Image: 1999 eclipse
CNES
French astronaut Jean-Pierre Haignere snapped this picture of the moon’s shadow passing over Europe from Russia’s Mir space station during the eclipse of Aug. 11, 1999.
By NBC News space analyst
Special to NBC News
updated 3/13/2006 1:03:26 AM ET 2006-03-13T06:03:26

When the shadow from a total solar eclipse sweeps over Earth on March 29, two skywatchers should have a guaranteed cloud-free view of the spectacle: NASA astronaut Bill McArthur and Russian cosmonaut Valery Tokarev, the current residents of the international space station.

The date with an eclipse will serve as a fitting celestial sendoff for the station's Expedition 12 crew members, just days before a new international crew arrives to take their place.

Between now and then, McArthur and Tokarev will be cleaning up and packing up for the transition. Also on the agenda for the weeks ahead are a couple of tests designed to assure space station operations for years to come.

The crew's date with an eclipse is something that NASA says is an unexpected "bonus." Only a handful of humans have witnessed such a phenomenon, and it's not clear whether the crew will be able to do anything beyond photographing the moon’s shadow on Earth beneath them.

At one point, it looked as if the space station would be flying right through the shadow.

The station's Zvezda service module had been scheduled to fire up its rocket engines on Wednesday to adjust the orbit in anticipation of the replacement crew's launch on March 30. According to German space engineer Gerhard Holtkamp, that maneuver would have put the station right in the path of the eclipse over the coast of southern Turkey.

"With the eclipse shadow moving at triple the speed of sound, and the ISS faster still, the whole thing is a little like shooting two bullets out of different directions and trying to make them hit each other," he told MSNBC.com in an e-mail.

Sources within NASA’s Mission Control told MSNBC.com that the maneuver had nothing to do with the eclipse. "It's to set up phasing and lighting for the next Soyuz exchange," one expert explained in an e-mail. "The eclipse is a bonus."

Maneuver deferred
Over the weekend, NASA said the maneuver wouldn't have to be done before the eclipse after all. Rob Navias, a spokesman at NASA's Johnson Space Center in Houston, said refined tracking of the station’s path had determined that the rocket firing "was not required at this time."

The Zvezda engine firing is now planned for April 19, Navias said. That would represent the first time Zvezda's engines have been used since it was put into orbit six years ago, and the first time they've ever been used for orbital adjustment. Past adjustments have been made by docked vehicles such as NASA's space shuttles or Russia's Progress cargo craft.

Zvezda's engines, which are protected when not in use by a hinged cover that swings across the engine’s mouth to shield against impacting objects, are the most powerful engines on the station. The Russians want to fire them every few years just to verify they are still functional. Nobody expects any problems during this test firing, and even if they don’t work, they’ll just fail to ignite — no explosion is expected.

It's not yet clear whether NASA's refined calculation of the station's path results in the "bullet-crossing" effect that Holtkamp initially noticed. But the path still appears headed very close to the March 29 eclipse.

Eclipse thrills and chills
Human spaceflights have rarely passed near the fast-moving shadows of solar eclipses. One Gemini mission in 1966 altered course to intercept totality over the eastern Pacific, and a 1999 Mir mission was able to observe from a distance as totality passed across Europe.

One encounter with an eclipse even sparked a few scary moments for the Russian space program.

On Nov. 3, 1994, as a Soyuz spacecraft carrying two Russians and a German astronaut was closing in for a docking to the Mir space station, the crew and Russian ground controllers were alarmed to see the spacecraft’s power falling sharply. They were off the coast of Brazil in full sunlight — yet their electrical system, powered by the Soyuz's solar panels, appeared to be failing completely. Then one of the cosmonauts noticed it was growing dark outside the porthole and realized what was happening.

Inexplicable oversight
By some inexplicable oversight, Moscow space controllers had forgotten to calculate the effect of a solar eclipse that was happening that very day —  perhaps because they never before had experienced any interference, or possibly because the undock-and-redock maneuver, at the very end of the crew's mission, was an urgently added experiment to test a malfunctioning guidance beacon.

In any event, the darkness soon passed, heart rates in space and on Earth returned to normal, and the crew completed the test docking. The following day they undocked for good and returned to Earth.

Russian space experts rarely make the same mistake twice. So even if the station's path will take it right through the eclipse shadow, they've probably already accounted for the momentary blackout of the station's solar panels.

For the crew, the eclipse will be a last, memorable space sight, perhaps a fitting spectacle to cap their flight careers (neither expects to go into space again). Out the window, as they soar across the bright sands of the Sahara, a dark smudge on the horizon will grow until it may envelop their entire station for several heartbeats, and then pass. They'll probably take a few photos — and then it will be back to preparing for the voyage home.

Going on a ‘camp-out’
In addition to the test firing of Zvezda's rocket engines and the eclipse watch, yet another break from the routine is in store for the space station. This experimental procedure is what mission planners call a "camp-out," which involves sleeping inside the U.S.-built Quest module's air lock while an automatic timer slowly reduces the air pressure. The experiment is to validate new spacewalk hardware and procedures, and if it works it will significantly shorten the preparation time for future spacewalks.

The exercise serves as practice for a technique that would reduce the dissolved nitrogen in spacewalkers' bloodstreams overnight, so that a spacewalk in spacesuits that are at an even lower air pressure can be safely done on the following day with less waking time wasted. This camp-out is just a test, and the astronauts involved won't be expected to perform a spacewalk.

While they sleep sealed in the airlock module, a computer-controlled valve will gradually drop the air pressure from the normal 14.7 psi ("sea level" air pressure) to the lower value of 10.2 psi (about the equivalent of a 10,000-foot altitude on Earth). If this were an actual spacewalk, the astronauts would then put on suits pressurized to a mere 5 psi.

For this experimental camp-out, the presence of two crewmen is required to put the control system to a proper test. If the procedure works as planned, it could shave hours off the schedule for extravehicular activities.

Test to be conducted soon
Controllers want the test completed soon, before the launch of the next supply ship, so that additional equipment can be sent quickly to correct any problems pointed up by this practice camp-out.

“We want to run through this to find any hiccups with the software or our procedures before we have to use it,” one space station official told MSNBC.com on condition of anonymity. “Time is more precious on docking missions, both shuttle and Soyuz, [so] we are doing this now so that we have sufficient time to fix any problems well in advance.”

McArthur and Tokarev had been scheduled to perform the camp-out around March 23, but Navias said the test would be "deferred a little while longer."

An internal NASA status report obtained by MSNBC.com now suggests that the test will occur after the new Soyuz has docked, so that two American astronauts — McArthur and his replacement, Jeff Williams — can do the camp-out together, leaving the station under the command of Tokarev's replacement, Russian cosmonaut Pavel Vinogradov.

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