NASA's Nancy Grace Roman Space Telescope may be able to operate for far longer than planned, after an unusually precise first course correction left the observatory with substantially more fuel than its mission was designed to need.
"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, centre director at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Roman was built around a five-year primary mission and a five-year extended mission, giving it a fuel budget intended to support 10 years of operations. Propellant is the spacecraft's main consumable resource, so every kilogram saved on the journey can be converted into observing time later.
The observatory carried out its first burn on 31 August, adjusting its path towards its eventual orbit. The manoeuvre was completed with more than 99% accuracy and consumed less than 10% of the fuel reserved for it: about 40 pounds, or 18 kilograms, against a planned allocation of 441 pounds, or 200 kilograms.
Those savings alone could provide roughly four additional years of potential science operations.
Roman also began its journey carrying more propellant than planners originally expected it to need. Engineers had calculated the spacecraft's fuel requirements using a conservative maximum weight of 21,605 pounds, or 9,800 kilograms. Roman ultimately weighed 17,760 pounds, or 8,056 kilograms, at launch.
Because the spacecraft was lighter, it required less fuel for its mid-course correction, and its lower mass also allowed the team to fill the propellant tanks to capacity rather than loading only what the original 10-year mission required. That additional fuel could support roughly four more years.
"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," she said.
Roman's successful first correction is also expected to make its second correction much smaller, potentially conserving still more propellant.
The reasoning behind the headline figure is worth restating, because NASA has stated it clearly. Propellant is the spacecraft's main consumable resource. The mission has more propellant than expected. The mission is therefore longer than expected, and by a margin of twelve years.
This newspaper accepts the logic and would like to extend it. If the limiting factor on a space telescope is the fuel, and the fuel was determined by the mass, then the observatory's operating life was decided not in orbit but on a set of scales. Roman weighed 3,845 pounds less than the maximum it had been permitted to weigh, and each of those pounds has now been converted into astronomy.
Our analysts put the rate at roughly one additional year of surveying the universe for every 320 pounds the spacecraft declined to weigh. On that basis the twelve extra years were not earned by the instrument, the detectors or the science team. They were earned by the parts that were never fitted.
The procurement implication is considerable. A mission's duration can be extended, at no cost, by budgeting it against a heavier hypothetical version of itself and then building something lighter. The heavier the imaginary spacecraft, the longer the real one lasts.
The Department of Unnecessary Research has accordingly recommended that all future observatories be designed against a maximum permitted weight of several thousand tonnes. There is no upper limit on how much a spacecraft can save by not being as heavy as it was allowed to be.