WASHINGTON — NASA has granted a second, and likely final, extension for the Dawn mission, allowing it to continue observations of the dwarf planet Ceres through 2018.

The extension, announced by the agency Oct. 19, means that the spacecraft will not leave orbit around Ceres, the largest object in the main asteroid belt, to visit another asteroid, an option project scientists said earlier this year was under consideration.

The latest extension, the agency said, could allow the spacecraft to make high-resolution observations of Ceres. The mission team is studying options to place Dawn into an elliptical orbit that would bring it within 200 kilometers of the surface of Ceres. Previously, the spacecraft came no closer than 385 kilometers.

Those close approaches, NASA said in a statement, would allow better measurements of the uppermost layer of the dwarf planet’s surface using the spacecraft’s gamma ray and neutron spectrometer, including determining how much ice it contains. Dawn will also collect high-resolution images and spectra on those close approaches.

Dawn’s lifetime is limited by its supply of hydrazine fuel, which the spacecraft has to expend for attitude control after the failure of three of the spacecraft’s four reaction wheels. Dawn’s current orbit, which keeps the spacecraft between 5,100 and 38,300 kilometers above Ceres, allows it to be “very frugal with its precious hydrazine,” said Marc Rayman, chief engineer for the mission, in a Sept. 27 statement. “When the hydrazine is expended, the mission will end.”

With the proposed shift to a lower orbit, Dawn will expend more hydrazine. The spacecraft should be able to operate until some time in the second half of 2018. That will allow observations of Ceres as it passes perihelion, the closest point in its orbit to the sun, in April 2018.

This extension, though, rules out proposals to send Dawn out of orbit to another asteroid. Project officials proposed such an extended mission in the senior review of planetary science missions in 2016, which would have sent Dawn on a flyby of the asteroid Adeona in 2019. NASA rejected that proposal, concluding that Dawn would provide more science by remaining in orbit around Ceres.

As that initial extended mission came to an end in June, project officials again suggested they were considering a proposal to send Dawn to another asteroid. “It’s an option,” Carol Raymond, deputy principal investigator for Dawn at the Jet Propulsion Laboratory, said at a June 13 meeting of NASA’s Small Bodies Assessment Group. She didn’t provide additional details about that option.

Dawn, part of NASA’s Discovery program of low-cost planetary science missions, launched in September 2007. Using an ion propulsion system, it entered orbit around the main belt asteroid Vesta in July 2011, remaining in orbit there before leaving for Ceres, arriving there in March 2015.

NASA said the extended mission will be designed so that, when Dawn depletes its hydrazine, it will be in a stable orbit around Ceres with no risk of crashing into the dwarf planet.