Webb’s First Glimpse of Jupiter, Its Moons & Rings

A long time ago when I was a budding amateur astronomer, one of the first targets I would observe each evening with my new telescope was the planet Jupiter. Even with the modest optical instrument I had, the cloud bands across the face of this distant gas giant were readily visible as were its four largest moons, which moved noticeably over the course of hours to days. Fast forward almost half a century later and NASA’s new James Webb Space Telescope (JWST) has done the same thing I did so long ago: observe Jupiter as an initial target.

The launch of the James Webb Space Telescope on an Ariane 5 rocket on December 25, 2021 from the ELA-3 Launch Zone of Europe’s Spaceport at the Guiana Space Centre in Kourou, French Guiana. (NASA/Bill Ingalls)

JWST was launched on Christmas Day 2021 and finally reached its station at the Sun-Earth L₂ Lagrange point about 1½ million kilometers from the Earth a month later. Over the course of the following weeks, JWST deployed its sunshade and its mirrors to begin the six-month commissioning phase with end-to-end tests and adjustments of the telescope’s systems and instruments. One of the early targets chosen for observation during this phase was the planet Jupiter as part of a series of tests to characterize Webb’s ability to observe moving targets. Jupiter, which was moving at a rate of 3.3 milliarc seconds per second against the background stars, was an especially taxing target for Webb’s systems. This is because Jupiter is such a bright object located only 140 arc seconds from the telescope’s Fine Guidance Sensors (FGS), which track faint background stars to keep to maintain accurate pointing.

An artist’s conception of the James Webb Space Telescope deployed in space. (NASA/GSFC/CIL/Adriana Manrique Gutierrez)

Observations made by JWST’s instruments confirmed the telescope’s ability to accurately track Jupiter and other moving targets across the sky during an extended observation session. As NASA and its partner space agencies released the first full-color image acquired by Webb, the Space Telescope Science Institute (STScI) also quietly released a report on July 12 detailing the results of tests performed to date during the commissioning phase. Included in the report were a pair of infrared images of Jupiter.

Here are a pair of images of Jupiter taken during the commissioning phase of JWST to test the telescope’s ability to track moving targets accurately. The images had an exposure time of 75 seconds each through the F212N and F323N filters centered at the wavelengths of 2.12 µm and 3.23 µm, respectively. The contrast of these images was stretched to reveal the faint rings and moons (as well as some instrument artifacts). Click on image to enlarge. (STScI)

These images were taken using the NIRCam (Near InfraRed Camera) which employs a set of ten, four-megapixel sensor arrays to observe in the 0.6 to 5 µm range of the spectrum. The images had an exposure time of 75 seconds each (during which time, Jupiter would have moved about a quarter of an arc second) through the F212N and F323N filters centered at the infrared wavelengths of 2.12 µm and 3.23 µm, respectively. Both images not only clearly show the disk of Jupiter, but also its tenuous ring as well as Europa and two of its tiny inner moons, Thebe and Metis. At these infrared wavelengths, Jupiter is primarily illuminated by scattered sunlight as are the moons and rings. While Jupiter’s cloud bands are visible in the F212N image, the F323N image is nearly featureless because of methane absorption at that wavelength. Although not as spectacular as some of the early images released by the JWST science team, they do show the promise of this new telescope observing Jupiter and other objects in our Solar System.

This is a version of the F212N image, released by ESA on July 15, that was processed to highlight the details of Jupiter’s cloud deck. Europa is the bright object on the left with its shadow visible next to the Great Red Spot. The rings and smaller moons of Jupiter are not visible in this processed image. Click on image to enlarge. (NASA/ESA/CSA/STScI)

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General References

Jane Rigby et al., “Characterization of JWST science performance from commissioning”, Space Telescope Science Institute, July 12, 2022 [Report]