A presidential panel assessing U.S. manned space flight presented five options to the White House today, ranging from NASA's current plan to build outposts on the moon to a "flexible path" approach to explore a variety of targets in the inner solar system.

But in its executive summary, the panel warned that without significant additional funding and a long-term commitment to exploration, none of the options is viable.

"The Committee has found two executable options that comply with the FY 2010 budget," the panel wrote in its executive summary. "However, neither allows for a viable exploration program. In fact, the Committee finds that no plan compatible with the FY 2010 budget profile permits human exploration to continue in any meaningful way."

The Review of U.S. Human Space Flight Plans Committee, chaired by former Lockheed Martin CEO Norman Augustine, was set up by the Obama administration to examine NASA's current plans for retiring the shuttle, completing the space station and returning to the moon, as well as alternative strategies for moving beyond low-Earth orbit.

The committee also considered how long NASA and its partners should operate the International Space Station. NASA currently has no money in its projected downstream budget to operate the space station beyond 2015.

In its executive summary - the group's final report is not yet complete - the panel did not make any recommendations. Instead, it listed five options, or architectures, and the pros and cons associated with each. The first two options assume NASA is forced to live within current 2010 budget projections.

In Option No. 1, essentially the Bush administration's Constellation program, the International Space Station would be deorbited in 2015 and a new Ares I rocket would be developed to launch Orion crew capsules to low-Earth orbit.

NASA then would develop a huge heavy lift Ares V rocket to boost Orion capsules and Altair lunar landers to the moon for long-term exploration starting in the early 2020s. The Augustine panel notes that this option was effectively derailed by previous budget cuts that withdrew expected funding.

If NASA is forced to live within the current budget projection, "Ares I and Orion are not available until after the ISS has been de-orbited," the panel concluded. "The heavy-lift vehicle, Ares V, is not available until the late 2020s, and worse, there are insufficient funds to develop the lunar lander and lunar surface systems until well into the 2030s, if ever."

A second "constrained budget" option calls for NASA to operate the space station until 2020. Under this scenario, the space agency would forego development of the Ares I rocket and instead rely on private industry to develop manned spacecraft to reach low-Earth orbit. NASA would develop a manned heavy lift rocket called Ares (Lite).

"The option assumes shuttle (retirement) in FY 2011, and it includes a technology development program, a program to develop commercial crew services to low-Earth orbit, and funds for enhanced utilization of ISS," the panel wrote. "This option does not deliver heavy-lift capability until the late 2020s and does not have funds to develop the systems needed to land on or explore the Moon."

The remaining three options, and several variants, all assume the White House and Congress approve additional funding, increasing NASA's budget to $3 billion a year above the current projections by fiscal 2014 and then growing with inflation at 2.4 percent per year.

Under that scenario, assuming the shuttle is retired in 2011 and the station deorbited in 2015, NASA could, in fact, press ahead with the current Constellation program. But the Ares I/Orion vehicle would not be ready before 2017 and manned flights to the moon would slip into the mid 2020s.

Option No. 4, known as "Moon First," would extend the station through 2020 and retain the moon as the nation's primary target beyond low-Earth orbit. But it would rely on development of commercial access to low-Earth orbit. Two variants were considered.

In one, Option 4A, the shuttle is retired and NASA develops the Ares V (Lite) rocket to boost manned spacecraft to the moon. In Option 4B, the shuttle program is extended through 2015 at a "minimum safe flight rate" and a new heavy lifter based on shuttle technology is developed to reach the moon.

In both of the "Moon First" variants, astronauts could return to the moon by the mid 2020s.

Option No. 5, the one the Augustine panel seems to favor, represents a so-called "flexible path" architecture that would explore the inner solar system with long-duration flights to a variety of targets, ranging from lunar orbit to the moons of Mars. The long-range goals could include lunar landings and eventual flights to Mars itself.

There are three variants to option No. 5, differing only in which heavy lift rocket is used: Ares V (Lite), an evolved expendable launch vehicle, or EELV, or a shuttle-derived rocket.

"All variants of Option 5 begin exploration along the flexible path in the early 2020s, with lunar fly-bys, visits to Lagrange points and near-Earth objects and Mars fly-bys occurring at a rate of about one major event per year, and possible rendezvous with Mars' moons or human lunar return by the mid to late 2020s," the panel wrote.

The panel concluded:

"Human exploration beyond low-Earth orbit is not viable under the FY 2010 budget guideline.

"Meaningful human exploration is possible under a less constrained budget, ramping to approximately $3 billion per year above the FY 2010 guidance in total resources.

"Funding at the increased level would allow either an exploration program to explore Moon First or one that follows a Flexible Path of exploration. Either could produce results in a reasonable timeframe."

But it will not be easy, even with additional funding. The project gap between the end of shuttle operations and the debut of a new spacecraft to replace it will stretch at least seven years, the panel said. A heavy-lift rocket is a requirement for any exploration scenario and in all but a few cases, so is development of commercial access to low-Earth orbit.

"Commercial services to deliver crew to low-Earth orbit are within reach," the panel wrote. "While this presents some risk, it could provide an earlier capability at lower initial and lifecycle costs than government could achieve. A new competition with adequate incentives should be open to all U.S. aerospace companies. This would allow NASA to focus on more challenging roles, including human exploration beyond low-Earth orbit, based on the continued development of the current or modified Orion spacecraft."

The panel stressed that Mars remains the "ultimate destination" for human exploration of the inner solar system. "But it is not the best first destination."

"Both visiting the Moon First and following the Flexible Path are viable exploration strategies," the panel wrote. "The two are not necessarily mutually exclusive; before traveling to Mars, we might be well served to both extend our presence in free space and gain experience working on the lunar surface."

The panel's full report is expected to be presented to the White House around the end of the month. When the Obama administration might act is not yet known.