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The years were worth the wait. JWST gives us an amazing view of Neptune's rings

But uh-oh, friction faults already starting on distant probe

Pic The James Webb Space Telescope has snapped the clearest picture of Neptune, capturing its dust rings and seven moons, in more than 30 years.

Neptune, designated the furthest planet in the Solar System after Pluto was demoted to a dwarf world in 2006, is more than 2.6 billion miles away from Earth. The only spacecraft to visit the faraway gas giant was Voyager 2 during a flyby in 1989; that probe beamed back images of bright rings around Neptune made up of clumps of dust shed by its nearby natural satellites.


Star trek ... Neptune capture by the JWST. Image Credit: NASA, ESA, CSA, STScI. Click to enlarge or click here for a closeup

Now, the JWST – the multi-billion-dollar machine that was delayed for years before launching last year – has provided astronomers with fresh observations of Neptune's dust rings and moons.

"It has been three decades since we last saw these faint, dusty rings, and this is the first time we've seen them in the infrared," Heidi Hammel, a Neptune system expert and interdisciplinary scientist for Webb, said in a statement.

The bright blue-looking spot with diffraction spikes in the upper left of the above snap is Neptune's biggest moon, Triton, which has an odd retrograde orbit and is one of the few geologically active natural satellites in the Solar System.

There are six other moons that appear as small blurry blobs; three of them, Galatea, Naiad, Thalassa, are on the left side of Neptune nestled in the dust rings, while Despina is on the right. Larissa is directly below Despina outside the rings, and Proteus is further away to the right. 

Neptune is rich in heavier elements, and looks blue due to small amounts of methane in its atmosphere. Comparisons of images from the Hubble Space Telescope and the James Webb Space Telescope show what the planet looks like at different wavelengths. Observations of Neptune in visible light by Hubble show the planet's color but misses other details, whereas viewing it infrared reveals its dust rings. 

The dark color in the JWST's images are due to methane absorbing infrared light. The bright spots and bands across the planet are due to methane-ice clouds reflecting sunlight before they're absorbed by methane gas. Neptune has at least 14 moons; Triton outshines its host planet and is the most luminous since it reflects 70 per cent of sunlight that reaches its surface.

All is not well, however

But NASA's boffins are concerned about a developing problem, and have temporarily paused some observations in mid-infrared light as one of its instruments undergoes a technical examination.

"The James Webb Space Telescope's Mid-Infrared Instrument (MIRI) has four observing modes," NASA explained in a statement. "On August 24, a mechanism that supports one of these modes, known as medium-resolution spectroscopy (MRS), exhibited what appears to be increased friction during setup for a science observation.

"This mechanism is a grating wheel that allows scientists to select between short, medium, and longer wavelengths when making observations using the MRS mode. Following preliminary health checks and investigations into the issue, an anomaly review board was convened Sept. 6 to assess the best path forward."

Only the medium wavelength of MIRI's observation modes is affected; its other modes supporting imaging, low-resolution spectroscopy, and coronagraphy are still working. ®

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