WASHINGTON -- NASA wants to have a better idea by year’s end of how it will accomplish the first leg of proposed human expeditions to the moon, Mars and other destinations -- getting large payloads off the Earth’s surface.

A presidential directive to send humans back to the moon by 2020 and eventually on to Mars has revived NASA’s interest in developing a heavy lift launcher able to boost large amounts of space hardware into orbit. But NASA is also considering making do with existing launchers like the Atlas 5 and Delta 4 to loft smaller bundles of ready-to-assemble hardware into space that would be put together in orbit before being sent on its way.

Some NASA estimates say an Apollo-style trip to the moon would require launching roughly 100 metric tons of hardware and fuel into space, about what it took the last time around. The agency and other experts agree that a Mars exploration mission could easily require three to five times that amount of hardware and fuel.

Today’s most powerful launcher, the U.S. space shuttle, can lift about 27.5 metric tons to low Earth orbit (LEO). The heavy-lift version of the Delta 4 expected to make its debut this year is designed to haul about 25 metric tons to LEO.

Launcher technology is just one piece of what NASA officials envision as an integrated space transportation for the exploration missions. The agency’s selection of launch capability will have significant repercussions for many other aspects of the exploration program.

Craig Steidle, NASA’s associate administrator for exploration systems, has said told industry audiences he wants to have the agency’s heavy lift decision made by the end of the year, possibly as early as October.

But in an April 6 interview, Steidle said he expects only to narrow considerably the field of candidates this year, not decide on one approach to the heavy lift question.

Steidle said he also expects to have, by year’s end, firmer estimates of how much mass NASA would have to put into space to accomplish its exploration goals.

A number of studies supporting those decisions are under way at NASA. A key participant in those studies, Robert Sackheim of Marshall Space Flight Center, Huntsville, Ala., told Space News the options for launchers run the gamut from relying on today’s stable of expendable rockets more or less as is, to designing a brand new behemoth rivaling the Saturn 5. NASA also is evaluating  new launch vehicle concepts derived from the space shuttle and the Atlas 5 and Delta 4 families of evolved expendable launch vehicles (EELVs).

Sackheim, a Marshall assistant director and the center’s chief engineer for propulsion, said one approach not getting much consideration at this point is fully reusable launchers like those NASA spent billions of dollars trying to develop in the 1990s.

“In my opinion it is highly unlikely we would pick a fully reusable launch vehicle at this point in time because of the low predicted launch rate,” Sackheim said.

Sackheim said NASA expects it will take six to 10 launches per year to meet the exploration goals. Even at 20-30 launches per year, he said, it would be hard to make the case for a fully reusable launcher of the caliber NASA likely would require for exploration missions. “Reusability only pays off when you have a high flight rate,” he said.

NASA is also looking at EELV designs that would mix and match the best components from the Atlas and Delta rockets to find another 10 metric tons of lift. Other options include adding strap-on boosters, enlarging the Centaur upper stage fuel capacity and improving the power output of the Centaur’s engine.

More radical approaches involving the EELV, Sackheim said, could include new and fatter core stages for the Atlas and Delta to yield as much as 40 to 60 metric tons of lift.

Some shuttle-derived designs could lift 80-100 metric tons to LEO, Sackheim said. Others would be designed to lift considerably less than that. Sackheim said NASA is studying about a dozen different shuttle-derived designs.

Despite advances in materials and propulsion since the Apollo program, Sackheim said, it still is a safe bet that sending a couple of people to the moon for a short stay is a 100 metric ton proposition. The first human excursions to Mars may well last two years, and would require launches of several hundred metric tons of material per year.

The U.S. space agency also is wrestling with how to get the nuclear-powered Jupiter Icy Moons Orbiter into orbit in 2015.

The unmanned probe, the biggest spacecraft on NASA’s drawing boards, could weigh around 30 metric tons including roughly 15 metric tons of xenon propellant. That would be too heavy for any of the launchers in service or under development. NASA could build a brawnier launcher or launch the Jupiter probe and its propellant to LEO in two or more flights. Sackheim said NASA is trying to take these issues into account before choosing a path forward.

“Rather than simply assert we are going to develop a 100 metric tons launch capability based on shuttle-derived or EELV-derived [designs], we are going to look at an optimized approach,” Sackheim said.

A Saturn 5-class lifter may not be necessary. NASA could opt to assemble and fuel spacecraft in orbit.

It could choose to launch astronauts and their gear separately, perhaps pre-positioning exploration equipment weeks, months or even years ahead of time.

Michael Griffin, NASA’s associate administrator for exploration from 1991-1993, says the most logical approach, all things considered, is to spend the $3 billion or $4 billion it would cost to build a shuttle-derived heavy lifter and forget about EELV-driven approaches.

“No matter what lunar or Mars architecture is chosen, a lot of mass will have to be moved through LEO, or through some other staging point,” Griffin told Space News. “I would argue that 100 [metric tons] represents a reasonable place to start, and that shuttle-derived systems can get us to that point more cheaply than other systems. No one would favor a clean-sheet approach more than would I, but unless more money is made available for it than I think likely, we won’t get it. I dislike giving up something we have in favor of something we might get.”

Griffin also said that while he takes a “dim view” of approaches that would rely on orbital staging and assembly operations, he thinks NASA is examining the right options“While I don’t think EELV is a competitive option, you need to make sure the issue has been thoroughly examined,” he said.

Sackheim, for his part, would not hazard a guess at which way NASA might go, saying the decision is in the hands of NASA Administrator Sean O’Keefe and the rest of the senior management team.