By Irene Klotz

CAPE CANAVERAL, Fla. Feb 2 (Reuters) - - NASA wants to hike spending by 50 percent on a program to help two commercial companies develop space taxis to ferry crew to the International Space Station, the agency's 2016 spending plan shows.

The move is part of the Obama Administration's proposal to boost NASA's budget for the fiscal year that begins Oct. 1 to $18.5 billion, a $519 million increase over the current year.

The plan includes an increase to $1.24 billion in spending to help Boeing and privately owned Space Exploration Technologies, or SpaceX, develop and test commercial space taxis needed to fly crew to and from the International Space Station.

Congress allotted $805 million for the so-called Commercial Crew program for the current fiscal year, which ends on Sept. 30. The White House wants additional money to keep both Boeing and SpaceX on track to take over station crew ferry flights from Russia before the end of 2017, NASA Chief Financial Officer David Radzanowski told reporters on a conference call.

If Congress does not fund the $1.244 billion, NASA will be unable to pay for development milestones planned for SpaceX and Boeing, he said.

"As a result we will not be able to certify the development of those services by the end of 2017. That's the outcome," Radzanowski said.

NASA figures it will pay an average of $58 million per seat on the U.S. space taxies, compared to the more than $70 million Russia currently charges for rides on its Soyuz capsules.

The United States has been without its own human space transportation system since it retired the space shuttles in 2011.

Obama's 2016 budget for NASA maintains development of the heavy-lift Space Launch System rocket and Orion capsule, intended to carry astronauts beyond Earth's orbit for the first time since the 1960s-era Apollo moon program. NASA plans to spend $2.5 billion on the program, with the rocket's debut test flight slated for November 2018. The budget is about $400 million less than what NASA is spending in 2015, a cutback that already sparked a sharp rebuke from Texas Republican Lemar Smith, who chairs the House Science Committee.

"I am disappointed that the budget request does not adequately support the programs that will take us farther into space to destinations like Mars," Smith said in a statement.

The new spending plan shifts responsibility for the country's non-military Earth-monitoring satellites to NASA from National Oceanic and Atmospheric Administration, which under the proposal would operate only weather satellites.

The 2016 budget also funds an airplane-based astronomical observatory that was revived by Congress after the White House cut funding last year and continues development of a robotic mission to Jupiter's ocean-bearing moon Europa, another project championed by Congress.

(irene.klotz@thomsonreuters.com; editing by Andrew Hay)