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Wednesday, August 04, 2004

Third Spacewalk a Breeze for Space Station Crew

The crew of the International Space Station (ISS) flawlessly completed a spacewalk Tuesday morning, preparing their orbital home for a new cargo ship and retrieving experiments attached to its hull.

ISS Expedition 9 commander Gennady Padalka and NASA ISS Science Officer Michael Fincke are safely back inside the space station after a 4.5-hour spacewalk that found them consistently ahead of schedule and so eager to work that ground controllers repeatedly urged them to take breaks and enjoys the scenery.

“It’s a great view,” Fincke said during a rest early in the activity as he watched the sun rise during one of the spacecraft's 90-minute orbit of the planet.

The spacewalk was the third extravehicular activity (EVA) for the Expedition 9 crew and began when Padalka and Fincke opened the outer hatch of the station’s Pirs Docking Compartment at 2:58 a.m. EDT (0658 GMT) Tuesday morning. By the spacewalk’s end at 7:28 a.m. EDT (1128 GMT), the astronauts had completed all of their EVA tasks.

Preparing for ‘Jules Verne’

Expedition 9’s primary task during the spacewalk was to prepare a docking port on the aft end of the station’s Zvezda Service Module to receive an unpiloted European supply vehicle expected to make its first delivery in the fall of 2005.

The cargo-carrying Autonomous Transfer Vehicle (ATV), named Jules Verne by its builders, is an unmanned spacecraft developed by the European Space Agency (ESA). It carries three times the cargo capacity of Russia’s unmanned Progress ships, and may also be used to move the ISS if needed.

It took the Expedition 9 crew about 30 minutes from exiting the space station to reach their Zvezda worksite, but once the astronauts arrived, they were all business.

Padalka and Fincke removed six obsolete laser reflectors used for docking, replacing four of them with more modern versions. The crew also installed two antennas - to allow the Jules Verne ATV to communicate with the space station - and removed a cable from a faulty television camera that will be retrieved in a later spacewalk.

The astronauts also replaced a series of space exposure experiments, including a folding replaceable cassette container filled with materials and a Kromka experiment that measured contamination caused by the station’s thrusters.

“I did not work much,” said an energetic Padalka during the spacewalk. “I’m not tired.”

Drifting in space

Despite their busy schedule, Padalka and Fincke snagged some 40 minutes of down time while outside the space station when mission controllers reoriented the station using its Russian thrusters.

The reorientation was required primarily to counteract the effect of the Expedition 9 crew, NASA spacewalk commentators said.

When the Expedition 9 crew first ventured out to the aft of Zvezda, U.S. station handlers put the ISS under U.S. momentum gyroscope control to prevent Russian thrusters on the module from firing while crewmembers were near.

But the heavy work activity on Zvezda’s aft by Padalka and Fincke increased the momentum load on the gyroscopes and the station began to drift slightly off course. The drift led ground controllers to institute power conservation procedures that temporarily cut off primary S-band communications with the Expedition 9 crew, though a backup system reestablished contact.

NASA officials said Russian and U.S. station controllers had anticipated the drift, but concluded it posed no danger to the crew or station. It was easily cancelled out at about 5:15 a.m. EDT (0915 GMT), when Russian thrusters were fired while Padalka and Fincke were at a safe distance from Zvezda’s aft.

By 6:00 a.m. EDT (1000 GMT),the spacewalkers resumed their work.

One more to go

Padalka and Fincke still have one last spacewalk to perform before they turn the ISS over to new tenants in October. Some of those activities will also be ATV-related, NASA officials said. Tuesday’s spacewalk was the 55th EVA made to support space station and the 12th staged from the Pirs compartment. The spacewalk marks Padalka’s fifth career spacewalk and the third for Fincke. The three Expedition 9 spacewalks to date total of more than 10 hours of EVA work for the astronauts.

Earth is Rare, New Study Suggests

Flip a coin. Heads, Earth is a common sort of planet. Tails, and ours is as unusual as a coin landing on edge. That's about the state of knowledge for scientists who ponder the question of our planet's rarity.

But lack of knowledge is a cornerstone of speculation, and several scientific papers and books have conjectured everything from our solar system being unique to it being run-of-the-mill.

The latest toss of the coin is tails, suggesting we may be very, very alone.

Jupiter, the outlier

Theorists involved in the new study acknowledge the distinct possibility that there could be many solar systems similar to ours, as others have suggested, and that we haven't seen them because technology has yet to allow their detection. But they raise an interesting counter-argument:

All known extrasolar planets are roughly as massive as Jupiter or much more so. In studying about 100 of these, the scientists found most are similar to one another in terms of orbital characteristics, and that Jupiter is the oddball. The results are 98 percent on track by a scientific measure known as the significance level.

It follows that perhaps the known extrasolar planets formed in a different manner, one by which rocky, Earth-like planets would not be created.

"We have shown that the solar system, as represented by Jupiter, is formally not part of the distribution of observed extrasolar planetary systems," Martin Beer of the UK's University of Leicester told SPACE.com. "From this result we suggest that we may be looking at two different methods of planet formation."

Those two methods have been laid out previously by others.

How to build a planet

The leading model of planet formation -- conjured before anyone knew there were planets around other stars -- is called core accretion. A rocky core develops first, then an object either becomes a terrestrial planet (like Mercury, Venus, Earth or Mars) or it attracts huge amounts of gas and grows into something like Jupiter or Saturn.

An alternative method forms a gas giant planet via the gravitational collapse of a knot of material.

Scientists disagree which way the outer planets of our solar system were born. The core accretion model has shortcomings. For one thing, when run on a computer, Neptune and Uranus typically don't show up. Further, observations reveal that Saturn has a solid core but Jupiter does not.

The gravitational collapse model has been invoked to explain some these discrepancies. It is also appealing as a method for making the gargantuan gaseous planets found around other stars. Most are a few to several times the mass of Jupiter and orbit incredibly near to their host stars on wildly non-circular orbits.

In all but a handful of these setups, rocky inner planets are not possible because they'd be consumed by a giant or gravitationally booted out of the "habitable zone," a comfortable region that can support life, Beer explained. Earth and Mars both orbit stabily in just such a temperate area, thanks to the fact that the outer planets are far away and on nearly circular orbits.

If gravitational collapse formed the known extrasolar planets, then there's no need for rocky cores.

"Without these rocky cores, terrestrial planets (that is, Earth-like) do not form," Beer points out.

Answers soon

"More observational work is required if the solar system is to be shown to be a 'normal' planetary system," Beer said.

The truth will soon be known.

Finding giant planets takes time. Scientists monitor the gravitational wobble a planet induced on its star. To make a detection, they must monitor a star for roughly the same number of years as it takes for a planet to make an orbit. So most of the discoveries so far have involved planets that orbit in days or months. But planet hunters are beginning to find Jupiter-sized planets that take years to go around their stars (Jupiter's orbit is nearly 12 years).

(This "selection" problem can be likened to a commercial fishing boat with 5-inch holes in its net. After catching a few large ones, the fishermen determine there are no fish smaller than 5 inches in the sea.)

A firmer answer to our uniqueness will likely come around the end of this decade from NASA's space-based Kepler mission, due to launch in 2007. It is designed to detect Earth-sized planets by searching 100,000 stars for four years, looking for tiny dips in stellar brightness that would indicate an orbiting object.

Beer's team included Leicester's Andrew King, Mario Livio of the Space Telescope Science Institute in Baltimore and Jim Pringle of the University of Cambridge. The study will be published in the Monthly Notices of the Royal Astronomical Society.

Supernova

a stellar explosion in which a star may be completely disrupted, leaving a compact stellar remnant such as a neutron star or black hole. At maximum light, the supernova can have luminosity about 108 or 109 times that of the Sun. The luminosity decays after the initial outburst, in certain classes of supernova, the decline being exponential with a half-life of about 80 days. In massive stars, the supernova occurs when the star has used up all its available nuclear fuel and it reaches a lower energy state through gravitational collapse to form a more compact star. In white dwarf stars in binary systems, accretion of mass onto the surface of a neutron star can be sufficient to take the star over the upper mass limit for stability as a white dwarf and it collapses to form a neutron star resulting in a supernova explosion