For 35 minutes, starting around midnight eastern time on Monday, it’s programmed to burn nearly all of it as it makes the most dramatically, bone-jarring entrance of any space vehicle in history.
At best, the effect of all the fireworks will be to shave about 1 per cent off the probe’s breakneck speed of slightly more than 200,000 kilometres an hour. But if all goes according to plan, it should be just enough to allow Juno to be captured by Jupiter’s immense gravitational pull and directed into a long, swooping orbit around the planet.
As far as scientists are concerned, that’s when the fun begins for a mission whose goal is to peel back Jupiter’s enigmatic cloud layers and probe the powerhouse that lies beneath.
Dr. Stevenson still recalls the moment in 1973 when Pioneer 10 sailed past Jupiter, giving scientists their first close-up look at the solar system’s largest planet. Five more probes have done the same since then (generally while on the way to somewhere else) and, in 1995, the Galileo mission arrived to orbit Jupiter from a safe distance. But while these various spacecraft have peered at Jupiter’s swirling cloud bands and inspected its large and diverse retinue of moons, none has been equipped to answer the most fundamental question about the planet: How did it get so big?
Scientists have good reason for wanting to know. Jupiter has 300 times Earth’s mass, much of in the form of hydrogen gas so compressed it behaves like a liquid metal. Its gravitational influence has made it the architect of the solar system and the key to understanding the origin and orbital stability of all the planets, including our own.
Over the past two decades, thousands of other solar systems have been detected around other stars, most of them with planets that are similar in girth to Jupiter or even bigger. Divining the role of these planets in shaping the destinies of smaller, Earth-sized worlds that form along with them is one way to learn if we are likely to ever find another civilization somewhere out there in the galaxy.
It’s a tall order for the $1.13-billion (U.S.) mission. Meeting it will require Juno to repeatedly venture within 4,200 kilometres of Jupiter’s cloud tops as it makes a year-long series of hairpin turns around the planet. That’s close enough to be fried by the high-energy particles darting around within Jupiter’s powerful magnetic field. To survive the extreme conditions, Juno’s computer circuits have been mounted inside a 200 kilogram “electronics vault” with solid titanium walls.
During the close approaches the probe will deploy its suite of science instruments, including a microwave radiometer that should be able to detect the presence of water more than 500 kilometres below Jupiter’s upper cloud layer, which is made of frozen ammonia crystals.
How much water Jupiter’s atmosphere contains is probably the single most important thing Juno will measure, said Steven Levin, project scientist for the mission.
“Just by measuring that one number we can learn a lot about how Jupiter formed,” he said.
The leading theory of how Jupiter came to be posits that it began with a core of solid material, composed of both rock and ice, that grew to such a large mass that it eventually began sweeping up most of the hydrogen gas left over from the sun’s formation.
The amount of water Jupiter carries in its atmosphere today should provide a clue to how far away from the sun it was when it first emerged from the spinning debris cloud that gave rise to the solar system. Its growth and movement after that would have had a profound effect on the other planets, which are thought to have formed somewhat later than Jupiter.
Another way that Juno will get at the question will be using subtle variations in Jupiter’s gravitational field to see whether it has a discernible core. This could prove tricky, Dr. Stevenson said. Unlike Earth’s metallic core which is well separated from its rocky mantle, Jupiter’s innermost concentration of heavy elements could be dissolved in the hydrogen that envelopes it.
If so, “it might be hard to decide what’s going on down there,” he said. But then other data from Juno, including precise measurements of Jupiter’s magnetic field, will help scientists understand what they’re seeing.
But all of that is for the months that lie ahead. Before scientists can get to work, Juno must arrive safely. It’s a task the probe will have to accomplish by itself, since Jupiter is so far away that Juno’s radio signals now take 48 minutes to reach Earth – far too long a delay for flight controllers to react and respond in real time.
Final commands to Juno were sent on Thursday, though engineers are continuing to monitor what the probe is doing as it speeds toward its destination, said mission manager Ed Hirst at NASA’s Jet Propulsion Lab in Pasadena, Calif.
From here on in, he added, “the spacecraft is on its own and is designed to take care of itself.”