It's been PowerPC in space for decades, from the Opportunity rover (a 20MHz BAE RAD6000, based on POWER1) to the James Webb Telescope (a 118MHz RAD750 "G3"). These are battle-tested processors in extremely hostile conditions, such as the Juno space probe in orbit around Jupiter where the RAD750 (a 132MHz part with 128MB of DRAM) operates in radiation levels a million times the human lethal dose. As evidence of its performance, it was supposed to be deorbited for destruction in 2021, but was extended to 2025 to examine the inner moons of Ganymede, Europa and Io.
Still, PowerPC never had a monopoly: the European Space Agency uses LEON, which is actually a free and open SPARC V8 core, and NASA has also used MIPS processors such as the 12MHz Mongoose-V (based on the R3000) used in New Horizons, which visited Pluto. A cluster of ARM-based "Rad-Tol Dependable Multiprocessors" (PDF) with OMAP 3503 cores will fly on the 6U CubeSat Lunar Flashlight nanosatellite scheduled for later this year after SLS Artemis 1 got scratched. For non-mission critical components, even some off-the-shelf ARM cores have made it to space; the Perseverance rover is another RAD750 system at 133MHz, but the Ingenuity helicopter drone it deployed was a regular Qualcomm Snapdragon 801.
BAE does have later generation Power parts available today: the RAD510 SOC, a system-on-a-chip with twice the performance of the RAD750, and the RAD5500 family with the RAD5545, derived from the ISA 2.06 NXP e5500. These are all Power ISA, all radiation-hardened, and all available from BAE's Manassas, Virginia facility, a U.S. Department of Defense Category 1A Microelectronics Trusted Source. (The RAD510 core is actually made by GlobalFoundries Fab 10 — one of IBM's former fabs.) With those on the shelf it's a bit puzzling that SiFive announced their X280 (U74-derived) core with vector extensions and AI/ML support will be the heart of NASA's next-generation High-Performance Spaceflight Computing (HPSC) processor instead, or more accurately eight of them (an additional four unspecified general-purpose RISC-V cores round out the total to 12). The chips are being developed on a radiation- and fault-tolerant process by Microchip Technology over the next three years, at a cost of US$50 million. More than just the added processing capability, probably what gives it a greater edge is lower expected power usage on a smaller process size and the ability to shut down silicon blocks for even greater power savings.
It would have to be indeed a significant technical leap to justify a complete break from a well-understood architecture and we'll see soon enough if it's worth it. That said, assuming it accumulates the same stellar track record as the BAE Power parts, the RISC-V HPSC will likely have its own decades-long run in space (assuming Jack Kang, SiFive's idiot senior vice president of business development, can stop smoking crack: there are still more PowerPC programmers than RISC-V programmers even now, chumpcakes). For that matter, the ESA is interested in RISC-V too and we approve of any free computing solution as long as it does the job. But don't cry for Power ISA in space yet; with three years of HPSC development to go, and several critical missions in progress, there's plenty of universe to explore no matter what CPU is doing the exploring. As HPSC will just be one choice of many even at NASA, Power ISA parts are likely to remain part of this very conservative industry for awhile, especially in commercial and military applications.