What’s Up with Jupiter and Saturn?

Jupiter and Saturn made a big splash last month due to the historic conjunction of our solar system’s two largest planets. From our vantage point here on Earth, looking out onto the flat plane of the sky, a conjunction occurs when two or more celestial bodies seem to slide right past each other in the sky. In the case of Jupiter and Saturn, this was seen just after sunset with the closest approach happening on December 21st. Of course, in reality, the two planets are millions of miles apart, but those distances appear compressed from our perspective.

The Conjunction of Jupiter and Saturn as seen from Monroe Lake, Indiana on December 21st, 2020. Photo courtesy of Zolt Levay.

Conjunction Junction – Opposition Proposition

This alignment of the planets all originates in the graceful dance of the celestial bodies of our solar system in their respective orbits around the Sun. Throughout the year, as Earth and the other planets orbit the Sun in the orbital plane of the solar system, also known as the ecliptic, they trace out different paths across the sky. At different times throughout each planet’s orbit around the Sun, the relative positions of the planets may come into some chance alignment, as was the case with Jupiter and Saturn’s conjunction.

Diagram showing the orbital alignment of Earth, Jupiter, and Saturn during conjunction in 2020.

Speaking of planetary alignments, once every year, Earth approaches something called opposition with each of the outer planets. Opposition occurs when Earth’s orbit takes it as close as possible to each of these planets. In this configuration, not only are the planets closer to Earth, but they are also fully illuminated by the Sun, which presents a great opportunity to train Hubble’s keen vision on our planetary neighbors.

Diagram showing the orbital alignment of Earth and Jupiter during opposition in 2020.

From Chance Alignments to Beautiful Images

A team of astronomers routinely takes advantage of this alignment through a Hubble program known as Outer Planet Atmospheres Legacy (OPAL) which aims to provide astronomers with an archive of Hubble imagery of the outer planets over time (now at seven years) in an attempt to better understand the dynamics and evolution of their atmospheres. The OPAL program produces Hubble’s best views of Jupiter, Saturn, Uranus, and Neptune on a yearly basis and is often featured in press releases detailing the amazing imagery and scientific results. This year was no exception as Hubble provided exceptional views of storms on Jupiter and summertime on Saturn.

Jupiter and Saturn from Hubble’s 2020 press releases.

Processing Hubble imagery of Jupiter and Saturn presents its own set of challenges unique to these objects. In both cases, as Hubble speeds through its orbit taking data, the planets are trucking along in their own orbits and also rotating surprisingly quickly given their massive size. The length of a day on both planets is about ten Earth hours, so they’re both rapidly spinning while Hubble tries to capture a portrait! Imagine that you’re ice skating around a large rink with a camera, trying to get a clear, sharply focused image of a figure skater in the midst of a rapid spin on the other side of the rink—that’s analogous to what Hubble is trying to do!

“figure skater in motion” by isla_yelo is licensed under CC BY-SA 2.0.

Constructing a color image when both the subject and the camera are moving relative to each other, and you’re limited to taking one color at a time (red, green, and blue), means that the data have to be carefully reconstructed to provide a clear image. For example, details in the Jovian atmosphere will change drastically between exposures in Hubble’s different filters. This creates a situation in which those features will be smeared out were we to simply combine the three filters together to make a color image—they actually appear in separate colors in the same image. 

Jupiter data from Hubble combined in a color composite without accounting for rotation effects. Note the color separation in the atmospheric features which moved between the different exposures that make up this color image.

Accounting for this, a process known as ‘derotation’ can be done by hand using photo manipulation tools in your favorite image processing software, but this method is very subjective, time consuming, and prone to errors. Given that we know the exact date and time of the observations, we can use this information to determine the exact amount of rotation in degrees between frames of data, and account for that in the processing of the data. Software such as WinJupos is designed to do just that. Hubble image processor and guest blogger, Judy Schmidt, recently described her own process of derotating Jupiter data using 3D software.

Once the individual frames have been derotated, the image assembly proceeds in the manner that has been described in several blog posts on this site. Careful attention is paid to the color balance of the image to ensure that the atmospheric colors are reproduced accurately, made all the more challenging by the selection of narrow-band filters which isolate specific colors of light in the final composite image. The tools available to image processors now can easily be taken to extremes, creating a garish ‘overcooked’ image, so great care is taken to wield available tools as needed and with a delicate hand.

Yikes! Although image processing tools can bring out more details and color in an image, sometimes less is more!

That’s No Moon!

Finally, many of the exposures of both Jupiter and Saturn also capture their respective moons, at least the ones large and bright enough to show up in these exposures. Of course, some of the moons are much dimmer compared to the planets. In most cases, it is nearly impossible to create an image that is adequately exposed for both the moons and the planet at the same time. If the image brightness is adjusted to show the planet in detail, the moons may not be visible, and conversely, if the image brightness is adjusted to show the moons, the planet will be greatly overexposed.

Raw image of Saturn taken during opposition in 2020 with brightness adjusted to show faint details; visible moons are circled in red.

This situation can sometimes require the creation of two separate images, balanced for each object, which are then composited for the final image. You may also wonder how we know which moons we’re seeing in a given image. This is made all the more difficult by the amount of cosmic-ray artifacts present in any one image, many of which can easily be mistaken for a moon. Thankfully, astronomers maintain very detailed lists of the positions of the planets and their moons, among other astronomical objects. This list of positions is known as an ephemeris. NASA maintains the Planetary Data System which includes tools designed to make use of ephemeris information to generate graphs showing the exact location of the planets and their moons on the sky, at any time, for any field of view—even Hubble’s!

The same image of Saturn with PDS diagram overlaid to identify the visible moons in the image.

We routinely make use of this tool to guide the creation of press release imagery that will feature moons so that we can be absolutely sure of which moons are present in the image.

Hubble’s compass image from the July 2020 opposition with moons identified.

So, the next time you see a beautiful Hubble image of one of our solar system’s outer planets, remember the OPAL program and the many scientists and engineers who choreograph in exquisite detail the dance between telescope and planet that makes such an image possible. Surely, that is as deserving of attention as the chance alignments that draw so many people to gaze upwards in wonder and awe. 

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