Could the CHARISMA be the basis for a Solar System Space Telescope?
Kunio M. Sayanagi
It’s been almost 32 years since the Space Shuttle Discovery released into Earth orbit the Hubble Space Telescope.
Since then the school bus-sized ‘scope has cruised around Earth at 17,000 mph making observations of the distant nebulae, galaxies and, just occasionally, objects in own Solar System.
In fact, Hubble has so far spent just 6% of its time studying Solar System targets during its life.
Despite that, Hubble is of critical importance to planetary scientists in a way that the new James Webb Space Telescope (JWST) never can be.
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Hubble sends back images of the giant planets, specifically Jupiter and Saturn, each year as part of its Outer Planets Atmospheres Legacy program (OPAL) so scientists can look for changes in their storms, winds and clouds.
As for the many smaller bodies of the outer Solar System there’s already scant time for astronomers to study them using Hubble.
Once-a-year glances aren’t much, and besides, Hubble entering old age (and beginning to fail). The new JWST will study some Solar System targets—such as Jupiter’s “Great Red Spot”—but it will work primarily in the infrared so will largely be focused on deep space. It’s primarily for astrophysics, not planetary science.
Either way there could soon be no “eye on the sky” capable of sending us images of the planets, comet, asteroids—and making new discoveries.
Imagine what planetary scientists could do with a telescope trained on Solar System objects 100% of the time.
Is it time for a dedicated Solar System Space Telescope (SSST)?
An image of Jupiter taken by NASA’s Hubble Space Telescope in ultraviolet, visible, and … [+] near-infrared light on Aug. 25, 2020.
NASA, ESA, STScI, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL team
What is the Solar System Space Telescope?
It’s a large/medium-class space telescope dedicated to planetary science and to observing targets across the entire Solar System. We’re talking high-resolution, high-sensitivity observations of in visible and ultra-violet wavelengths.
That’s currently possible from Hubble, but not Webb, which primarily sees in the infrared.
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It’s all detailed in a white paper now being considered as part of the upcoming Decadal Survey for Planetary Science and Astrobiology, a report compiled by the National Academy of Sciences that will set out the priorities for NASA for the next 10 years. It will be published on April 19, 2022.
The authors wants the SSST to be considered for one of NASA’s New Frontiers missions, which usually cost under $1 billion. SSST is estimated to cost $815 million.
Why do we need a Solar System Space Telescope?
To help astronomers understand time-dependent phenomena in our solar system—such as seasonal changes on planets, dust storms and even rare comet-strikes—as well as a detailed study of (and a hunt for more) minor bodies across the Solar System.
“What’s interesting about solar system targets is that they evolve on human timescales—very quickly—sometimes on a daily, but definitely on an annual, basis,” said Kunio Sayanagi at Hampton University, Virginia, who co-authored the proposal for the SSST. “Hubble’s OPAL program has been capturing a good view of Jupiter, Saturn, Uranus, Neptune once a year to see the changes.”
Sure, Juno at Jupiter and Cassini at Saturn have taken far higher-resolution images during Hubble’s lifetime, but such missions don’t last long.
A Solar System Space Telescope could study:
Volcanic activity on Venus and Titan, a large moon of Saturn.
Look for water vapor on the moons of Uranus (Miranda and Ariel), Neptune’s massive moon Triton, and Pluto.
The changing surfaces of Enceladus, a moon of Saturn, and Io, a volcanic moon of Jupiter.
Study Jupiter’s Trojans, irregular satellites, Centaurs, and Kuiper Belt Objects (KBOs).
This artist’s concept shows the NASA/ESA Hubble Space Telescope in orbit around the Earth. The … [+] telescope’s current appearance is the result both of its original design, and the modifications made during five servicing missions. In particular, the most recent set of solar panels, visible here, substantially changed the appearance of the telescope when they were installed during Servicing Mission 3B in 2002. They are smaller than the older panels, and produce significantly more power.
ESA/Hubble (M. Kornmesser)
Why can’t we use the Webb Space Telescope?
Webb is incredible. It’s got a 6.6 meter mirror—compared to the Hubble Space Telescope’s 2.4-meter mirror—which means six times more collecting power.
However, we shouldn’t compare Webb with Hubble.
While the latter looks at the Universe mainly in visible light, Webb is an infrared telescope. Webb’s sensitivity to infrared light means it will be able to see the light from the first stars—300 times more massive than the Sun—and the faintest, earliest galaxies.
“JWST is not a successor in the strict sense that it would continue any of the measurements that Hubble has been making,” said Dr. Cindy L. Young, a Research Physical Scientist at NASA Langley Research Center in Hampton, Virginia. After all, there’s little overlap in the in the wavelength range. JWST’s shortest wavelength coverage is 700 nanometers while Hubble goes down to almost 200 nanometers. So in terms of the wavelength JWST is not a continuation of Hubble. “For the UV light measurements that are so critical to uncover temporally dynamic phenomenon in the Solar System we need high frequency and long duration missions,” said Young.
In short, JWST is just not designed for planetary astronomy, but to answer questions in astrophysics. Besides, it won’t have the time to spare. “Using astrophysical assets for Solar System science is important, but cannot fully address the science described here, given the number and time-varying nature of solar system targets,” reads the paper. JWST is for “first look” science, as Hubble generally is—neither telescope is intended to look at objects more than once.
However, for as long as it lasts Hubble is going to stay very valuable to the planetary scientists until a successor—in terms of UV wavelengths—comes along.
Cue the SSST.
NASA’s Hubble Space Telescope captured this image of Saturn on July 4, 2020. Two of Saturn’s icy … [+] moons are clearly visible in this exposure: Mimas at right, and Enceladus at bottom. This image is taken as part of the Outer Planets Atmospheres Legacy (OPAL) project. OPAL is helping scientists understand the atmospheric dynamics and evolution of our solar system’s gas giant planets. In Saturn’s case, astronomers continue tracking shifting weather patterns and storms.
NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team
How big would the Solar System Space Telescope be?
We’re talking a 10 meter telescope. Although it’s designed to be different from the JWST, in some ways the SSST builds on the technologies developed for JWST. Except it can be far less sensitive—because Solar System objects are generally bright—so it uses a sparse aperture design to achieve a really sharp, detailed image. There’s also no need for a sunshield. “We’re hoping that it will be cheaper so that it can be built faster,” said Sayanagi.
The SSST is built on the concept of the Caroline Herschel high-Angular Resolution In-Space assembled Multi-Aperture (CHARISMA) telescope.
What could a Solar System Space Telescope do?
It could respond to events on fast-changing targets.
Take Europa, one of Jupiter’s largest moons, which is a prime target for astrobiology. “Hubble has looked at Europa a couple of times and it looks different every time, but there have been tantalizing signs that it is spewing water out into space,” said Sayanagi. However, Hubble is barely sensitive enough to return an interesting signal. “If we could look at Europa more often then we could see how often its plumes are happening, where they are happening on the surface, and see if they’re associated with interesting geological features on its surface.”
Another area ripe for exploration by a space telescope that largely gets overlooked are small, cold and dark bodies in the outer Solar System, which JWST may locate, but won’t have time to properly characterize. Some of those small bodies known to science have rings that have never been studied—and don’t forget the ring systems of Jupiter, Saturn, Uranus, Neptune.
There are many, many other uses for a SSST, such as auroras and weather systems on planets, as well as studies of asteroids and comets.
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Then there’s Venus and Saturn’s moon Titan. “The jury’s still out on whether there’s active volcanism on Venus, so having longer time to target it would help us answer that question,” said Sayanagi. How energy moves around Venus is important is scientists are to make parallels to Earth’s weather as well as to the atmospheres of exoplanets. But how long can JWST stare at Venus and Titan to watch how quickly their clouds move, and therefore measure their wind speeds?
In the case of Venus it’s actually a lot more fundamental a problem than pure observation time because JWST can never look anywhere near Venus. As a general rule Hubble doesn’t look at Venus either—it’s generally way too close to the Sun.
“Looking toward the Sun is a dangerous business for telescopes so you have to build in a special capability,” said Young, suggesting a carefully designed optical baffle that will enable the SSST to look at Venus, and anything else within 30º of the Sun.
The Hubble Space Telescope has been magnificent, but to call JWST its successor is an error. There’s about to be a huge gap in our abilities to study what’s going on in our own Solar System that can only be solved by launching a space telescope dedicated to planetary science.
Wishing you clear skies and wide eyes.