Russian Soyuz spacecraft, like the TMA-6 vehicle shown here as seen by ISS astronauts in April 2005, ferry spaceflyers to the space station every six months.
CAPE CANAVERAL, Fla. ? The crew of the International Space Station will get a go-ahead next week to perform spacewalking inspections as part of a probe into back-to-back ballistic re-entries by Russian Soyuz spacecraft.
Two veteran cosmonauts, meanwhile, say the type of steep trajectories flown by consecutive Soyuz crews are safe-but-rocky rides back to Earth.
"Imagine you drive a luxury car with fine shock absorbers, not feeling the road at all," said Pavel Vinogradov, who served on Russia's Mir space station and commanded an expedition to the new outpost.
"And then suddenly, one of the shock absorbers breaks and you start feeling all the dents and unevenness of the road," he said. "It doesn't mean that your life is in danger. You can still safely drive the car."
A Russian commission aims to pinpoint the cause of the re-entry difficulties. In October and in April, Soyuz crew capsules did not separate cleanly from attached propulsion modules prior to re-entering the atmosphere on the way to landing in Kazakhstan.
That type of failure could cause a Soyuz to plunge with its hatch, rather than its heat shield, facing forward ? a lethal situation with re-entry temperatures up to 3,000 degrees.
"Probably the thing that concerns us the most is that the instrumentation and propulsion section did not separate correctly from the spacecraft," NASA space operations chief Bill Gerstenmaier said.
Similar trouble almost incinerated a Soyuz during a January 1969 re-entry with cosmonaut Boris Volynov onboard.
NASA is concerned because the Soyuz will be the only spacecraft capable of flying to and from the station after the 2010 retirement of its shuttle fleet. The spacecraft also double as emergency lifeboats at the outpost.
A ballistic re-entry is one in which the spacecraft relies solely on atmospheric drag to slow the vehicle. The U.S. and the former Soviet Union relied on the mode for early human spaceflight. Yuri Gagarin and others flying Vostok spacecraft made ballistic re-entries. So did John Glenn and the rest of the Mercury astronauts. Gemini, Apollo and Soyuz capsules were designed to use aerodynamic lift for more gradual descents that reduce high gravitational forces on crews.
The ballistic mode serves as a backup for the Soyuz. Cosmonaut Sergei Krikalev says the capability shows the Soyuz design is sound and robust.
Krikalev, who logged 803 days in space and holds the world record for most time tallied in orbit, said it guarantees that Soyuz will land intact.
That's not to say the ride won't be sensational.
The Expedition 16 crew encountered forces eight times normal gravity during a ballistic re-entry on April 19. That's almost triple the 3 G's astronauts experience on shuttles.
The trajectory increased the rate of deceleration and pressed American astronaut Peggy Whitson and her crewmates into custom-molded seats, crushing their lungs, making it hard to breathe.
"I saw 8.2 G's on the meter and it was pretty, pretty dramatic," Whitson said. She called it "an interesting ride."
Soyuz spacecraft comprise three sections.
An orbital module at the front end is equipped with rendezvous and docking systems. A central capsule is known as the descent module; crews strap into its seats for launch and landing.
The back of the spacecraft is an instrumentation and propulsion module. Its contents include steering thrusters and guidance systems.
Investigators think explosive bolts failed to separate the crew capsules and the propulsion modules on the last two missions.
The Soyuz is designed so aerodynamic forces will break the modules apart in any case. But engineers still fear a hatch-first re-entry.
Cosmonauts Sergei Volkov and Oleg Kononenko will venture outside the station July 10 to inspect the bolts on the Soyuz now moored at the outpost.
Gerstenmaier said investigators are making good progress.
"I've made one trip to Russia to go understand how they are progressing with the analysis. They were very open with me. They showed me all the data. They showed me the drawings," he said.
"I saw the physical hardware. I went through a detailed discussion of their engineering processes; they are the same as ours."
Bottom line: "It's not easy flying in space."
Yuri Karash contributed to this report from Moscow.
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