SPYING ON THE NEIGHBOURS
Hubble’s successor, the James Webb Space Telescope, will look farther back in time and space than ever before. But this giant telescope could also be turned to targets right in our own cosmic backyard, as Benjamin Skuse reveals
The James Webb Space Telescope won’t only peer into the distant Universe; here’s what we hope to learn about our Solar System.
Bigger and more powerful than any space observatory ever launched, the James Webb Space Telescope’s (JSWT’s) infrared gaze will stretch to the very first stars and galaxies being born, offering new insight into the Universe’s origins. Its eyes will also scan exoplanets in the search for the building blocks of life beyond our cosmic doorstep, looking for answers to the perennial question: ‘Are we alone?’
What many do not realise though is that JWST will not solely be peering at the farthest reaches of the Universe. In fact, with some clever reconfiguring, Webb will be able to cast its spying eye on our closest cosmic neighbours, hoping to uncover some of the secrets hidden within our Solar System.
Adapting JWST for the local nature of Solar System science, however, is fraught with difficulties. The biggest is that the telescope is designed for detecting the faintest, most distant objects. Its extremely sensitive sensors therefore need to be protected at all times from the overpowering light and heat from the Sun, which is why it is equipped with a tennis court-sized sunshield. This would not be a problem but for the fact that Webb will be located at the second Lagrangian point (L2), some 1.5 million km beyond Earth’s orbit. As it is, the sunshield permanently shrouds Mercury, Venus, Earth and the Moon from Webb’s gaze.
The closest of our neighbours Webb will be able to track are near-Earth objects (NEOs) like Eros and Halley’s Comet.
“The Earth’s atmosphere makes it very difficult to observe NEOs in certain wavelength regions, some of which are very informative and diagnostic of things like water and organics,” says NASA research scientist Cristina Thomas. “If we want to focus on origins of life questions, then going outside the atmosphere helps us.”
The brightness dilemma
The second nearest target, Mars and its moons, will only be within JWST’s spyglass every two years. Webb will add an infrared view to the Mars toolbox of rovers and satellites tasked with studying the planet and its potential for hosting life.
NASA planetary scientist Geronimo Villanueva believes this capability will be invaluable: “JWST will open a new window into the planet’s current and past habitability,” he says. Villanueva should know. Among other achievements, he was the co-discoverer of methane on the planet (a possible biosignature) and mapped deuterium to hydrogen ratios in Mars’s atmospheric water – leading to the realisation that the Red Planet had an ancient ocean. “New observations
“What many do not realise is that JWST will not solely be peering at the farthest reaches of the Universe”
are urgently needed to confirm these findings,” he says.
The Red Planet brings us to the second main challenge in using Webb to look over the garden fence: overexposure. Essentially, Mars is far too bright for the Webb’s sensitive detectors to cope with. “Even Pluto is bright enough that if we took full-frame data with our widest filters it would saturate,” says John Stansberry, a Space Telescope Science Institute (STScI) scientist. “So bright has a different definition for JWST!”
To get round this, NASA will command the instrument to just process a tiny square right in the middle of the full detector
array. “Instead of having a 4-megapixel image, we’ll take a much smaller postage stamp in the middle,” says NASA space scientist Conor Nixon. “That way we can read that out really quickly before it becomes overexposed.”
Beyond Mars is where JWST will really have to start getting busy. With
A full-size mock up of JWST’s 21x14m sunshield, the device that will keep the space scope’s infrared instruments cool enough to work properly
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Mars and its moons will be too bright to process using the capacity of JWST’s detectors (NIRCam imager shown left) so only a tiny section of the sensor will be used to image them