On 5 June 2024, after many delays, Boeing (Arlington, VA, USA) launched the inaugural crewed flight of its conical Starliner spacecraft (
Fig. 1) [
1]. But the mission did not finish as planned. Multiple technical problems emerged, and the National Aeronautics and Space Administration (NASA) three months later had the spacecraft return from the International Space Station (ISS) empty, further stranding its crew of two astronauts at the orbital outpost until they could be rescued with another craft.
The Starliner misfire marks a major setback for Boeing. The company hopes to advance as a competitor to SpaceX (Boca Chica, TX, USA), whose Dragon spacecraft, funded by lucrative contracts with NASA’s commercial crew program, has been ferrying astronauts to and from the ISS since 2020 [
2]. Until Boeing can demonstrate Starliner’s safety and reliability, NASA remains dependent on SpaceX, a potential issue as the agency prepares its ambitious program of Moon missions and plans for commercial space stations.
NASA originally intended to have the Starliner return to Earth with its crew in June after a short stay at the ISS. But as engineers conducted batteries of tests, NASA leadership ultimately concluded the risks of bringing back astronauts Butch Wilmore and Suni Williams on the spacecraft were too high. The decision was prompted by uncertainty with the craft’s systems, which “does not meet the agency’s safety and performance requirements for human spaceflight,” according to a NASA statement [
3]. The spacecraft eventually flew back and parachuted down on 6 September 2024 in New Mexico’s White Sands Space Harbor. “It is great to have the Starliner home. A safe and successful landing is exactly what we wanted,” said Joel Montalbano, a deputy associate administrator at NASA headquarters, at a press conference the following day [
4]. “It is important to remember this was a test mission.”
Now NASA and Boeing engineers have the capsule on hand to collect data, analyze its systems, and run more tests before deciding how to proceed. NASA has yet to purchase more Starliner rides to the station and has not announced whether it believes another uncrewed test flight is necessary before astronauts can board it again.
Issues with the brand-new Starliner became apparent early in the mission [
5]. NASA detected a helium leak in the propulsion system as it blasted upward, following its launch atop a United Launch Alliance Atlas V rocket, and two more leaks were spotted after it reached the ISS. That helium is necessary to pressurize the fuel lines of Starliner’s thrusters. To make matters worse, five of the 28 reaction control system thrusters malfunctioned as Starliner approached the station and prepared to dock. The capsule did dock successfully, but the thrusters are again needed for maneuvering during reentry. If the craft is not pointed in the right direction as it descends into the atmosphere, the flight could end in disaster.
“The thrusters were getting hotter than anticipated, which caused problems with the seals on the valves that feed propellant into the thrusters,” said Don Platt, an associate professor of aerospace, physics, and space sciences at the Florida Institute of Technology (Melbourne, FL, USA). According to Platt and others [
6], this was partly due to the aft-facing thrusters being exposed to direct sunlight, and the atmosphere lacking air that would cool them.
These issues did not arise to such a degree during Boeing’s successful uncrewed flight to the ISS in May 2022, when two thrusters failed [
7], and they cannot be easily simulated or modeled, Platt said. While the latest Starliner did successfully fly back to Earth, Boeing’s engineers will need to study it and resolve the problems. In the meantime, astronauts Wilmore and Williams have extended their unscheduled stay in orbit, alongside five other astronauts and cosmonauts at the ISS. They are currently scheduled to return to Earth on a SpaceX Dragon in March 2025 [
8].
The Starliner and Dragon are similarly shaped, though the former is slightly shorter and heavier, and both use a similar hydrazine-based propellant [
9]. They are both designed to carry crew and cargo, they have solar panels to generate electricity, and they can remain docked to the ISS for up to seven months. They do, however, have some significant differences: for one, SpaceX develops all of Dragon’s components in-house, while Boeing relies on subcontractors, and importantly, from NASA’s perspective, Starliner is currently more expensive than Dragon [
10].
The Starliner woes add to Boeing’s troubles, as the aviation powerhouse also deals with problems with its commercial aircraft. Boeing 737 Max 8 airplanes crashed in 2018 and 2019, killing all on board [
11]. Then, in January 2024, a door plug flew off a 737 Max 9, which managed to land with no casualties [
12]. The Federal Aviation Administration then grounded those aircraft for weeks, and during that time, Alaska and United Airlines found more loose parts on some of their jets. The mounting problems prompted Boeing’s CEO to step down at the end of 2024 [
13], and the company to explore jettisoning its space assets. In August 2024, Boeing and Lockheed Martin (Bethesda, MD, USA) began negotiating a potential deal to sell their joint rocket-launch firm, the United Launch Alliance, or ULA, to Sierra Space (Louisville, CO, USA) [
14]. Then in October, the incoming CEO announced that the company may sell its entire space division, except for the massive Space Launch System rockets being developed for NASA’s Artemis Moon program [
15].
Encountering technical problems on the first crewed flight of a new spacecraft is not surprising, said Phil Metzger, a research professor in physics at University of Central Florida (Orlando, FL, USA), who formerly worked at NASA’s Kennedy Space Center. “There are always anomalies because these vehicles are so complicated. It is normal to fly with hardware problems,” he said, pointing to occasional problems with NASA’s previous space shuttle missions. “If you demanded perfection, you would never fly.”
NASA flew 135 missions with its space shuttles, which, although mostly successful, included two fatal accidents with the Challenger shuttle in 1986 and Columbia in 2003.
That history played a role in the decision to send back Starliner without the astronauts on board, said NASA administrator Bill Nelson in August 2024 [
16]. The US government terminated the shuttle program in 2011, and for most of the next decade, NASA depended on Russia’s Soyuz spacecraft for sending its astronauts to the ISS. However, the Russian space program has had its own technical problems in recent years, with the formerly dependable Soyuz and cargo ships springing coolant leaks, and the Soyuz rocket having a booster failure [
17]. The Russian space program has also faced political problems and budget shortfalls, especially with the continuing Russia-Ukraine conflict and related economic sanctions.
For travel to and from the ISS, NASA currently depends solely on SpaceX’s Dragon, which launches atop the company’s reusable Falcon 9 rocket [
18]. To date, the spacecraft has carried 10 crewed flights to orbit, encountering only minor problems, such as with the rocket’s ignition system and a pressure sensor in the upper stage, each of which caused brief launch delays [
19].
While SpaceX has rapidly come to dominate as NASA’s chief contractor, Boeing and other companies are seeking to rival it in the coming years. NASA would like contingency options, as other NASA programs have lacked that over the past few decades, Metzger said. Indeed, the agency’s leadership was glad to have the option of returning Williams and Wilmore on SpaceX’s Dragon. “The beauty of having Dragon and Starliner in two different diverse space transportation systems is that we can use those as backups,” said Steve Stich, NASA’s head of the commercial crew program, at a July 2024 press conference [
20].
Beyond Boeing, Rocket Lab (Long Beach, CA, USA) has enjoyed considerable success, having launched more than 50 of its Electron rockets since 2017 [
21]. Though less than a third the height of the SpaceX Falcon 9, the Electron has already lofted numerous smaller payloads, like groups of satellites, to orbit. Blue Origin (Kent, WA, USA) has been developing its New Glenn rocket for years, and its first launch is slated for December 2024 [
22]. The New Glenn, which has more thrust and a much larger payload volume than Falcon 9, would build on Blue Origin’s success with nearly 30 crewed suborbital flights on its New Shepard spacecraft [
23]. For its part, Sierra Space plans the first flight of its Dream Chaser spaceplane sometime in spring 2025 [
24].
So far, neither Boeing nor these others appear close to challenging SpaceX’s dominance. But many hope to see that changing within a few years, to the benefit of NASA missions and commercial satellite operators. “People would like to see some level of competition, because there is always the concern of a monopoly and having to rely on not only one company but one rocket, the Falcon 9,” Platt said. “If there was ever an issue that came up with the SpaceX hardware, essentially everything would be grounded.”
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