On Nov. 2, NASA will mark 20 years since the Expedition 1 crew reached the International Space Station, kicking off an unbroken succession of astronauts and cosmonauts living aboard the orbiting outpost.

But the chain may be starting to thin. The Expedition 63 crew, which arrived at the station on April 9, will be short-staffed. NASA has yet to announce astronauts for follow-on missions, including who will be onboard when Expedition 63 Commander Chris Cassidy and his two crewmates return home in October.

• Boeing aims for October reflight
• NASA pins hopes on SpaceX

NASA had hoped to be flying astronauts onboard Boeing’s CST-100 Starliner and SpaceX’s Crew Dragon by now, but technical problems delayed both programs. SpaceX is in position to emerge from the quagmire first, though analysis of a Falcon 9 premature engine shutdown during a March 18 launch and a botched Dragon 2 parachute test on March 24 remain underway.

If those issues are resolved and remaining work completed, NASA astronauts Robert Behnken and Douglas Hurley could launch in late May or June for a flight test to the International Space Station (ISS). No launch dates have been scheduled, U.S. Air Force Brig. Gen. Douglas Schiess, commander of the Eastern Range, told reporters during an April 9 conference call.

Behnken and Hurley are training for a possible longer stay on the ISS to help bridge the staffing shortfall.

NASA last year began training the Boeing Starliner flight test crew for a potential extended ISS stay of up to six months, but those astronauts are extremely unlikely to reach orbit this year. Boeing instead will repeat an uncrewed Starliner flight test following a troubled trial run on Dec. 20-22, 2019. The Orbital Flight Test 2 (OFT) is targeted for October.

“Flying another uncrewed flight will allow us to complete all flight test objectives and evaluate the performance of the second Starliner vehicle at no cost to the taxpayer,” Boeing said in an April 6 statement. “We will then proceed to the tremendous responsibility and privilege of flying astronauts to the International Space Station.”

Boeing’s investigation into software glitches that marred the orbital flight test of its CST-100 Starliner commercial space taxi is nearing completion, with no additional major errors discovered, spokesman Joshua Barrett said.

“To date, we have not found any additional errors with similar or anywhere near the same mission-impacting significance as the two declared issues,” Barrett wrote in an email to Aviation Week. “There were a few cases where nonmission-impacting system adjustments will be made, but these are as expected during any first-time flight test.”

OFT-1 failed to dock at the ISS as planned due to a software error that mis-synchronized the Starliner’s mission-elapsed timer with the actual mission-elapsed time. The error cost Starliner its first attempt to fire thrusters to reach the station’s orbit. A communications problem then scotched a follow-on effort to position Starliner for an ISS rendezvous.

Instead, Boeing conducted a two-day free-flying test, demonstrating Starliner’s systems and extending and retracting its docking ring. The capsule then successfully deorbited and parachuted to a touchdown in New Mexico.

During the abbreviated flight, Boeing engineers uncovered a second software issue that could have caused Starliner’s jettisoned service module to collide with the capsule after the deorbit burn. Those problems prompted NASA, on March 6, to declare the mission a “high-visibility close call,” enabling the agency to broadly share lessons learned from the investigation.

Boeing and NASA decided to reverify Starliner’s entire flight software—1 million lines of code—prior to continuing with the flight test program. The software verification process remains underway. “The work scope is well-understood, and we are making good and steady progress,” Barrett said. 

The independent review team also is homing in on the likely root cause of the communications problem, which prevented OFT flight controllers from communicating with Starliner via NASA’s Tracking and Data Relay Satellites as planned. Recommended corrective actions are pending, Barrett said.

In January, Boeing reported a $410 million pretax charge in its 2019 fourth-quarter results to cover the cost of a possible OFT reflight, which would require the purchase of another Atlas V launch service from United Launch Alliance, among other expenses. Boeing is covering the costs associated with corrective actions needed after the OFT, Barrett said. 

The delays with both of NASA’s Commercial Crew providers are hitting the ISS program just as U.S. paid rides on Soyuz are coming to an end. Cassidy’s April 9 launch was the last seat currently reserved under NASA’s existing agreements with Russia’s state space corporation Roscosmos. As Cassidy and his crewmates began their 196-day mission, negotiations for a ride for his backup, Stephen Bowen, were still underway.