The Planetary Society, a non-profit organization involved in space advocacy, is deeply concerned about the future of U.S. space exploration and is urging Congress to reconsider NASA's current plans.

In an Aug. 18 letter, the society asks Congress to revisit the current human space exploration plan. The letter was sent to the leaders of four subcommittees currently discussing policies that affect NASA's future and also highlights what the society contends are oversights in the current space exploration bills under discussion.

"We are concerned about omissions and a lack of coherence in the four committees' versions of this bill," members of the Planetary Society stated in the letter.

The Planetary Society list of problems in the bills includes apparent vagueness surrounding what will replace NASA's retiring space shuttles, and future goals for human spaceflight.

NASA's current plan is to complete construction on of the International Space Station next year after two final space shuttle missions, though a potential third shuttle flight is being discussed in Congress. [NASA's New Direction: FAQ]

Once the shuttles are retired, NASA will rely on Russian, Japanese and European spacecraft to ferry astronauts and cargo to the station until American commercial spacecraft are available. President Barack Obama canceled NASA's previous moon-oriented Constellation exploration program. In its place, he has called on NASA to mount a manned mission to an asteroid by 2025 as part of the new plan.

Planetary Society officials said the bills drawn up in Congress have so far gone against the president's directives.

"The bills reject the President's new plan, as well as the old Constellation plan, and instead come up with a patchwork of proposals," said Louis Friedman, executive director of the Planetary Society.

The letter is signed by Friedman, as well as incoming executive director, Bill Nye (TV's Science Guy), society president Jim Bell, and vice-president Heidi Hammel.

The letter asks the senators and House representatives to step back from each of the congressional bills that have been passed by the Authorization and Appropriations committees, and instead work on refocusing the whole initiative.

The Planetary Society was founded in 1980 by the late famed astronomer Carl Sagan, Friedman and planetary scientist Bruce Murray.