Juno’s Jupiter journey continues
With its mission extended by four years, the Juno spacecraft still has a lot to learn about the gas giant, as Ezzy Pearson discovers
What secrets of the gas giant will the extended mission uncover?
For the last five years the undisputed king of the planets has been watched over by the Juno spacecraft. During its swooping passes, the NASA orbiter has imaged the upper cloud deck and peered into the planet’s depths, mapping out the gas giant’s magnetic and gravitational structure to create a three-dimensional picture of the Solar System’s largest planet.
The mission was originally intended to meet its fiery end this August, crashing into Jupiter’s atmosphere. But in January Juno was granted a reprieve and the mission has been extended to September 2025.
Juno’s original end date was predicted based on the number of orbits required to map out Jupiter before the intense radiation created by the planet’s massive magnetic field ravaged the probe’s electronics. To protect Juno as long as possible, the most important components were locked inside a vault made of 1cm-thick titanium, designed to stop all but the most energetic radiation.
Scott Bolton from the Southwest Research Institute, Juno’s Principal Investigator, is upbeat about how well the craft’s protection has performed. “So far, we have not seen any signs of significant radiation related degradation that’s important to us,” he says. “We built an armoured tank and it turned out our shielding is holding up.”
The mission can now delve even deeper into the planet’s mysteries – many of which the probe itself has helped reveal. Over the next four years, Juno will conduct 42 additional orbits of Jupiter, on top of the 34 from the prime mission. These eliptical orbits swing out wide for most of the time to avoid the worst of Jupiter’s radiation, before quickly diving in close to get a good look at the planet. This point of closest approach (the ‘perijove’), was initially close to the planet’s equator, but has been slowly creeping northwards at about 1˚ per orbit.
The northward drift has turned out to be hugely beneficial to the extended mission
though, as it will now be able to take a closer look at several interesting features that the prime mission revealed. As Juno passes over the poles, rather than circling the equator, planetary scientists attained their first ever close-up of these regions, revealing they were dominated by huge, swirling storms.
“We’ll be able to study the storms in the north in ways never before possible,” says Bolton. “We really don’t understand how they form, why they’re stable or what happens over time. We’ll be able to look deep into the atmosphere, underneath the giant vortices and see how they compare to the large vortex storms at lower latitudes, like the Great Red Spot.”
During the extended mission, Juno will get close enough to use its Microwave Radiometer, which can pierce up to 400km down through the clouds to look at both the storms’ root structures and the distribution of water and ammonia within them
– both important chemicals for understanding how the planet’s atmosphere behaves.
“We know that Jupiter’s deep atmosphere changes, but how does that change happen as you go further north? What happens when the stripes that we call the zones and belts – where the winds go back and forth – start to change?” says Bolton.
Creating a storm map
One of the areas they’ll be paying particular attention to is where these stripes transition into the giant polar cyclones. Juno’s northward creeping will allow it to create a gravitational map of these storms. It does this by taking careful measurements of its orbit, which can be compared to its predicted path to see where gravity has pulled it off course. If the spacecraft is deflected, then gravity is stronger and that part of the planet is denser than expected, and vice versa.
“The zones and belts go through some kind of transition at northern latitudes, so getting more gravity data closer up will help investigate what happens during that transition,” says Bolton.
Another area Juno will look into is the lightning that jumps between Jupiter’s clouds. At lower latitudes Juno observed this lightning occurring at high altitudes, suggesting there must be liquid water clouds far above where they were expected.
“It should all be frozen at the altitude we saw the lightning,” says Bolton. “Most theories of lightning, the kind we’re seeing, suggest it needs three phases of something – liquid, gas and ice. There must be some sort of liquid at the altitude of the lightning, but it can’t be water because that would all be frozen. We came up with the idea that the ice was being melted by ammonia, which would act like antifreeze.”
As most of the lightning was at northern latitudes, a closer look at the northern hemisphere will help pin down exactly how much it is happening on Jupiter and where it is.
Juno will also get closer to some of Jupiter’s most dramatic features – the aurorae. Like Earth,
Jupiter’s magnetic field captures and accelerates particles, which crash into the atmosphere at high latitudes, creating a glorious light show. But, unlike on Earth, Jupiter’s aurorae are largely outside the visible spectrum, shining in the ultraviolet and infrared wavelengths. Juno has already detected some of the accelerated particles, but something else seemed to be missing.
“Even though we were pretty close, we couldn’t correlate the [number of] particles that we observed with the aurora that we were seeing,” says Bolton. “Some of the particles must be being accelerated closer to Jupiter than we were sampling.”
The extended mission will see Juno get 10 times closer to the aurora, allowing it to hunt for these missing particles. It will also be able to look at another piece of the puzzle, the planet’s magnetic field.
“The magnetic field up in the north has really got a lot of structure, it has more complexity than the south. We don’t fully understand that. All of Jupiter is asymmetric, which is surprising to us, because it’s a giant ball of gas, spinning around really fast. You would think it would get more symmetric, but we see asymmetry in the gravity field, the magnetic field and the atmosphere,” says Bolton. One of the magnetic features already seen by Juno further south, at the equator, is a patch of strong magnetism known as the Great Blue Spot, which shows signs that the magnetic field is being pushed around by Jupiter’s deep jets or winds. “When we go to the north, there’s a whole bunch of these little features,” says Bolton. “They’re not quite as big as the Great Blue Spot, but we’ll get higher and higher resolution to be able to investigate them.”
Lunar flybys
However, not all of Juno’s attention will be focused on the planet itself. As the craft’s perijove moves northwards, the point at which Juno’s orbit crosses the plane of the moons moves inwards, meaning it will make close passes of three of Jupiter’s icy moons – one of which has already occurred.
Juno flew past Ganymede’s surface on 7 June 2021, using the manoeuvre to adjust its orbit, reducing the time to loop around Jupiter from 53 days down to 43. Flying just 1,038km from the surface, it was able to take images with a resolution of 600m–900m per pixel. The team are planning to study these images before making any scientific judgements.
The next flyby on 29 September 2022 will see the spacecraft pass within 320km of Europa. During this flyby, Juno will be able to use its Microwave Radiometer to look at the top 10km of ice. This will search for where the ice is thin, or where cracks might appear in the icy crust, allowing water to escape from a subsurface ocean.
Next, Juno will make two flybys of Io on 30 December 2023 and 3 February 2024, passing either side of the moon 1,500km apart. The moon is known for being the most volcanically active body in the Solar System, leading many to wonder if the surface is covered by a liquid magma ocean. The dual flybys will measure the gravitational layout of the moon, revealing any surface magma.
Finally, the path of the spacecraft will take it far enough in to reach the planet’s rings. While you might expect this dust-filled region to be hazardous to the spacecraft, that isn’t the case.
“The rings look like they’re full of material, but most of it’s empty space,” says Bolton. What dust there is, however, Juno will try to study. “We’re using the dust impacts for science; we have cameras onboard and plasma wave instruments that will detect when the solar panels are getting hit by dust. We’ll use that to learn about the dust, but a large pebble might hit us.”
By that point in the mission, it’s expected Juno will be running out of the fuel it needs to keep its antenna pointed towards Earth and able to transmit back data. That’s if it makes it to that point – though Juno is faring well against the radiation, it is still constantly being bombarded and cannot hold out forever.
The initial plan for ending the Juno mission was to crash the spacecraft into the clouds of Jupiter, to make sure it doesn’t end up contaminating the potentially habitable Europa. Fortunately, the new path will take it well within Europa’s orbit. While that means it won’t be purposefully crashed, the spacecraft won’t avoid its meeting with Jupiter’s atmosphere entirely. Eventually its path will collide with Jupiter and it will burn up in the atmosphere, meeting its end at the hands of the planet whose secrets it has spent years uncovering.
When Earth is directly between the Sun and another planet, that planet is at opposition. This is a great time to view as the planet appears at its largest and brightest.
Gas giants Jupiter and Saturn are both coming to opposition in August and are impressive to view through a telescope for different reasons. Jupiter’s dynamic atmosphere shows lots of detail and changes markedly over time. Saturn’s atmosphere is subtler, but still has the potential to surprise. And, of course, although Saturn’s globe doesn’t show the drama present on Jupiter, its rings are a constant draw. Over previous years, both planets have appeared low from the UK, which has made observing them harder. The conditions are now slowly changing and it’s a great time to discover how to make scientifically useful observations of them.
Observing the gas giants
Despite their great distances, Jupiter and Saturn are physically large enough to appear bright and with tangible size through a telescope. Jupiter shows a wealth of detail, which is fascinating to record, either by sketching or imaging. As it rotates in less than 10 hours, you don’t have to wait long for its appearance to change. Indeed, wait too long and the planet’s fast rotation hides features and reveals new detail.
Saturn rotates quickly too, but it’s a different world in terms of visual appearance. Indeed, Saturn’s banded
atmosphere is shrouded by a haze layer of ammonia clouds, making it hard to see detail. Here, visual and imaging skills need to be honed as much as possible. One way to accomplish this visually is to make intensity estimates for different parts of the planet’s disc and rings (see box, page 71). Also, if you can image the planet, you'll find that animation can help to emphasise subtle features on the edge of visibility.
Atmospheric variations
When we look at Jupiter and Saturn through a telescope, we’re looking at their atmospheres. Jupiter’s is most distinctive, appearing as a series of light and dark banded regions. The famous storm known as the Great Red Spot (GRS), nestles into a scalloped-out region on the southern edge of the South Equatorial Belt (SEB). The atmosphere is rich in shorter-lived phenomena too, such as light and dark spots, festoons and barges. In addition, the visibility and appearance of belts and zones varies over time. In 2010, when the SEB disappeared completely, the GRS appeared odd as it floated, detached, around the planet.
Saturn has many belts and zones, but they are subtle and can be difficult to identify uniquely, whether visually or within images. Saturn’s atmosphere does exhibit intricate detail but thanks to a high haze layer surrounding the planet, this detail is difficult to pick out. Bright, light-coloured storms do occur in Saturn’s atmosphere from time to time. These can become quite extensive, so it’s important to make accurate recordings of their estimated size and changes in shape. Large, long-lived Saturnian storms may spread until they virtually encircle the globe.
Jupiter and Saturn’s coordinate systems are equivalent to Earth’s latitude and longitude. However, as there are no fixed surface features to use as anchor points, longitude-zero is more complex to define than the Greenwich Meridian, Earth’s line of zero longitude. Latitude is easy: measured in degrees, it varies from
0˚ at the equator to 90˚ at either pole. By convention, northern latitudes are positive, southern negative.
Longitude determination is done by timing. Visualise an imaginary line running from the planet’s north pole to its south pole, and this imaginary line is known as the planet’s Central Meridian (CM). As the rotation period of each planet’s atmosphere is known, it’s possible to define the Prime Meridian (zero longitude) as the longitude at the CM at an agreed, defined start time. As the planet rotates, the CM longitude increases until it’s reset as the chronological Prime Meridian is reached again, one rotation later.
A complication occurs due to the equatorial regions rotating faster than the rest of the atmosphere. This is addressed by dividing regions into two longitude zones: System I is used for the equatorial region between latitudes +10 and –10, while System II covers everything else. However, a further longitude system also exists, known as System III, which reflects the time it takes for the planet’s magnetosphere to rotate. Although System III reflects the official rotation period of each planet, amateur astronomers normally use System I and II.
Keeping track
As Jupiter and Saturn rotate quickly, over several nights it’s relatively easy to see a specific atmospheric feature passing the CM. For small features, accurately recording the time (in Universal Time) when this happens allows you to determine the longitude of that feature. If you make longitude measurements of the same feature over a long period of time, ranging over months or even years, you will be able
to determine how much the feature is drifting within ▶ the planet’s atmosphere. By plotting time versus longitude you can generate a drift chart to tell you a great deal about how the planet’s atmosphere works.
Features with tangible widths can be measured using the planet’s rotation. Imagine the leading (preceding) edge of a feature reaching the CM at time Tp. The feature’s trailing (following) edge will pass the CM at time Tf. The feature’s width can be determined by converting these to longitudes Lp and Lf using WinJupos (http://jupos.org). The physical width in kilometres is calculated with the following formula:
(Lf–Lp) x 1217.4 x cos(latitude)
All timings should be done in Universal Time and it’s important to use the correct longitude System, I or II.
This brings us to Jupiter’s Great Red Spot (System II), an extended feature (see box, opposite). CM timings of its preceding and following edges, along with timings that record its centre crossing the CM, give valuable information. While edge timings give its width, the CM timing reveals its central longitude. Over the years the GRS’s physical width changes and its longitude drifts.
If you can master these useful scientific observing techniques, it will help to enrich your planetary observing for many years to come.