Soyuz TMA-11 lands short
Published on Apr 19, 2008 at 10:04 am.
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Filed under space exploration, space station.
Earlier today, the Soyuz TMA-11 spacecraft returned to Earth, having spent about six months at the International Space Station. Now, that doesn’t mean that the entire crew spent that long there. Peggy Whitson and Yuri Malenchenco rode the craft up and back. However, Malaysian astronaut Sheikh Muszaphar Shukor flew to the ISS aboard TMA-11, but returned with TMA-10, the Soyuz that had been docked at the space station. Returning aboard TMA-11 was South Korea’s Yi So-yeon, who flew to the space station aboard TMA-12, which is now docked with at the ISS. TMA-12 will return in about another six months, when it is replaced by another Soyuz craft. The TMA configuration (transport modified anthropometric) is the latest generation of the venerable Soyuz. The first Soyuz flew in 1967, and this has been one of the most successful spacecraft designs around. It isn’t the most sophisticated, by any means, nor the most capable, but it is robust. But, it has its problems.
The earliest manned spacecraft were designed to reenter the Earth’s atmosphere in such a way that they did very little on the way in. The atmosphere itself did almost all of the work in slowing the craft. This is called a ballistic reentry. Such a reentry results in a rough ride for the astronauts and cosmonauts, and considerable force on their bodies. Later designs, such as Apollo and Soyuz still use the force of the atmosphere to slow the craft upon reentry, but they are also designed to be a bit more aerodynamic. As they plunge through the atmosphere, they generate some lift. This prolongs the passage through the reentry interface some, meaning that engineers need to be careful to design such craft to safely dissipate the heat of reentry, but the forces are much easier on the craft and crew. The Space Shuttle would be an example of this idea carried even farther. But, unlike the Space Shuttle, the Apollo and Soyuz craft were designed to be able to still do a ballistic reentry if necessary. It is a backup reentry mode if something goes wrong with spacecraft guidance, but it is not the preferred way to reenter.
Nonetheless, the first manned TMA Soyuz mission had a control failure and went into its backup ballistic reentry mode when it returned to Earth in 2003. This being a steeper reentry angle, the craft landed hundreds of kilometers short of its intended touchdown position. The problem was supposedly found and fixed. However, the last Soyuz prior to this one (TMA-10) also experienced a problem and the craft defaulted to its backup ballistic reentry mode, also landing hundreds of kilometers short. Now, Soyuz TMA-11 has apparently done the same thing. If I were an astronaut or cosmonaut heading to the ISS on a future mission (or one of the ones up there now!), I’d be worried. Granted, all three failures resulted in nothing more than a rougher than usual ride for the crew. Still, this is not the planned reentry, and the reentry phase of a mission is dangerous. You really don’t want to be going to the backup reentry profile if you can avoid it. This is the second time in a row that the TMA configuration has gone into its backup ballistic reentry mode. One would hope that they are trying to figure out why and how to avoid it again. Three missions out of eleven is 27%. That is a worrying statistic. Of course, it is important to remember that the Soyuz is designed with this mode of reentry as a possibility, so in none of those cases were the crew actually in grave peril, but I am sure it was quite frightening for them. It is my understanding is that another generation of Soyuz craft is to soon replace the TMA configuration. Maybe that one won’t have these problems.
-Astroprof
(The image at the top of the post is a Wikimedia file picture of TMA-3’s successful landing.)
