A recent report on the French "Go-Ariane" website -- quoted by "NASA Watch" on April 10 -- claimed that engineers Rosetta's prime contractor, the European multinational Astrium Ltd., had discovered to their dismay that Rosetta cannot even be drained of its potentially explosive hydrazine fuel so that the spacecraft can be safely stored until its launch next January or February.

Hydrazine is highly reactive, and notorious for reacting with the metal walls of fuel tanks. To quote the article: "Apparently chemical reactions have altered the fuel tank linings. So long as the hydrazine stays inside, everything is okay and Rosetta can wait.

"But if it drained, the reservoirs' internal surfaces would be damaged, so that Rosetta cannot be refueled. The only solution would be to replace the tanks, but these are placed at the heart of the spacecraft", so that Rosetta would have to be largely disassembled and then reassembled just to defuel it.

The article ends, "In the words of an Astrium engineer, Rosetta is still 'essentially a 1700-kg bomb.'" (The total weight of propellant inside Rosetta -- including its nitrogen tetroxide oxidizer -- comes to 1700 kg.)

However, Rosetta chief scientist Gerhard Schwehm and BBC science reporter David Whitehouse both tell "SpaceDaily" that this story needs to be seriously modified. Whitehouse says: "There was a concern with defueling Rosetta in January, but that has been resolved, and now Astrium says it can be defueled with no damage to the linings of the fuel tanks. Rosetta is set to be defueled in May.

"Apparently the fuel can cause so-called 'stress corrosion' to the titanium fuel tank, so an inhibitor [chemical] is added to the fuel to prevent this. The problem is that, when the fuel is removed, the inhibitor comes out first and leaves the uninhibited fuel to cause its corrosion in the tank. They say, however, that they have now solved this problem."

Schwehm confirms this, but adds that Astrium has concluded that it really is impossible to remove the spacecraft's equally large supply of nitrogen tetroxide oxidizer without replacing the tanks -- and this, in turn, prohibits the team from being able to move the spacecraft to any other launch site than the Ariane launch site at Kourou in French Guiana.

He adds that this factor, in turn, has finally and firmly decided the ESA's choice of a cometary target for Rosetta. To launch it to Wirtanen at the backup opportunity next January, a more powerful booster than the regular Ariane 5 would be needed.

And since there is almost no chance that the new, second-generation Ariane 5 ECA (which exploded disastrously on its first test flight last December) will be ready by then, such a launch would have to be done on a Russian Proton booster -- which would require shipping Rosetta halfway around the world to the launch site at Baikonur.

And so, the new mission for Rosetta will be a launch next February -- on a regular Ariane 5 -- from the Kourou site to the comet Churyumov-Gerasimenko. Since this comet's nucleus is distinctly larger than Wirtanen's, some redesign of the procedure by which Rosetta drops its little secondary lander spacecraft onto the comet's surface will be necessary.

The decision will not be officially made by the ESA until May, but Schwehm describes it as a virtual certainty -- especially since the regular Ariane 5 has now been cleared to resume flights, and made a perfect launch with two onboard comsats on April 9. Meanwhile, the ill-starred new Ariane 5 ECA will not make its second test flight until spring 2004 -- probably with a dummy spacecraft.

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