-popping mysteries of Pluto, or the European Space Agency’s Rosetta mission which landed a probe on a comet, Juno seems to be going into well-charted territory. Jupiter can be seen without too much effort even in amateur telescopes, and its bleary red eye—a storm three times the size of the Earth—is a familiar sight, no matter how glorious its baroque swirls.
And Juno’s objectives—to investigate the composition of Jupiter’s atmosphere and interior, and study its magnetic field—sound like stolidly unglamorous planetary science. Of course, this is all a build-up to my saying that you should not shrug off Juno as the prosaic face of space exploration. Truly, though, we might be in for some spectacular sights and surprises from this low-key mission. For one thing, Jupiter is really the key to the entire solar system. Easily the most massive of the planets, it is primarily a ball of gaseous hydrogen and helium—the same stuff from which the Sun is made. If Jupiter were only twice as massive, it wouldn’t be formally a planet at all, but one of the intriguing objects called sub- brown dwarfs or rogue planets: halfway between planet and star, made from gravitational collapse of their own gas clouds rather than the offspring of some parent star. At thirteen times the mass of Jupiter, such bodies become genuine brown dwarfs: quasi-stars dense enough to ignite nuclear fusion reactions and emit a dull glow.
Jupiter is thought to have changed its orbit somewhat over the lifetime of the solar system, and these wanderings gave rise to the asteroid belt and limited the size of Mars—and maybe of the Earth and Venus too. Jupiter’s huge mass makes it a gravitational magnet for comets: some, like Comet Shoemaker-Levy 9 in 1994, smash into the planet, but more are dispatched like slingshots to the outer solar system before they can collide with Earth. It’s possible that this might have helped life here to develop; but Jupiter’s gravity is also capable of nudging the asteroids towards the Sun, raising the possibility of their hitting our planet on the way. Either way, our fate seems closely dependent on the nearest and greatest of the gas giants.
The study of Jupiter, then, has led us to appreciate the interdependence of worlds in our solar system, and the contingent and somewhat unusual nature of its evolution. That evolution is a focus of Juno’s objectives. Simply, we don’t yet know quite how Jupiter formed. Did this immense mass of gas condense around a small solid core, or did the gas simply pull itself into a ball? Surprisingly, we don’t even know if Jupiter has a solid core or not. By measuring the amounts of water vapour and ammonia in the atmosphere, Juno hopes to find out. That will tell us not just about the giant planet but about the key formation processes of the entire solar system.
What’s more, we don’t know much about the planet’s atmosphere at all beyond what we can see of its dramatic surface—it’s just too thick. The swirling fog is patterned into “zonal bands” parallel to the equator, but it’s not clear how deeply they penetrate, nor what the temperature, composition or movements are like below the clouds. By zooming in closer than any previous mission and using cameras that register cloud-penetrating light wavelengths, Juno should find out.
It’s thought that, very deep inside the planet, the hydrogen gas becomes so densely squeezed by the pressure of the gases above that it will condense into a fluid that is electrically conducting: what physicists would call a metal, although that seems an unfamiliar label for this conventionally non-metallic element. Electrical currents circulating in this metallic hydrogen are thought to produce Jupiter’s strong magnetic field, just as that of Earth comes from currents in our planet’s liquid iron-alloy outer core. Juno will travel over Jupiter’s poles, where the magnetic field is concentrated just as it is at the Earth’s poles. Here the spacecraft will look at the charged particles of the solar wind focused by the magnetic field, which stream down into the atmosphere to create aurorae like those above the Arctic and Antarctic.
Jupiter’s aurorae, visible most clearly at ultraviolet wavelengths, are among the most stunning sights in the solar system. Frankly, I can’t wait to see what they will look like in Juno’s ultraviolet camera. It’s going to be quite a ride.
It’s not easy to get very excited about NASA’s Juno spacecraft mission to Jupiter, which entered orbit successfully around the giant planet on 4th July after a five- year voyage. Unlike the New Horizons mission unveiling the eye-popping mysteries of Pluto, or the European Space Agency’s Rosetta mission which landed a probe on a comet, Juno seems to be going into well-charted territory. Jupiter can be seen without too much effort even in amateur telescopes, and its bleary red eye—a storm three times the size of the Earth—is a familiar sight, no matter how glorious its baroque swirls.
And Juno’s objectives—to investigate the composition of Jupiter’s atmosphere and interior, and study its magnetic field—sound like stolidly unglamorous planetary science. Of course, this is all a build-up to my saying that you should not shrug off Juno as the prosaic face of space exploration. Truly, though, we might be in for some spectacular sights and surprises from this low-key mission. For one thing, Jupiter is really the key to the entire solar system. Easily the most massive of the planets, it is primarily a ball of gaseous hydrogen and helium—the same stuff from which the Sun is made. If Jupiter were only twice as massive, it wouldn’t be formally a planet at all, but one of the intriguing objects called sub- brown dwarfs or rogue planets: halfway between planet and star, made from gravitational collapse of their own gas clouds rather than the offspring of some parent star. At thirteen times the mass of Jupiter, such bodies become genuine brown dwarfs: quasi-stars dense enough to ignite nuclear fusion reactions and emit a dull glow.
Jupiter is thought to have changed its orbit somewhat over the lifetime of the solar system, and these wanderings gave rise to the asteroid belt and limited the size of Mars—and maybe of the Earth and Venus too. Jupiter’s huge mass makes it a gravitational magnet for comets: some, like Comet Shoemaker-Levy 9 in 1994, smash into the planet, but more are dispatched like slingshots to the outer solar system before they can collide with Earth. It’s possible that this might have helped life here to develop; but Jupiter’s gravity is also capable of nudging the asteroids towards the Sun, raising the possibility of their hitting our planet on the way. Either way, our fate seems closely dependent on the nearest and greatest of the gas giants.
The study of Jupiter, then, has led us to appreciate the interdependence of worlds in our solar system, and the contingent and somewhat unusual nature of its evolution. That evolution is a focus of Juno’s objectives. Simply, we don’t yet know quite how Jupiter formed. Did this immense mass of gas condense around a small solid core, or did the gas simply pull itself into a ball? Surprisingly, we don’t even know if Jupiter has a solid core or not. By measuring the amounts of water vapour and ammonia in the atmosphere, Juno hopes to find out. That will tell us not just about the giant planet but about the key formation processes of the entire solar system.
What’s more, we don’t know much about the planet’s atmosphere at all beyond what we can see of its dramatic surface—it’s just too thick. The swirling fog is patterned into “zonal bands” parallel to the equator, but it’s not clear how deeply they penetrate, nor what the temperature, composition or movements are like below the clouds. By zooming in closer than any previous mission and using cameras that register cloud-penetrating light wavelengths, Juno should find out.
It’s thought that, very deep inside the planet, the hydrogen gas becomes so densely squeezed by the pressure of the gases above that it will condense into a fluid that is electrically conducting: what physicists would call a metal, although that seems an unfamiliar label for this conventionally non-metallic element. Electrical currents circulating in this metallic hydrogen are thought to produce Jupiter’s strong magnetic field, just as that of Earth comes from currents in our planet’s liquid iron-alloy outer core. Juno will travel over Jupiter’s poles, where the magnetic field is concentrated just as it is at the Earth’s poles. Here the spacecraft will look at the charged particles of the solar wind focused by the magnetic field, which stream down into the atmosphere to create aurorae like those above the Arctic and Antarctic.
Jupiter’s aurorae, visible most clearly at ultraviolet wavelengths, are among the most stunning sights in the solar system. Frankly, I can’t wait to see what they will look like in Juno’s ultraviolet camera. It’s going to be quite a ride.
