How the space station flipped out of control—and why that's a big problem - MrLiambi's blog

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Friday, 6 August 2021

How the space station flipped out of control—and why that's a big problem

The International Space Station orbiting above Earth.

The International Space Station flipped over on its back on July 29.

This was a significant, though fortunately not disastrous, nearly one-hour episode for humanity's largest and oldest space outpost. The station slowly turned over one and a half times. (Or as NASA describes it, the space station experienced a "total attitude change" of around 540 degrees, with "attitude" being jargon for a spacecraft's orientation.) The new Russian module "Nauka" had docked to the sizeable 356-foot-long station, but Nauka's thrusters fired when they shouldn't have, causing the space station to start unexpectedly spinning.

"It was quite an event," said Keith Crisman, an assistant professor of space studies at the University of North Dakota who researches safety systems for human spaceflight. "It was a potentially serious issue," Crisman added, noting that an out-of-control spacecraft is one of the highest-risk events in space.

Later on July 29, after flight engineers had righted the space station, NASA held a media briefing to address the unusual event. The agency's summary: All is OK, the space station had returned to normal, and nobody aboard was in danger. In fact, a NASA public affairs officer said in an email that the station's spin was "slow enough to go unnoticed by the crew members on board" (until they received warning messages), and everything else operated normally.

While it's fortunate the astronauts and cosmonauts aboard are OK, the event still carries questions about what happened, along with future concerns about space station safety.

"When spacecraft misfire it's a serious thing."

"When spacecraft misfire it's a serious thing," said Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics who tracks rocket and spacecraft launches. "I cannot imagine there aren't some very serious conversations going on at NASA."

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What went wrong

As noted above, thrusters on Nauka started firing after the module docked to the space station, forcing the station to (slowly) spin at a maximum of half a degree per second. It ended up upside down, before the correction. On spacecraft, these types of misfires do sometimes happen, and more easily than engineers would like, explained Kurt Anderson, a professor of mechanical aerospace engineering at Rensselaer Polytechnic Institute.

In 2016, for example, a thruster on Japan's 46-foot long astronomy satellite Hitomi misfired. Hitomi spun uncontrollably and broke apart. And perhaps most famously, the small spacecraft Gemini VIII (piloted by legendary astronaut Neil Armstrong in 1966) violently spun out of control after a thruster problem, but Armstrong impressively stopped the wild tumble, and narrowly avoided national tragedy.

(The space station, fortunately, is a big, over 925,000-pound object with lots of material to turn, so Nauka's thrusters didn't have a chance to get the station spinning treacherously.)

Life on the space station in 2020.
Life on the space station in 2020. Credit: NASA
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But what triggered Nauka's mishap? A software glitch likely played a role. Nauka experienced some minor software issues before arriving at the space station, noted Crisman. The day after the unexpected flip, the Russian space agency Roscosmos officially blamed the event on a software glitch, causing thrusters to fire out and try to withdraw Nauka, which had just docked several hours before. That's as much as we currently know, which comes from a four-paragraph Roscosmos press release.

After the thrusters began misfiring, the space station soon entered an official "Loss of Attitude Control," the NASA representative told Mashable. The blasting thrusters were countered by other space station thrusters firing in the opposite direction to regain the station's normal orientation, NASA said. The station had to flip completely over â€" by 180 degrees â€" to right itself.

Real concerns

The space station excitement comes with some notable concerns, according to experts outside of NASA.

1. The space station is old and not meant for acrobatics. People first inhabited the station over two decades ago. "The ISS is an older piece of equipment. We call it legacy equipment," said Crisman.

It's not a spry vehicle intended to flip around, though the flipping in this case wasn't nearly violent. However, the station experienced thrusters fighting with each other for control of the craft, noted McDowell, which is undoubtedly somewhat strenuous for a spacecraft with attached instruments, like huge solar arrays branching out from the station. "You've got torque on relatively old parts," explained Crisman.

2. Wasted precious fuel: Stopping the station's flip required firing propellant from thrusters, which is problematic because propellant in space is finite, and at times necessary in order to maneuver the space station. There's no other way to purposefully move.

"Propellant is blood."

"Propellant is blood," said Anderson. Unlike for most spacecraft, however, NASA can launch more propellant to the space station, though at a cost.

3. The misfiring thrusters couldn't immediately be turned off. To stop the station from spinning, ground control operators in Russia needed to tell the automated Nauka to stop firing. But this didn't work, necessitating the counter-thrusting. "That they couldn't get the thrusters shut off immediately bothers me," said the aerospace engineer Anderson.

4. Things could have been worse â€" much, much worse.

Any mishap on the space station has the benefit of happening under the watchful eye of NASA's space station team in Houston. "They have a really excellent flight control team," said McDowell, of the Center for Astrophysics.

NASA's flight controllers, like flight director Zebulon Scoville, immediately noticed the station's unexpected behavior, and soon declared a "spacecraft emergency."

Yet there shouldn't have ever been an emergency, emphasized Crisman. Yes, errors are inevitable, but the system shouldn't allow such an issue to percolate down into a potentially serious, active problem. "We should have systems in place to mitigate those errors," he said.

Broadly, these systems should follow the "Swiss Cheese Model," Crisman explained. Different layers of naturally imperfect departments (or layers of Swiss cheese) like mission control, computer programmers, engineers building spacecraft, etc. should make it extremely difficult for an error to find its way through the small holes in each department's slice of Swiss. In the case of the space station flipping, an error slipped through many, many layers of international Swiss cheese.

"Humans are perfectly fallible, and machines are perfectly fallible."

Generally, the space station is a quiet place, tranquilly orbiting some 250 miles above Earth. It's an afterthought to many of us. But things can go wrong. It's hugely fortunate, for example, that Nauka didn't start misfiring as it was docking, potentially leading to an impact with the space station.

This recent flip wasn't terrible, but it's a poignant warning of our vulnerabilities in the harsh realm of space, even on the dependable space station.

"Space is dangerous," said Crisman. "Humans are perfectly fallible, and machines are perfectly fallible."



Source : http://feeds.mashable.com/~r/Mashable/~3/F9COgzHmgx4/international-space-station-flipped-why

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