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"  UPDATE: Since we published this story last week, Russia announced its intention to de-orbit the ISS and drop it into the ocean in 2020, at the end of the station&amp;#039;s planned mission.  The final curtain drops today on the space shuttle program, with Atlantis&amp;#039;s long last fall to Earth from orbit. After the wheels stop permanently on the Kennedy Space Center runway, what is next for the International Space Station (ISS), so much of which was built by the shuttle?  ISS Could Fly Higher The operating altitude of the International Space Station was always a balance between the payload performance of the spacecraft that needed to reach it and the operational considerations for the facility. The lower the altitude of the ISS, the more payload that can be delivered to it. Smaller expendable systems, such as the Russian Soyuz or Progress tankers, are less sensitive to this rule than the shuttle was, but even they have their limits. On the other hand, if the station&amp;#039;s altitude is too low, it loses orbital velocity to atmospheric drag more rapidly, requiring more propellant more often to reboost the ISS and keep it from falling into the atmosphere. There are other disadvantages of being too low. It means more time in Earth&amp;#039;s shadow, which reduces the amount of sunlight received by the solar panels and increases the size of storage batteries. Low orbiting altitude also causes more degradation of the structure by corrosive monatomic oxygen. Since assembly first began, almost a dozen years ago, the altitude has varied between 330 and 400 kilometers (about 205 to 250 miles), slowly drifting downward because of drag and then getting reboosted once or twice a year. Over the last few years, it has never gone above about 360 kilometers or so, due primarily to lessened atmospheric drag at lower altitudes from reduced solar activity, which also ensured that the Shuttle could get there with its heavy final construction payloads.  The upshot of all this is that every factor other than launch-vehicle performance would suggest keeping the station at higher altitude, as long as it remains below the radiation of the Van Allen belts. And now that the ISS no longer has to account for the shuttle, expect the station altitude to go (and stay) higher, to reduce corrosion and propellant needed, and to get more time in sunlight. This will also reduce disturbances to the microgravity environment, as it moves from the construction phase into finally doing serious orbital research. In the new era, the target will be 410 kilometers (Soyuz has an upper limit of 425) with a requirement that it not decay to lower than 278 kilometers in six months without reboost. This will still make it reachable by the vehicles, present and future, that need access. What if Something Goes Wrong? During the shuttle era, we had backups for access to space. When the shuttle went down for two and a half years after the loss of Columbia in 2003, the Soyuz was available to maintain access to the station for crew, and other vehicles (such as the Russian Progress tugs) could still provide cargo flights. But with the shuttle retired, there is no such redundancy. If a problem occurs with Soyuz for a significant period of time, the astronauts and cosmonauts may be stranded there. Or worse: the ISS itself may be in danger of uncontrollably entering the atmosphere if it can&amp;#039;t get reboosted in a timely manner, converting a hundred-billion-dollar investment into a flaming inferno and hazard to people on the planet below. Soyuz has a long record of reliability, but, setting aside the fact that the U.S. should have built some sort of shuttle successor to avoid forking over millions of dollars to Moscow for Soyuz seats, it simply would be prudent to get a backup in place just in case. For cargo, we have some. The European ATV has been operational since 2008, and the Japanese H-2 transfer vehicle (Kounotori, Japanese for &amp;quot;white stork&amp;quot;) had its maiden voyage a little less than two years ago. If the SpaceX Dragon docking now planned for late this year is successful, that will make three (with more to come in future years, from Orbital Sciences and others). That would pretty much eliminate the scenario of the ISS falling from space. The much bigger issue is crew. Soyuz serves two functions: It is used to transfer crew between the ISS, but it has served as a lifeboat for a decade. The shuttle was never able to perform this latter function, because it couldn&amp;#039;t stay in orbit for more than a couple of weeks at a time. In fact, the Soyuz crew size of three has restricted ISS permanent crew size to either three, or six when two vessels were docked. Private Space to the Rescue? The new commercial crew systems under development by SpaceX, Boeing, Sierra Nevada and others are capable of carrying seven crew, which means that more people could be permanently based in the station. While most estimate it will be at least three years or more before any of these new spacecraft can carry crew to orbit because of the need to develop a launch-abort system, the Dragon\u2014which flew unmanned and returned successfully last December\u2014could potentially serve as a lifeboat much sooner. Once it demonstrates the ability to dock, all it would need for an Earth return mission would be couches and a minimal life-support system. If NASA really wanted, that could probably be ready next year. Moreover, if there were a Soyuz problem, Dragon might even be usable for performing an emergency crew delivery. NASA doesn&amp;#039;t want to fly astronauts on the Dragon without an abort system, but the agency flew the shuttle for decades without one, and if there were an emergency it might be worth the risk. The end of the shuttle era is by no means (as so many have falsely claimed for the last year and a half) the end of American human spaceflight. It&amp;#039;s at most a brief lull, just as we had for half a dozen years between the last Apollo flight in 1975 and the first shuttle flight in 1981. It&amp;#039;s likely that, once more than one new crew-delivery system is available in the next few years, the amount of passenger traffic to not only the ISS but perhaps other private facilities, such as those planned by Bigelow Aerospace, will dramatically increase. It&amp;#039;s quite likely in fact that historians may call the end of the shuttle the beginning of the golden age of humans in space. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/a6756/the-iss-after-the-shuttle-analysis/>