KENNEDY SPACE CENTER, Fla.--Even using shuttle-derived hardware, established contractors, and long-standing engineering expertise, NASA's projected budget will not cover the costs of developing a congressionally mandated heavy-lift booster and a manned capsule for deep space exploration by 2016 as ordered, agency officials informed lawmakers this week.

NASA managers promised to continue studying alternative approaches and designs for a new Space Launch System heavy-lift booster and Multi-Purpose Crew Vehicle, but insisted any such program must be "affordable, sustainable and realistic."

"To date, trade studies performed by the Agency have yet to identify heavy-lift and capsule architectures that would both meet all SLS requirements and these goals," NASA said in its report to Congress. "For example, a 2016 first flight of the SLS does not appear to be possible within projected FY 2011 and out-year funding levels."

An early concept for a possible heavy-lift rocket intended for deep space exploration compared to a space shuttle.

As directed in its 2011 appropriations language, NASA focused on a rocket that would utilize extended shuttle boosters, main engines, and an advanced Saturn 5 upper-stage engine. The Orion capsule initially designed for the Bush administration's now-canceled Constellation moon program, was selected as the basis for a new Multi-Purpose Crew Vehicle.

"However, to be clear, neither reference vehicle design currently fits the projected budget profiles nor the schedule goals outlined in the Authorization Act," NASA's report concluded. "Additionally, it remains to be determined what level of appropriations NASA will receive in FY 2011 or beyond -- a factor that will impact schedule as well."

Sen. Bill Nelson, a Florida Democrat who flew aboard the shuttle in 1986 and who played a major role in adding the near-term requirement to build the new launch systems, said in a statement late Wednesday that NASA's answer was not good enough.

"I talked to (NASA Administrator) Charlie Bolden yesterday and told him he has to follow the law, which requires a new rocket by 2016," Nelson said late Wednesday. "And, NASA has to do it within the budget the law requires."

In a letter to Bolden that was released late Thursday, Nelson and Kay Bailey Hutchison, a Texas Republican, said "the report contains no specific justification or analysis to validate the claim that 'none of the design options studied thus far appeared to be affordable in our present fiscal conditions.' We expect NASA to work with Congress to identify the basis for the claims made in the report, how existing contracts and technologies will be utilized, and where any additional congressional action may be needed to ensure successful implementation of the law."

Nelson also plans to introduce legislation eliminating a requirement for NASA to continue spending money on Constellation. Due to a provision in the continuing resolution currently funding the space agency, NASA must follow a House directive in its 2010 budget that blocks the program's termination.

The continuing resolution expires March 4. But NASA's inspector general said today that unless Congress acts, NASA could end up spending $215 million on the program by the end of February.

"Without congressional intervention, by the end of February 2011 NASA anticipates spending up to $215 million on Constellation projects that, absent the restrictive appropriations language, it would have considered canceling or significantly scaling back," the inspector general's report said. "Moreover, by the end of FY 2011 that figure could grow to more than $575 million if NASA is required to continue operating under the current constraints and is unable to move beyond the planning stages for its new Space Exploration program."

John Logsdon, a space policy analyst who serves on the NASA's Advisory Council, said the near-term issue facing the agency's plans for deep space exploration is more a matter of schedule than budget and that NASA already had indicated its belief that a new heavy lifter could not be deployed by 2016.

"This should not come as a surprise to Mr. Nelson and his compatriots," he told CBS News today. "Charlie Bolden told him the same thing last year when they first passed the authorization bill. So there is a small, or maybe not so small, element of posturing here. It seems to me that more than the budget...NASA is saying that there's no way they can do a development this large and have the thing flying by the end of 2016.

"This doesn't mean there's not going to be an HLV (heavy-lift vehicle)," he said. "There will be an HLV, and there will be work at the Cape to do it, among other places. Going back to the authorization bill and now this report, they are steps in a dialogue between NASA and the White House and the Congress on what makes sense...If the country is serious about having a good space program, Congress has to do its part."

The Obama administration's fiscal 2011 budget charts a controversial new course for NASA. The agency has been told to rely on private industry for future manned and unmanned rockets and capsules to service the International Space Station in low-Earth orbit.

The administration ruled out an immediate return to the moon, concluding the Bush administration's Constellation program was not affordable, and instead ordered a "flexible path" approach to a variety of deep space targets. But development of heavy-lift rockets to facilitate deep space exploration was deferred and no timetables were specified.

Space advocates immediately protested this approach and the president eventually agreed to begin development of a new heavy lifter in the 2015 time frame. Nelson and others then campaigned to begin development immediately and to have a system ready for first flight in 2016. Along with providing access to deep space for U.S. astronauts, the new system would serve as a backup in case untried commercial rockets run into problems or delays.

Rep. Gabrielle Giffords, an Arizona Democrat who was severely injured during a shooting spree in Tucson, on Saturday, strongly disagreed with the Senate's requirement during budget discussions late last year, favoring instead the continued development of the Constellation program's Ares rockets.

Married to shuttle commander Mark Kelly and as chairman of the House space and aeronautics subcommittee, Giffords urged her colleagues not to go along with plans for a new rocket that was designed "not by our best engineers, but by our colleagues over on the Senate side. By NASA's own internal analysis, they estimate this rocket will cost billions more than the Senate provides."

"In short, the Senate bill forces NASA to build a rocket that doesn't meet its needs, with a budget that's not adequate to do the job and on a schedule that NASA's own analysis says is unrealistic," Giffords said. "That is not my idea of an executable and sustainable human spaceflight program."

In a report ordered by Congress in NASA's funding authorization, the agency said it "recognizes it has a responsibility to be clear with the Congress and the American taxpayers about our true estimated costs and schedules for developing the SLS and MPCV, and we intend to do so."

"Currently, our SLS studies have shown that while cost is not a major discriminator among the design options studied, none of the design options studied thus far appeared to be affordable in our present fiscal condition."

Operational costs are another factor, the agency said, along with funds needed to pay for development of other exploration systems, including habitats and landers.

"A feature of the Shuttle/Ares-derived reference vehicle is that it enables leveraging of current systems, current knowledge base, existing hardware and potentially current contracts, thereby providing schedule and early-year cost advantages," the report said. "However, a 2016 first flight does not appear to be possible within projected FY 2011 and out-year funding levels, although NASA is continuing to explore more innovative procurement and development approaches to determine whether it can come closer to this goal."

In the meantime, NASA said, "it is clear that successful development of SLS and MPCV will be dependent on sufficiently stable funding over the long term, coupled with a successful effort on the part of NASA and the eventual industry team to reduce costs and to establish stable, tightly managed requirements."