CAPE CANAVERAL, Florida (Reuters) - The U.S. plan to return astronauts to the moon by 2020 will not happen without a big boost in NASA’s budget, leaving only the International Space Station as a viable target for the country’s human space program, according to a presidential review panel.

The moon passes between the sun and the earth amid monsoon clouds during a solar eclipse in New Delhi July 22, 2009. REUTERS/B Mathur

The Human Space Flight Plans committee, which presented its preliminary findings to the White House on Friday, concluded that a human mission to Mars currently would be too risky.

Developing new spaceships to replace the retiring space shuttle fleet and bigger rockets to reach the moon would require about $3 billion more per year, the panel headed by former Lockheed Martin chief Norm Augustine said.

The only human space program affordable under NASA’s existing budget is an enhanced space station, one that has a side benefit of seeding a commercial passenger-launch services market, said the panel, which completed a series of public meetings this week.

NASA spends about half of its $18 billion annual budget on human space flight to fly the space shuttles, build and operate the space station and develop new vehicles in a follow-on program called Constellation.

The committee said the new U.S. exploration initiative -- aimed at landing astronauts on the moon by 2020 -- is doomed because its 10-year, $108 billion budget has been shaved by about $30 billion.

“We can’t do this program in this budget,” said panel member Sally Ride, a former astronaut. “This budget is simply not friendly to exploration.”

ULTIMATE GOAL

Even with additional funds, heading to the moon may not be the best choice for human missions, the panel concluded. More economical and potentially more galvanizing to the public would be flights to asteroids and other destinations, it said.

The ultimate goal of future U.S. endeavors should be excursions to Mars, but the money and technologies needed to do so are not currently available, the panel said.

“We think to go direct to Mars with today’s technology and money is riskier than we would want to be associated with,” Augustine said. “It would likely not succeed.”

NASA for decades has explored Mars with unmanned spacecraft and rovers that have roamed the Red Planet’s surface.

NASA already has spent about $9 billion on Constellation, a project to build a capsule, rockets and lunar landers like those developed for the 1960s-era Apollo moon program.

The outlook for missions beyond Earth’s orbit turned even bleaker when the committee added funding to keep the space station operational through about 2020, as the station partners, including the United States, have said they would do.

NASA currently has no funding in place after 2015 for the space station, a $100 billion project of 16 nations.

Construction of the station is scheduled to be finished next year after seven more flights of the space shuttle, which orbits 225 miles above the planet.

After the shuttles are retired, NASA plans to pay Russia to transport crews to the station. The panel’s recommendations include adding $2.5 billion into NASA’s budget between 2011 and 2014 for commercial launch services to the space station.

“We’d like to get NASA out of the business of flying people to low-Earth orbit,” Ride said.

The board’s final report is due on August 31.