NASA launches telescope to map the invisible universe - critical summary review

The telescope that maps instead of zooms

A space telescope is usually imagined as an instrument for looking closely: astronomers pick a distant object, the telescope turns to it, gathers light, and comes back with a sharper image than anything possible from the ground. That is broadly how the Hubble Space Telescope and the James Webb Space Telescope are used — someone chooses the target, and the observatory points at it.

NASA's Nancy Grace Roman Space Telescope, which lifted off this weekend, is built around the opposite instinct. It is a bus-size observatory carrying a 300-megapixel infrared camera with a field of view more than 100 times larger than Hubble's, which lets it survey the sky more than 1,000 times faster. Rather than studying a few objects extremely well, it sweeps enormous slices of sky, again and again.

Shawn Domagal-Goldman, director of NASA's Astrophysics Division, compared the effect to taking in a whole forest while also seeing the leaves and the birds on individual trees. "Of course, this isn't a forest we're talking about; it's the known universe, and those birds and trees are supernova galaxies, planets and stars," he said at a briefing on August 5.

That is the shift worth understanding: the question stops being how sharply we can see one thing and becomes how many unknown things are out there. Nicky Fox, associate administrator for NASA's Science Mission Directorate, called Roman "a discovery machine that will bring us closer than ever before to answering humanity's most profound questions about our cosmic history."

The observatory is named after Nancy Grace Roman, NASA's first chief of astronomy in the 1960s, who pushed the idea of putting telescopes above Earth's obscuring atmosphere and became known as the mother of the Hubble Space Telescope.

Launch, and three months of travelling

Roman lifted off aboard a SpaceX Falcon Heavy rocket at 7:26 a.m. Eastern time on Sunday from the Kennedy Space Center in Florida, reaching orbit about ten minutes later. The estimated cost is $4.3 billion; the telescope has been in development for roughly a decade, and the scientific community made it a priority about 16 years ago.

Nothing scientific happens yet. Roman will take around three months to reach its working position, about 1 million miles (1.6 million kilometres) from Earth, at a spot known as Lagrange Point 2, or L2 — a place where the gravitational pulls of Earth and the Sun balance a spacecraft, roughly four times farther from Earth than the Moon. The James Webb Space Telescope sits in the same orbit.

During the trip the observatory goes through commissioning: testing its instruments in space and deploying the antenna that will send its images home. That antenna matters, because the telescope is expected to transmit one terabyte of data per day — the equivalent, said Jeremy Perkins, integration and test scientist at NASA's Goddard Space Flight Center, of streaming a million songs from a million miles away.

The primary mission runs five years, and the team said after the launch that it could be extended up to ten.

Chasing two things nobody has seen

The scale of the survey is what makes Roman useful for the two biggest unknowns in cosmology.

Astronomers have known since the 1920s, from the work of Georges Lemaître and Edwin Hubble, that the universe has been expanding since its birth 13.8 billion years ago. Research beginning in the 1990s showed something began accelerating that expansion about 6 billion years ago, and the cause is unknown. It is called dark energy.

Exploding stars, or supernovas, act as reference points for measuring that expansion, but they happen too quickly for a narrow-field telescope to catch reliably. Roman's panoramic vision can see several at once, according to Julie McEnery, the mission's senior project scientist.

Dark matter is the other unknown. It has never been detected, but it is believed to make up 85% of the total matter in the universe and to help hold stars and galaxies in stable orbits. Roman will measure how gravity bends light across large cosmic distances, building a three-dimensional map of where that unseen mass sits, and map the distribution of galaxies and galaxy clusters looking for its gravitational fingerprints.

"Roman is designed to make the measurements that will determine whether there's some sort of fundamental difference in the way the universe works compared to the way we think it works," said Dominic Benford, the mission's program scientist at NASA.

The same repeated sweeps should also turn up things nobody is looking for. Fox said the mission will enable the discovery of billions of galaxies, thousands of supernovas and tens of billions of stars. McEnery describes Roman's planet survey as the largest census ever taken of worlds across the Milky Way, potentially finding tens of thousands of previously unknown ones — including rogue planets that orbit no star at all. Benford added that small, wandering black holes are among the faint objects it could pick up, and that Roman could provide the first definitive proof of exomoons, moons orbiting planets around other stars.

"Roman's vast reach will allow us to find the weird, the rare and the unusual," McEnery said. "We'll redefine what it means to find a needle in a haystack."

The coronagraph, and a different way to see planets

Alongside the wide-field camera, Roman carries a technology demonstration: a coronagraph, an instrument that blocks the blinding light of a star so that fainter things beside it become visible. Seen from far enough away, Earth and the Sun would appear almost on top of each other, with the Sun's glare swamping the planet entirely.

Hubble and Webb both have coronagraphs. Roman's experimental version is 100 to 1,000 times better than any built before, according to Bertrand Mennesson, deputy project scientist at NASA's Jet Propulsion Laboratory, who has worked on the mission for 14 years. The difference is that the existing ones are passive. "You send the coronagraph into space, and it's set it and forget it," Mennesson told CNN. Roman's is active: it uses mirrors that change shape in real time to compensate for vibrations, temperature shifts and other errors.

If it works, Roman should be able to see the glow of a Jupiter-size or possibly Saturn-size planet next to another star, and perhaps even light reflecting off the tops of its clouds.

Data anyone can dig through

Roman's output will be released publicly as soon as it is processed on Earth. That follows directly from the survey approach.

"Instead of having a telescope that astronomers have to choose to point at a certain object, we just map lots and lots of sky," Benford said. "Anybody who's interested in star-forming regions, dying stars or distant galaxies, they're already there in the data somewhere."

In practice, discoveries are less likely to arrive as a single announcement about a single target and more likely to emerge from someone searching the archive for something the mission was not designed to look for. That is what Benford means when he says part of the point is "to give people a way to ask the universe questions that we have yet to conceive."

What to do with this information

The next months are procedural, and the milestones are already known. Commissioning and instrument testing run through the cruise phase; arrival at L2 is expected around three months after launch; the first images are expected in January.

January is also when the open-data arrangement starts to matter. Because everything is released as it is processed, there is no need to wait for a press conference to see what Roman found — NASA's public archives are where the survey lands, and they are open to anyone willing to learn how to query them, not only to professional astronomers.

Two things are worth tracking beyond the pretty pictures. The first is whether the coronagraph performs as designed, since it is a technology demonstration rather than a guaranteed capability. NASA has tied it to the Habitable Worlds Observatory, at present only a concept, which would be the first observatory designed to photograph Earth-like planets and look for chemical signs of life in their atmospheres. "We're not going to get there with Roman, but we're going to get there because of Roman," Benford said.

The second is the pace of the surveys themselves. A telescope that revisits the same patches of sky repeatedly produces its value cumulatively — the comparisons between passes are what reveal things that move, flare or explode. Roman's first year of data will not settle what dark energy is. It will show how fast the catalogue of unknown objects grows once someone stops choosing where to point.