STOCKHOLM, Sweden (AP) -- U.S. space agency NASA plans to launch several massive helium balloons from a northern Sweden launch pad this summer, officials said Wednesday.

The balloon campaign will begin either in May or June when a large -- and heavy -- telescope will be launched, along with a test flight of a newly developed long-last balloon vehicle.

NASA plans to launch balloons for westward flights, from Esrange to Alaska.

The giant helium-filled balloons have a volume of up to 1.3 million cubic yards.

The campaign is part of the space agency's way of using the balloons to lift heavy payloads _ astronomical telescopes and other heavy equipment for astrophysical experiments and research into cosmic radiation _ to an altitude of about 25 miles.

From Esrange, NASA's balloons will be able to remain aloft for longer periods than from other launch sites, because of prevailing winds in the northern region and because the balloons' trajectory on their way to Alaska would not cross Russia, which has not granted permission for the balloons to pass through its air space.

The flights are being conducted by NASA and the Palestine, Texas-based National Balloon Facility in cooperation with the Swedish Space Corp. at Esrange.

Because of the balloons' size, Swedish Space is in the process of doubling the size of its launching pad to 2,640 by 1,650 feet.

"This campaign is a milestone for SSC. Large investments have been made at Esrange, regarding increased infrastructure and facilities. We are well prepared for the mission and the plan is to carry out similar balloon campaigns together with NASA and also European scientists every summer," Esrange director Olle Norberg said.

Since flights started in 1974, about 450 scientific balloons have been launched from Esrange, which is situated near Kiruna, 765 miles north of the capital, Stockholm.

The first launch will carry the BLAST telescope aloft. The 5,940 pound telescope will reach an altitude of 25 miles and the flight will take between six to nine days.

"It's the first time such a flight will be made, regarding payload weight in combination with altitude, duration and flight trajectory," Norberg said. "BLAST will address some of the most important cosmological and galactic questions regarding the formation and evolution of stars, galaxies and clusters."

The NASA Balloon Program's capabilities are being expanded with the development of the ultra-long duration balloon vehicle, or ULDB. The balloon is made of advanced materials and uses a pumpkin-shaped design to achieve flights with duration of up to 100 days.