PASADENA, Calif. Budget constraints may force NASA to shuffle the launches of three upcoming missions. The two that could be postponed are a spacecraft set to zip through the suns upper atmosphere and another to explore Jupiters moon Europa; a probe to faraway Pluto in 2004 would fly as scheduled.

The separate missions Europa Orbiter, Pluto-Kuiper Express and Solar Probe have three distinct destinations, but are being implemented as a single project under the aegis of NASAs Jet Propulsion Laboratory (JPL). Each individual mission costs about $250 million, but shares with the others some avionics, software, telecommunications and propulsion features to save money.

But the higher than expected cost of the advanced rockets either the Delta 4 or Atlas 5 that will launch the three spacecraft has led NASA to consider spreading out the missions, or perhaps even canceling one of the trio, said Jay Bergstralh, program scientist for the Europa Orbiter.

"The game here is to make this all fit within the budget profile we have forecast for the next five years or so," Bergstralh said.

Originally, the American space agency had hoped to launch the Europa Orbiter to plumb the icy jovian moon for signs of a liquid ocean in November 2003. Launch of the Pluto-Kuiper Express, the first spacecraft to ever study the minuscule planet from up close, was to have followed in December 2004. In February 2007, NASA hoped to send the Solar Probe to fly through the suns corona.

NASA Headquarters is now examining three options for schedule changes to the Outer Planets/Solar Probe Project:

• Launch the Pluto-Kuiper Express in December 2004 and the Solar Probe in February 2007, both as originally planned, but delay the Europa Orbiter by 26 months, to January 2006.
• Launch the Pluto mission as planned, but delay the Europa spacecraft to 2007 and the Solar Probe to 2008.
• Cancel one of the three missions outright.

Project members from JPL briefed NASA officials in Washington last week. A decision on any changes to the program is expected sometime this spring or summer.

"The sooner, the better," Bergstralh said.

As of now, the only definitive change will be a delay for the Europa Orbiter. The spacecraft will determine whether a salty ocean sloshes beneath the shield of ice that caps Europa, one of Jupiters 16 moons. The mission will also identify candidates for future landing sites for lander missions that could explore that ocean, thought to be a potential abode for extraterrestrial life.

The Pluto-Kuiper Express will characterize the geology and morphology of Pluto and its lone moon, Charon. The ninth planet is the only planet in our solar system never to have been visited by a spacecraft, although four have crossed its orbital path on their way out of the solar system.

The Solar Probe will dart within 1.25 million miles (2 million kilometers) of the sun, to investigate the source of the solar wind and what accelerates it, as well as examine our stars poles.