By Irene Klotz

CAPE CANAVERAL, Fla. Nov 19 (Reuters) - Despite budget uncertainties, NASA on Tuesday issued a solicitation for a commercially operated space taxi to ferry astronauts to the International Space Station, an attempt to break Russia's monopoly on crew transport by 2017.

The United States has been without a human space transportation system since 2011 when NASA retired its three-ship shuttle fleet due to high operating costs and fundamental safety questions. NASA's so-called Commercial Crew program is intended to address both cost and safety concerns, as well as return the capability to fly people to space from U.S. soil.

The agency wants to be able to purchase rides on a commercial basis before the end of 2017 to fly four crewmembers to and from the station about every six months.

The new solicitation asks for proposals for final design, development, test, evaluation and certification of a human space transportation system, including ground operations, launch, orbital operations, return to Earth and landing.

Rather than designing a replacement space shuttle and hiring contractors to build it, NASA decided to partner with industry, offering money, technical advice and oversight.

A precursor program for cargo ships spawned two new supply lines to the International Space Station, a $100 billion research outpost that flies about 250 miles (about 400 km) above Earth.

So far, privately owned Space Exploration Technologies, or SpaceX, has made one test flight and two cargo runs to the station. Orbital Sciences Corp. completed its test flight in September and is preparing its first resupply mission in December.

NASA contributed about $800 million toward the development of SpaceX's Falcon 9 rocket and Dragon cargo capsule and for Orbital Sciences' Antares rocket and Cygnus capsule. The companies also developed launch sites in Florida and Virginia, respectively, ground control stations and support services. Both firms also now offer orbital launch services commercially.

NASA followed up the development work with contracts worth a combined $3.5 billion for SpaceX and Orbital Sciences to fly cargo to the station.

Since 2011, SpaceX, Boeing and privately owned Sierra Nevada Corp. have been NASA's partners in a sister program to develop commercial space taxis to fly astronauts to and from the station. Since the shuttles' retirement, only Russia has the spaceships to ferry station crewmembers, a service that costs NASA more than $60 million per person.

FULL FUNDING URGED

The Obama administration is requesting $821 million for NASA's Commercial Crew program for the fiscal year that began on Oct. 1.