Roman Space Telescope Launch: NASA’s New Eye on Space

At Kennedy Space Centre on a Sunday morning^[1], a SpaceX Falcon Heavy launched through the coastal fog and directed NASA’s latest observatory towards a destination 1.5 million kilometres from Earth. The Nancy Grace Roman Space Telescope, being named in honour of NASA’s first chief astronomer, was due to arrive in mid-2027 but left eight months early^[2], was launched on budget^[3], and carried an instrument package which should change the way astronomers map the universe.

That is the most important reason: Roman has a wide field of view. Its 300-megapixel Wide Field Instrument is able to capture infrared light from a region of the sky that is 100 times larger than the area that Hubble’s cameras can cover in a single exposure^[4]. During its five-year primary mission, this field of view should result in images of about a billion galaxies^[5][6] and tens of billions of individual stars. The launch of the Roman Space Telescope, as stated by SpaceNews, marks the end of more than a decade of engineering work on one of the most ambitious telescope designs ever flown.

The primary mirror for Roman, which has the same diameter as that of Hubble^[7], was initially made for a completely different purpose. It was provided by the National Reconnaissance Office as part of a set of surplus mirrors offered to NASA in 2012. Members of the optics team at L3Harris^[8] took many years to modify both the main mirror and its nine supporting mirrors so that they could be assembled into an Optical Telescope Assembly capable of operating two science instruments at the same time.

To achieve that degree of optical precision is by no means an easy accomplishment. The main mirror has to stay in shape by only a fraction of the wavelength of infrared light, about 100 nanometres, even though it is subjected to temperature changes as the telescope orbits the Sun. This has been accomplished by using a low-expansion glass together with an active thermal control system which keeps the mirror stable to within a tenth of a degree Celsius.

The Coronagraph, which is Roman’s second instrument, carries out an intuitive action by intentionally blocking out the starlight. It achieves this by means of a number of masks, deformable mirrors, and wavefront sensors, thus reducing the star’s glare by a factor of about a billion^[9] and enabling the very faint light reflected by orbiting planets to be detected directly. Should the technology demonstration succeed, it will help to guide the design of future missions that might some day take photographs of Earth-like planets orbiting nearby stars.

Roman is now in the stage of commissioning, and the first scientific observations are due within a few months. Because of its wide survey capability the telescope should enable measurements of dark energy, the mysterious force which is causing the expansion of the universe to accelerate, to a precision greater than that which can be achieved with ground-based telescopes. It is not clear whether or not it will resolve the question, but the data set that it will produce will be the largest infrared sky survey ever put together.