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Spying on the Neighbors
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.
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 realize 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 neighbors, 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 neighbors 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 realization that the Red Planet had an ancient ocean. "New observations 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 an observing window of around 50 days approximately every six months, the giant planets Jupiter, Saturn, Uranus and Neptune will all be viewable, as well as their associated rings and 170 known moons.
While the planets themselves will be monitored by JWST, some of the most interesting science will concern their satellites. From helping to solve the tidal heating conundrum on Jupiter's moon Io to taking over the task of watching the Saturnian moon Titan after the Cassini mission comes to an end or even establishing whether Neptune's retrograde-orbit moon Triton has a subsurface ocean, JWST offers the chance to view and try to understand the most dynamic processes of the Solar System's satellites.
Focus on the small things
However, the bread and butter for JWST's Solar System science will be even less studied, smaller and distant bodies: comets, the main belt asteroids situated between Mars and Jupiter, the Trojan asteroids that share Jupiter's orbit, and the Kuiper Belt objects — including dwarf planet Pluto and the yet-to-be-seen Planet Nine. All could yield clues to how the Solar System came to be the home we know.
"Because they retain material from the very start of Solar System history, they reveal the chemical makeup of the planets and how planets form," says Andy Rivkin, planetary astronomer from Johns Hopkins University.
For these smaller distant bodies and ring systems, NASA has another trick up its sleeve: stellar occultations, where a star is temporarily blocked by a passing Solar System body.
"If you can take data very quickly as an object passes in front of a star, you can measure various things about the object itself," explains Stansberry. By looking at the changes to the star's light as it disappears behind a planet, Webb will be able to look at ring microstructures, and may discover rings around minor planets or even find atmospheres around various Kuiper Belt objects.
All of these proposed targets for Webb suggest the Solar System's most well-hidden mysteries may soon be solved, but one paper really sticks out as having the potential to captivate the public's imagination. In it, the authors propose using JWST and Hubble together to create stereo 3D movies of the planets and moons amateur astronomers have been fascinated by for centuries.
"I worked with a vision scientist colleague to understand the limits of human depth perception," says Joel Green, a project scientist at STScI, who led the study. "It turned out that if you had eyes one million miles apart, and the resolution of Hubble and Webb (roughly 1,000 times better than 20/20 vision), you could actually see objects like Mars, or Jupiter's moon system or Saturn's rings in stereo 3D!"
Not only might this be a boon to astronomers, offering stereo data on weather changes, collisional studies, ring system shocks, and many more, but would also be a first for science education, making ancient astronomical bodies come to life in the classroom. As Green notes: "These are the sorts of images that could inspire a generation."
ABOUT THE WRITER
Dr. Benjamin Skuse is a mathematician turned science writer based in Bristol, UK.
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