NASA's Wide Field Infrared Survey Telescope (WFIRST) is moving forward on development even as President Donald Trump's administration tries to cancel the mission, for the third budget in a row.

The mission recently passed "a critical programmatic and technical milestone" and is en route to starting hardware development and testing ahead of the telescope's planned launch in the mid-2020s, NASA said in a statement.

But that would be the case only if funding persists to keep the telescope on track. The Trump administration's federal budget request for the 2021 fiscal year, issued last month, proposes terminating the telescope entirely to allow for funds to move forward on the James Webb Space Telescope, a flagship mission that would study the early universe.

Related: What would it mean for astronomers if the WFIRST space telescope is killed?

A simulation of a WFIRST observation of the Andromeda galaxy, or M31. NASA's Hubble Space Telescope spent more than 650 hours imaging the areas outlined in blue. WFIRST will be able to image the entire galaxy in only 3 hours. (Image credit: DSS/R. Gendle/NASA/GSFC/ASU/STScI/B.F. Williams)

"The administration is not ready to proceed with another multi-billion-dollar space telescope until Webb has been successfully launched and deployed," NASA said in the statement. Webb has been delayed numerous times for technical and budget reasons.

While NASA said the mission is on track to leave Earth in 2021, in January 2020, the Government Accountability Office (an independent, nonpartisan agency that works for Congress) said there is only a 12% chance Webb will make its revised March 2021 launch date, and that's due to a very tight development schedule. The GAO warned that the launch could slip at least another three months, to July 2021.

Cancellations of WFIRST were also proposed for the 2019 and 2020 fiscal years. And in the first budget the Trump administration oversaw, for fiscal year 2018, the administration proposed cuts to the telescope's instrumentation to move it back under the mission cap of $3.2 billion. The fiscal 2021 budget is not finalized yet, as it is undergoing review by House and Senate representatives.

Should WFIRST move forward, the mission will use a repurposed spy telescope with capabilities similar to those of the Hubble Space Telescope but with a viewing area about 100 times larger. The mission's scope includes hunting for exoplanets, seeking infrared (heat signature) signals in cosmic phenomena and creating panoramas of the sky.

"With the passage of this latest key milestone, the team will begin finalizing the WFIRST mission design by building engineering test units and models to ensure the design will hold up under the extreme conditions during launch and while in space," NASA said. The agency noted that the WFIRST program is funded until September 2020, reiterating its position that it will not launch another big telescope until Webb is ready for operations in space.

WFIRST's development budget remains at $3.2 billion, NASA added, although other costs will bring the final tally up to $3.9 billion. These extra costs include five years of operations and science, as well as a technology demonstrator to image exoplanets.

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