TRENTON, New Jersey (AP) -- Would-be space travelerGregory Olsen remains optimistic that his flight to the International SpaceStation on a Russian rocket will proceed even though it is temporarily on holddue to an undisclosed medical issue.

The 59-year-old New Jersey physicist plans to doexperiments on his trip and is hoping Russian officials will give him a go-aheadat the end of next week, said Robert Volmer, spokesman for Space Adventures,which brokered the trip.

"We still have every confidence that he will end upflying," Volmer said Thursday. "He's extremely eager to go."

Volmer has advised Olsen, the co-founder of infraredcamera maker Sensors Unlimited Inc. of Princeton, not to give interviews ordisclose the "potential condition" to prevent jeopardizing the eight-day, $20million trip.

"Whether or not that condition exists remains to beseen," Volmer said, adding that dozens of U.S. astronauts and Russian cosmonautshave failed rigorous physical exams over the last 40 years and still flown onmissions.

Russia's Interfax news agency reported June 23 thatan official at Russia's cosmonaut training center in Star City said Olsen hadbeen rejected for health reasons. Space Adventures of Arlington, Virginia,quickly disputed that, saying no final decision had been made. The journey wasscheduled for April 2005, although there was a possibility Olsen could gosooner.

He left Russia last week for a previously plannedvacation at his ranch in Montana.

Olsen plans to return to Russia, probably next week,said Marshall J. Cohen, co-founder and president of Sensors Unlimited. Cohensaid Thursday that Olsen had done well in tests during his physical training, asix-month program that began in April.

"He's passed the tests with flying colors," Cohensaid.

According to Volmer, Olsen's training and testsinclude working inside a Russian space suit, enduring spurts of zero gravity ina plane that climbs and dives at high-altitude, and working inside a model ofthe space station in a "neutral buoyancy tank" that simulates zerogravity.

Olsen also has been studying Russian and training ina mockup of the space station, learning the controls, safety procedures and whathe should or should not do, Volmer said. In addition, Olsen has been training ina centrifuge to ensure he can endure the tremendous gravity force, or G-force,as the Soyuz craft leaves and returns to the Earth's atmosphere.

"He went up to 8 1/2 Gs in the centrifuge," more thannormal re-entry force, Volmer said.

Once in space, Olsen plans to use his company'shigh-tech infrared cameras to study distant stars, chemical reactions such asozone formation in the atmosphere, and the health of agriculturalcrops.

Space Adventures has previously arranged with theRussian space program for trips on Russia's Soyuz spacecraft to theInternational Space Station for two other space tourists, American Dennis Titoand South African Mark Shuttleworth. Space Adventures also charged them $20million. The company has not elaborated on its financial arrangement with theRussian government.

Volmer said Thursday the issue of whether Olsen isentitled to a refund has not yet been broached, which he called a good sign thatthe flight will proceed. Cohen said he believes Olsen has a pay-as-you-goarrangement for the bill.