Origins Space Telescope May Answer the Big Questions
"Are we alone?" That's one of the very big questions that the proposed Origins Space Telescope hopes to answer.
If the Origins Space Telescope gets the green light as a new NASA flagship space mission, its instruments will be tuned to the mid and far infrared, a spectrum in the cosmos that is rich with information that could answer that question. But that's a big if. Origins is one of four proposed large missions that will be reviewed by the 2020 Decadal Survey, along with Lynx, which is an x-ray mission, and LUVOIR and HabEx, which are both UV optical telescopes. They've all got great science goals—but Origins speaks to the curious child in all of us.
David Leisawitz is the NASA study scientist for the Origins Space Telescope Science and Technology Definition Team, a large group of volunteers who worked together to design the mission concept and instruments for the proposal. The team started with some pressing science questions that were informed by previous missions and current technology capabilities. "It's a back-and-forth bootstrapping between what you can do and what you want to do," he says.
And what they want to do is gather some data that may answer The Big Question. "The only life we've found so far after a lot of looking is right here on Earth," says Leisawitz. "But there are all sorts of hints that conditions are right for life to exist elsewhere."
Biosignatures are one such indication that "conditions are right." There are pairs of molecules—like oxygen and methane, for example—that chemically self-destruct through an oxidation reaction. But when life is present, like on Earth, those molecules can co-exist. So if you can detect a planet that has both oxygen and methane in the atmosphere, then there's a good chance you're looking at a planet with life. The oxygen-methane pairing is just one example of a number of possible biosignatures.
One of the planned instruments aboard Origins, called MISC-T, will look for those biosignatures in the spectra of transiting exoplanets in the 2.8–20 µm midinfrared wavelength range. The Origins team would start with a catalogue of Red Dwarf star systems with known transiting exoplanets-including data recently provided by the Transiting Exoplanet Survey Satellite (TESS)—and then do an initial characterization of the atmospheres of a few dozen of those planets. They'll then progressively narrow the sample to those that best justify a deep search for biosignatures.
"If we do that and find that we've observed a dozen and found zero [biosignatures], that's a pretty good indication that life is rare," says Leisawitz. "But of course we hope that we actually find a biosignature!"
Almost as exciting as discovering life elsewhere in the universe is understanding how life might come to be. If a planet is going to be habitable, at least as we understand it, then it needs to have water on it. And it can't be frozen or vapor. It needs to be liquid. So how did Earth acquire this unique life-giving resource? Scientists have a well-formed hypothesis: Earth's water may have caught a ride on a comet or asteroid.
The water in Earth's oceans is characterized by a unique ratio of deuterium atoms to hydrogen atoms, and after years of observations of thousands of comets, a few have been found with the right magic ratio, which gives some observational validation to the comet delivery idea. However, the sample size is small, and the instruments on Origins could help to greatly expand that sample to a statistically significant number. It will also be able to investigate the distribution of water vapor in the swirling discs of gas and dust in protoplanetary nebulae. By following the water trail, scientists will gain an understanding how life-giving conditions might develop.
The third goal, which is no less ambitious than the first two, is to learn how galaxies formed and evolved over time throughout cosmic history. From earlier space missions, we know that the universe began with essentially no heavy elements-just hydrogen and helium, for the most part. But over time, the furnace of stars built up the heavier elements, which were ejected from those stars into the interstellar medium, which became the reservoir for succeeding generations of stars. But there's still a lot that we don't know about this process. "You learn something, and then you learn what you don't know," says Leisawitz. "And you want to know more."
Origins would be designed to address these big "how did we get here" questions, which are complementary to the soon-to-launch James Webb Space Telescope (JWST). But whereas Webb's instruments are focused primarily on the near-infrared wavelengths, where a distant galaxy's starlight is found, Origins will focus on the far infrared spectrum, which is the right wavelength range to study the gas and dust in the interstellar medium. The interstellar medium is a hotbed (figuratively speaking-much of it is actually quite cold) of clues and data about how galaxies, stars and planets came to be.
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If approved to move forward as a large mission, Origins Space Telescope will join a legacy of transforming missions, including the Herschel Space Observatory, which was the latest and greatest mission in the far infrared. But technology has come a long way since Herschel, which was operated by the European Space Agency from 2009 to 2013. Origins will be about a thousand times more sensitive than its predecessors, and this improvement is due largely to the fact that it will be cryocooled to a very, very cold temperature.
And although space is already quite cold, anything you stick in space in the vicinity of Earth will heat up. And if you try to make observations in the far infrared with a warmish telescope, it would be like observing the universe through a fog. But if you can dial down the telescope's temperature, then the universe's photons will not be overpowered by those from the telescope. How cold is cold enough to avoid the photon fog? 4.5 Kelvin, or –268 °C. "You wouldn't want to stick your tongue to it," jokes Leisawitz.
4.5 Kelvins is considerably colder than Herschel, which operated at a balmy 80 Kelvins, or –193 °C (and produced the dreaded fog of photons). Origins will cool itself using mechanical solar-powered cryocoolers, a recent technological advancement that will also cool one of the JWST instruments. Previous telescopes had to rely on expendable tanks of coolant like liquid helium, which take up valuable space and weight. Origins will be able to dedicate that space to its telescope and instruments.
To improve likelihood of selection by the decadal, the Origins mission concept team put a big emphasis on simplicity and flexibility to help reduce risks. This is in contrast to JWST, which will have a complicated deployment that requires an origami-like unfolding of its mirrors and big sun shields. That requirement was necessitated by limitations in the size of the launch vehicle, and it added years of complexity and testing to the project.
But that limitation has been removed with the emergence of private companies like Blue Origins and SpaceX, who are actively competing to create large launch vehicles. NASA is developing a large launch vehicle as well, and these developments change the landscape for future space missions. The Origins team expects to be able to fit a large 5.9 m tubular-shaped telescope into an 8.4 m fairing with only very simple deployments-like an antenna and solar panel.
They also have a plan to service the telescope, which is now required for all of NASA's large flagship missions, just in case they want to extend the lifetime of the mission. Origins would fly in a Sun-Earth L2 orbit, a million miles away from earth, which is about four times the distance from Earth to the Moon. Rather than send astronauts, they're planning for robotic servicing by utilizing a modular instrument bay. Old or malfunctioning instruments can be removed and new instruments plugged in. They've even planned for the unknown by including four instrument chambers for just three planned instruments [the Mid-infrared Spectrometer and Camera Transit (MISC-T) spectrometer, the Origins Survey Spectrometer (OSS), and the Far-infrared Imager Polarimeter (FIP)], which would allow the possibility to add a fourth instrument at a later time, even after launch.
The Origins Space Telescope Science and Technology Definition Team seems to have thought of everything. Now they just have to wait. The upcoming Astro2020 decadal survey, which is facilitated by the National Academies, occurs every ten years, as the name suggests. This group of volunteer experts helps set the astronomy community's priorities for the coming decade. James Webb Space Telescope emerged from Astro2000, and with its nearing launch, the timing is good to start work on another large mission.
In the next 20 years, the Origins Space Telescope may give us an answer to The Big Question, "are we alone?" It's hard to imagine a more intriguing query. But we'll have to wait for the response of the decadal survey. Their support is needed to make it so.
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