A NASA team tasked with fleshing out President Barack Obama’s overhaul of the nation’s human spaceflight program has briefed NASA Administrator Charles Bolden and his deputy on a deep space exploration plan that rejects some key assumptions of the president’s strategy but aligns with a version of the NASA authorization bill approved by the full Senate in August.

Dubbed the Human Exploration Framework Team (HEFT), the group of NASA field center employees spent four months looking at the hardware that would be needed to heed the president’s call for sending astronauts to an asteroid by 2025.

In outlining his proposal in April, Obama proposed spending up to five years and some $3 billion researching heavy-lift propulsion technologies before choosing a design for the United States’ first exploration-class rocket since Saturn 5. The president also reversed an earlier decision to cancel the Orion Crew Exploration Vehicle, directing NASA to use the capsule as the basis for a space station crew lifeboat.

In outlining a human asteroid mission that would launch several years past Obama’s 2025 deadline, the HEFT recommended NASA begin work immediately on a space shuttle-derived heavy-lift rocket, skipping the five years of exploratory research Obama proposed. A set of the HEFT’s charts, dated Sept. 2, were posted on the NASA Watch website.

“There is no benefit to delaying work” on the heavy-lifter, the charts say, adding that waiting until 2015 to select a basic design would limit NASA’s options and hamper exploration planning.

NASA spokesman Michael Braukus said Sept. 10 that Bolden, NASA Deputy Administrator Lori Garver and Doug Cooke, NASA associate administrator for exploration systems, have been briefed on the HEFT’s findings. He said no decisions have been made.

But the HEFT’s preliminary assessment found key elements of the president’s plans untenable and recommended NASA begin immediately building a space shuttle-derived heavy-lifter and retaining Orion for deep space missions, according to the briefing charts.

Before proposing a shuttle-derived design that represents NASA’s best choice for an exploration-class rocket capable of boosting 100 metric tons into orbit, the HEFT considered a heavy-lift design that would incorporate kerosene-fueled main engines and boosters of the sort the Obama administration has talked about developing under its $3 billion heavy-lift propulsion research initiative.

However, building such a rocket, the HEFT charts say, would cost more than NASA could afford without help from the U.S. Defense Department, which currently relies on the Russian-built RD-180 kerosene engine to power the Atlas 5 rocket.

The HEFT plan also incorporates the Orion crew capsule, but says building a crew lifeboat for the station “diverts near-term resources” that could be better aligned with advancing human space exploration beyond low Earth orbit. Instead, the HEFT proposes developing an Orion-derived direct-return capsule and a separate, in-house NASA effort to build an exploration spacecraft.

To pay for the architecture, the HEFT recommends focusing technology development on sending humans beyond low Earth orbit and drawing on proposed investments central to Obama’s vision  including flagship technology demonstrations, robotic precursor missions, heavy-lift propulsion technologies, exploration technology demonstrations and human research.

In August, the Senate approved a three-year NASA authorization bill that calls for spending $2.75 billion next year to begin building a heavy-lift launcher initially capable of delivering between 70 and 100 metric tons to low Earth orbit and continuing development of a crew vehicle for space exploration missions by 2016.

Former NASA Administrator Mike Griffin said in a Sept. 9 speech on Capitol Hill that the Senate bill should remove the 70 to 100 metric-ton minimum and retain language that requires an operational capability of at least 130 metric tons, though he said “150 metric tons would be enormously better, if we are to build the technical means for manned journeys to Mars in this century.”