NASA credits a light spacecraft and a precise launch for Roman's 22-year fuel margin

Four of the extra 12 years come from the tanks. Roman weighed 8,056 kg at launch against the 9,800 kg gross/wet mass of its matured design ^[7], 1,744 kg under it ^[1], so the propellant tanks were filled to capacity rather than to the level the 10-year requirement needed ^[9]. The phys.org account puts that fuller load at four more years of observing ^[10]. Take those out of the 12 and eight years are left to the accuracy of the launch and the burns that followed ^[3].

"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 ^[8]. The budget was written against a worst case the hardware came in under.

How far the plan moved is visible in an earlier document. A 2015 design reference projected a gross/wet mass of 4,166 kg carrying only 107 kg of propellant, a low figure that reflected maintenance burns not yet well defined at the time ^[6]. In flight, the allocation for the first midcourse correction alone was 200 kg ^[11], 93 kg more than the entire 2015 propellant projection ^[4].

That burn ran three minutes on Aug. 31 and was completed with better than 99% accuracy, consuming 18 kg ^[11]. That is 9% of the allocation, and 182 kg went unspent ^[2].

Two maneuvers are still ahead. NASA expects the second midcourse correction to need less fuel than planned because the first was so accurate ^[12], and it projects the same for the insertion into a halo orbit at L2 in early December ^[13]. Roman is still in transit to L2 ^[21]. "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 ^[3].

After insertion the recurring demand is a stationkeeping burn about every 28 days ^[14]. That is roughly 13 a year, or near 290 across a 22-year mission ^[5].

The first five years are largely committed, with the bulk going to the Core Community Surveys, including the High-Latitude Time-Domain Survey ^[16]. The planned second five belong mostly to the General Observer program, which opens the telescope to the wider astronomical community ^[17], and about 50 candidate GO programs are already prepared ^[18]. NASA has not said how the extra 12 years will be used; the phys.org report expects GO to absorb much of it, with some possibly going to the core surveys ^[19].

The 22-year figure describes propellant. NASA's framing is that fuel is Roman's only consumable resource and the limiting factor on mission length ^[4], and the announcement covers that and stops there; how long the detectors and mechanisms will keep working is a separate question. Any observing that runs past the original decade also runs the coronagraph longer, an instrument flying as a technology demonstrator for an eventual planet-finding mission ^[20].