From 10 to 22 years: The Nancy Grace Roman Space Telescope’s mission has just been extended

The Nancy Grace Roman Space Telescope hasn’t yet reached its Sun–Earth L2 orbit, and it already has some good news. NASA has announced that the mission has enough fuel to potentially double its mission length. Though initially scheduled for five years of observations, followed by a five-year extended mission, the entire mission could now reach 22 years.

“As a result of exquisite planning by our orbital dynamics team, brilliant execution by the operations team, and a precise launch from SpaceX, Roman has fuel for at least 22 years of potential science operations,” said Jamie Dunn, center director at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Fuel is Roman’s only consumable resource and is the limiting factor for mission length. Several things contributed to its new, extended mission length, starting with the precise launch. The SpaceX Falcon Heavy helped set the stage with its accurate low–Earth orbit departure burn. The more accurate this is, the less fuel is required for Roman’s subsequent burns.

The next reason is the spacecraft’s actual weight compared with its planned weight.

“A spacecraft’s mass changes throughout the design and build process, so we base the propellant budget on a set maximum value so we won’t come up short,” said Alison Rao, the Roman propulsion lead at NASA Goddard. “We track the propellant needed based on actual mass throughout integration and testing as well, to make sure we have wiggle room. Since Roman’s was lower than we budgeted for, we were able to fill the propellant tanks to their capacity rather than only filling them as much as we needed to for the 10-year requirement.”

Telescopes like Roman take a long time to come to fruition, so it’s no surprise that fuel payload and spacecraft weight changed over time. Other aspects of mission architecture also changed, affecting planned fuel consumption.

In a 2015 design reference document, Roman’s gross/wet mass was projected to be 4,166 kg, with only 107 kg of propellant. That low propellant amount reflected the mission’s less well-defined maintenance burns at the time. Years later, the design and mission had matured, and Roman’s gross/wet mass was 9,800 kg. By the time it was launched, it weighed less, only 8,056 kg. That meant more fuel could be loaded into its tanks, which had extra room.

The extra fuel could allow four more years of observations.

The second reason is the efficiency of the spacecraft’s first midcourse correction. It took place on Aug. 31 and was completed with more than 99% accuracy. Not only was it extremely accurate, but the spacecraft also weighed less. That meant that in the three-minute course correction, NASA burned less than 10% of the allotted fuel for the maneuver. Two hundred kg was allocated for the first course correction, but only 18 kg was consumed.

The third reason is upcoming. Roman will have to burn fuel for another midcourse correction, and because the first burn was so accurate, the second correction will require less fuel than planned, too.

Its final maneuver into a halo orbit at L2 will take place in early December, and NASA projects that it will require less fuel, too.

Once in orbit, the space telescope will only need periodic stationkeeping burns every 28 days or so. It all adds up to an extra 12 years of observations.

The bulk of Roman’s initial five years of observing time is dedicated to its Core Community Surveys, including the High-Latitude Time-Domain Survey. The five-year extended mission will be dominated by its General Observer (GO) program. The GO program will make the powerful telescope’s observing capabilities available to the wider astronomical and astrophysical community.

There are already about 50 candidate GO programs ready to go. Though NASA hasn’t said how the extra 12 years of observing time will be used, GO seems likely to account for much of it. But the core community surveys may also see some extra time.

Roman also has a coronagraph instrument that’s serving as a technology demonstrator for an eventual planet-finding mission. If that instrument performs well, it will also see more use during the extra years of observations.

The Nancy Grace Roman Space Telescope was initially named WFIRST, the Wide-Field Infrared Survey Telescope, but was eventually renamed in honor of NASA’s first chief of astronomy. Its initial design called for a 1.5-meter mirror, but eventually it ended up with a 2.4-meter mirror that was originally designed for a reconnaissance/spy satellite.

After years of design changes and haggling over funding and science goals, the spacecraft is approaching actual operations. When it reaches L2, it will undergo commissioning and testing. Before we know it, results will begin to arrive.

Now we can look forward to an extra dozen years of observations.

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Gaby Clark

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Andrew Zinin

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