As weather systems go, Jupiter’s Great Red Spot is an interplanetary heavyweight. It could swallow the Earth whole and still have room for Mars.
And at 12:06pm (AEST) on Tuesday, NASA’s Juno spacecraft will hurtle right over the top of it.
The gargantuan hurricane has been raging for 200 years or more – and we’ve never been this close to it.
Travelling at about 50 kilometres per second, Juno will flit within 9000km of the billowing brick-red cloud tops.
“It’s going to be incredible,” said the Juno mission’s principal investigator, Scott Bolton, from the Southwest Research Institute in Texas.
The storm itself is a good 16,000km across; Juno, if you include its outstretched solar panels, is the size of a tennis court.
“We’re going screaming past, but we’ve got cameras that know how to work at that speed,” Dr Bolton said.
“I can’t wait to see what it looks like.”
These will be the first-ever close-up snaps of the solar system’s biggest storm.
“Nobody knows exactly what kind of features we’ll see inside, what the kind of colours and swirling of the clouds are,” Dr Bolton said.
As it watches the Great Red Spot skid past, Juno will also be drifting upwards away from Jupiter because its closest approach – a mere 3500km above the cloud tops – happens about 11 minutes earlier. If Jupiter were the size of a basketball, this brings it within millimetres of the surface.
‘Through the gates of hell’
It’s the sixth time the NASA probe has buzzed the giant planet since putting itself into a precise, lopsided orbit almost exactly a year ago.
“There’s high risk in every flyby. We’re going through the gates of hell, every time,” Dr Bolton said.
“And each time we go by, we’re going through a worse region. More hazard, more radiation.”
Jupiter’s magnetic field, 20,000 times stronger than Earth’s, endows the gas giant with vast and punishing radiation belts that Juno must thread a path between. So the probe’s inner workings are shielded by a thick titanium wall, which has successfully withstood the onslaught so far.
Based on current calculations, Dr Bolton said, the team is confident Juno will succeed again – but they’re not taking anything for granted.
“We will be on the edge of our seats, just keeping our fingers crossed that everything works and we get the close-up pictures that we all want,” he said.
In order to have all its instruments staring down at Jupiter, during the flyby Juno faces the wrong way to communicate with Earth. It will be a few hours before the team even receives confirmation that the craft is intact and still functioning.
Then begins the process of downloading actual data: first from Juno’s magnetometer, then from its camera. And the photos will be sent in chronological order, starting with the ones taken at greater distances.
Our first close-up views of the Great Red Spot won’t arrive until at least the weekend, according to Dr Bolton.
Seeking the secrets of a storm
When they do arrive, Juno’s measurements will open a new chapter in our understanding of this famous anticyclone (so named because it spins in the opposite direction to most storms).
The Great Red Spot has been studied continuously since the early 1800s, and even some of the earliest views of Jupiter through telescopes, in the late 1600s, reported a big spot on its surface. Many astronomers think this was probably the very same storm.
What keeps a planet-sized hurricane spinning for 450 years?
“That’s a big puzzle; nobody knows,” Dr Bolton said.
“There are some scientists who believe that in order for a storm to have lasted that long, it must have very deep roots. Maybe the source of energy that’s creating that storm comes from deep inside the planet.
“Of course, up till now, we’ve only had the ability to look at the top part of Jupiter. We just see this thin veneer, which is gorgeous – it has these beautiful zones and belts, and this great storm on it, and a bunch of cyclones – but the key is what’s underneath.”
That is one of the strengths of the Juno mission: the craft carries a variety of instruments that are peering beneath Jupiter’s clouds for the first time.
So its flyby this week should reveal the Red Spot’s deep roots, if they exist.
The team will also be looking for lightning, which has been seen elsewhere on Jupiter and suggests the presence of water clouds.
“Lots of storms on Earth have a lot of lightning, but it usually takes water,” Dr Bolton said.
“This storm may be more ammonia than water, but we don’t know.”
Enjoy it while it lasts
Juno’s visit comes at an opportune time. After all those centuries, the Great Red Spot appears to be vanishing before our eyes and telescopes.
“It’s been observed for hundreds of years, but now in the last couple of decades, we’ve noticed that it seems to be getting smaller and changing in shape,” Dr Bolton said.
“So we may be catching it at the right time. It’s always exciting when you can watch something in transition – you can learn a lot more.”
Among their total of 32 planned orbits, Dr Bolton and his team hope to see Juno make at least two more passes of the Red Spot. That will depend on orbital calculations and – critically – where exactly the storm travels on the planet’s surface. It’s a moving target.
“In the future, we’ll have a pass that looks more at the gravity shield, to see if there’s a clump or a blob of mass underneath this thing. If it really has deep roots, there could be some deep mass that’s underneath, that’s moving around Jupiter with it,” Dr Bolton said.
“Most people would think that’s not possible, but I’ve learned not to be too confident guessing how Jupiter works. In the last year I’ve been surprised by so many things.”