NASA's Next-Generation Telescope: Ace Comet Hunter?
How does a comet come to life and then fade away? This is one of the questions comet-hunters hope to answer with the James Webb Space Telescope.
When NASA's James Webb Space Telescope finally sees first light in 2018, comet-hunters will be eager to get some time on the orbiting observatory. That's because the 6.5-meter (21.5-foot) mirror on the telescope will give unprecedented resolution of comets swinging close to Earth.
In new research led by Michael S.P. Kelley, an associate astronomy research scientist at the University of Maryland, goes into some of the reasons James Webb will be an awesome comet observatory. (The paper will be published in the Publications of the Astronomical Society of the Pacific.) The challenge is comet observers will be competing with astronomers with all sorts of research priorities, such as looking at galaxies or probing the beginning of the universe.
Kelley spoke with Discovery News to discuss some research priorities once comet hunters get some observing time on NASA's next-generation space telescope.
Our challenge with looking at comets is they look like fuzzy snowballs from a distance. They look even more distorted when looking through the Earth's atmosphere. James Webb, fortunately, is parked far from Earth and has a mirror several times larger than that of the Hubble Space Telescope, opening our eyes to extremely detailed views of comets in our solar system.
James Webb is also equipped with a spectrometer and is sensitive in the wavelength range of 3 to 5 microns. This happens to also be where the emissions from cometary gases show up well -- such as water vapor, carbon dioxide and carbon monoxide. By studying these gases, we will begin to understand what the heart of the comet (the nucleus) looks like.
"We want to know what the comet's nucleus is made out of," Kelley told Discovery News. "From a distance, all we see is this coma, this bright unbound atmosphere. We can't study the nucleus directly."
Carbon dioxide in comets is practically invisible from Earth, because so much of the gas is in our atmosphere that it cancels out what we can see in space. With James Webb, we can see it quite well. Why is that important? It turns out that carbon dioxide is one of the chief indicators of how much mass the comet is losing as it drifts close to the sun, and how it sheds its outer layers.
Carbon dioxide and its close cousin, carbon monoxide, are volatile molecules. This means that they react quite easily to the sun's energy, compared even to water ice. So when a comet starts to get close to the sun, scientists like to study these two elements to see how the comet is eroding. This lets us better understand how comets are constructed.
The asteroid belt between Mars and Jupiter also happens to have some comets in it. Telescopes from Earth have spotted dust trails from some of these comets, suggesting there is some sort of activity happening on them. But that activity is likely very faint. Given the comets are relatively close to the sun and have been in a stable orbit for billions of years, likely most of their gas has leached away.
James Webb's sensitivity, however, may allow scientists to finally see how much gas remains around these "main belt" comets, Kelley said, particularly gas that is related to water. "It's about the story of water in the asteroid belt," he said. "We want to know where does it come from, how much is out there, and how much was (previously) brought to Earth."
Most comets are only observed when they get close to Earth, because we only have so many spacecraft available to study them up close. (The Rosetta spacecraft, in fact, is the first one to spend a long time at a comet; it's been orbiting Comet 67P/Churyumov–Gerasimenko for more than a year.) This gives scientists a skewed view of comets. We only see them when they are very active -- not when they are warming up or cooling down.
With James Webb, Kelley hopes that the telescope could be used to watch comets flying away from us into the outer solar system. It can keep an eye on the gases and activity occurring as the comet goes far from the sun and quiets down. This will give a more complete idea of the life cycle of a comet, from when it is quietest to when it is most active.
There's an extensive list of good comet targets for James Webb, but these are some of Kelley's favorites:
103P/Hartley 2 (2023):