Roman Space Telescope
Exoplanets, gravitational lenses and dark energy lurk in the infrared universe
Exoplanets, gravitational lenses and dark energy lurk in the infrared universe
Each time humanity launches a new space telescope, it opens up a new window to the universe. One of NASA’s latest is the Nancy Grace Roman Space Telescope, previously the Wide Field Infrared Survey Telescope (WFIRST) before being renamed after NASA’s first chief astronomer. It’s due to launch in 2025 and will attempt to answer questions about dark energy through observations of gravitational lenses, supernovae and baryon acoustic oscillations – fluctuations in the density of the visible matter in the universe. It will also survey exoplanets, directly imaging those of Jupiter size, and use gravitational microlensing to find bodies only a few times the mass of Earth’s Moon. And it will do all this using infrared light.
Roman can trace its origins back to 2011, when a 1.3-metre (4.2-foot) infrared telescope was proposed. The next year this was scrapped in favour of a plan to use two secondhand telescopes donated by the National Reconnaissance Office (NRO), a US intelligence agency that also uses space telescopes, but points them in the other direction. The NRO considers these telescopes obsolete for its purposes, but for NASA they’re a way to continue with space science in an era of strained budgets, where the $10 billion (£7.2 billion) cost of the James Webb Space Telescope is starting to raise a few eyebrows. Michael Moore, the agency’s acting deputy director for astrophysics, has stated that using both telescopes may ultimately save NASA $1.5 billion (£1.1 billion) in the long term.
Roman was earmarked for cancellation itself in the 2019 US budget proposal due to the higher priority of Webb, but this was rejected by Congress and the mission continued, and should now cost no more than $3.9 billion (£2.8 billion). While the NRO telescopes, constructed between the late 1990s and early 2000s, are structurally intact and have never been used, they’ve been stripped of all their electronics, so these will need to be replaced at NASA’s cost. Their 2.4-metre (7.9-foot) primary mirrors are
“If you’re interested in the majority of stars in the local universe, the best place to look for them is in the infrared” David Clements
still attached, however, and are the same size and quality as those within Hubble, though the shorter focal length of the telescopes means they have a wider field of view, imaging about 100 times the area Hubble can. The mirror is so finely polished that the average bump on its surface is only 1.2 nanometres tall. If scaled to the size of Earth, those bumps would rise to just over half a centimetre. The mirror and some parts for a third telescope also exist, but NASA currently only has firm plans to use one of them, though the idea of sending the other to Mars to photograph the surface in high detail has been floated.
Aboard Roman will be two instruments: the Wide-Field Instrument, a 300-megapixel camera that captures light in visible and near-infrared wavelengths, and a coronagraph dedicated to studying distant stars in infrared. A system of prisms, masks and mirrors allows it to block out the glare from a star, essentially placing a dark spot over it, to reveal planets in orbit.
“What we’re trying to do is cancel out a billion photons from the star for every one we capture from the planet,” says Jason Rhodes, project scientist for Roman at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This may be the most complicated astronomical instrument ever flown. With [Roman] we’ll be able to get images and spectra of these large planets, with the goal of proving technologies that will be used in a future mission to eventually look at small rocky planets that could have liquid water on their surfaces, or even signs of life, like our own.”
To do this, Roman will orbit at a point known as Lagrange point 2 (L2). This is a spot 1.5 million kilometres (932,000 miles) from Earth in the opposite direction to the Sun where gravitational forces cancel out and objects will naturally hold their position relative to the large bodies. This point is four times as far from Earth as the Moon orbits, and will be shared with other space observatories, including Webb.
One benefit of L2 is that much of the Sun’s radiation is blocked by the Earth, appearing in a constant state of partial eclipse, and therefore the observatory is able to maintain a constant temperature while absorbing solar energy. It’s also extremely stable. Hubble, by comparison, sits in low-Earth orbit only 547 kilometres (340 miles) above our planet, and due to atmospheric drag is expected to re-enter the atmosphere between 2028 and 2040.
The first coronagraph in space – and still the only one at the time of writing – is on Hubble.
“The shorter focal length of the telescopes means they have a wider field of view, imaging about 100 times the area Hubble can”
After Roman launches, however, there will be three: Webb, due to blast off in 2021, will have one too, but it lacks the starlight-suppression technology of Roman. When it comes to discovering exoplanets, Roman has two methods available. The first is gravitational microlensing, which occurs when a star passes in front of another, its gravity bending the light from the farther star and magnifying it. Planets orbiting the closer star can have the same effect, just on a smaller scale, and the telescope should be sensitive to planets as small as Mars. For more distant stars, Roman should be able to find large planets in close orbits – like so-called hot Jupiters – and even the failed stars known as brown dwarfs. Rather than the dip in brightness associated with a planet when using the more common transit method of detection, a microlensing exoplanet actually causes its star to appear brighter, with peak brightness achieved when the foreground and background stars are perfectly aligned.
Roman’s detectors are tuned to light in the infrared part of the spectrum, but just why do astronomers want to look there? Dr David Clements, a reader in astrophysics at Imperial College London who studies the universe in the far-infrared, explains: “The infrared stretches all the way from one micron in wavelength to about a millimetre, so that’s a huge chunk of the electromagnetic spectrum. It gives us access to things that you can’t see from the ground. Starlight is a thermal emission, and if you take a star like our Sun, with a surface temperature of around 5,800 Kelvin, its emission spectrum peaks, as if by coincidence, at about the wavelength our eyes are most sensitive to. And the atmosphere is transparent at those wavelengths too.
“If you’re interested in things that are cooler than the Sun – such as late-type stars, M stars [red dwarfs] and brown dwarfs – then their peak emissions are at longer wavelengths,” Clements continues. “The majority of stars in our galaxy, and other galaxies, are M stars with a temperature of a few thousand Kelvin. Their black-body spectrum – and in fact the blackbody spectrum of the average galaxy – peaks at a wavelength of about 1.6 microns. If you’re interested in the majority of stars in the local universe, the best place to look for them is in the infrared.”
And it’s important that these observations take place in space, too. “Trying to do midinfrared astronomy from the ground is like trying to do optical astronomy with a telescope and a building that are glowing red hot,” says Clements. “That mid-infrared band is known as the thermal infrared, and everything there is glowing as if it’s been in a furnace. And that’s also true of the atmosphere. It can be done, but it’s very, very hard. In the far-infrared the background problem is even worse – there’s no point doing it from the ground because the atmosphere is opaque at those wavelengths. That’s why we build telescopes like Herschel and Planck to get above the atmosphere to do anything at all.”
Then there’s the problem of obscuration. “Star formation takes place in clouds of gas, and where you’ve got gas, you’ve got dust. Dust absorbs
starlight as a function of wavelength, just as there’s dust and smoke in our atmosphere that gives you the red Sun as it goes down – it absorbs short wavelengths better than long ones. If you want to see what’s going on in a region that’s got lots of dust, you can’t do that in the optical because there’s too much obscuration, so you look in the near-infrared. If you want to go to a region like the hearts of galaxies that are forming stars at an extremely high rate, you have to go into the far-infrared.”
There’s also redshift to consider. “Look at things that are farther and farther away,” says Clements, “and the whole spectrum is shifted to longer wavelengths. The most prominent emission line from any galaxy that’s got emission lines is the Lyman-alpha line of hydrogen. At zero redshift that’s way into the ultraviolet, but by the time you look at a quasar with a redshift of six you’re into the photographers’ infrared, and if it redshifts still further you’re into the astronomers’ near-infrared. And that’s true for every emission line down to hydrogen-alpha. H-alpha at a redshift of one is 1.3 microns in wavelength, so you’re into the infrared in the relatively local universe.”
With simple distance pushing many potential targets into Roman’s sensitivity band, there’s certainly a lot for it to look at. Another part of its mission, however, is to observe the effects of something we can’t see. Sensitive enough to detect infrared light from farther away than any other telescope, it will show the universe in its early stages, helping to unravel how it has expanded over the past 13.8 billion years and how it will continue to evolve. “We’re going to try to discover the fate of the universe,” says NASA’s Jeffrey Kruk, a Roman project scientist. “The expansion of the universe is accelerating, and one of the things the Wide-Field Instrument will help us figure out is if the acceleration is increasing or slowing down.”
One potential driver of the expansion is dark energy, the mysterious substance that makes up about 70 per cent of the total content of the cosmos, which may also be changing as the universe evolves. Roman will measure millions of distant galaxies to show how matter is distributed across the universe. Widely thought to be constant, dark energy was shown to have possibly varied over time thanks to data from the Chandra X-ray Observatory. It’s an exciting time to be chasing this invisible driver of change.
A disused spy satellite getting a second lease of life as an observatory that gives scientists a new viewpoint on the cosmos? It’s a story that will only get better as Roman sends back pictures of the universe to rival those of Hubble – and it may help to crack one of the universe’s final, most enduring, mysteries, too.
Ian Evenden Space science writer
Ian has been writing about the universe for over ten years, focusing on cosmology and space exploration. He’s also a keen astrophotographer.
“Look at things that are farther and farther away and the whole spectrum is shifted to longer wavelengths” David Clements