WASHINGTON — NASA could be forced to delay two approved, top-tier Earth science missions by up to one year due to congressional inaction on the president’s 2011 spending request, which has U.S. federal agencies operating at their 2010 spending levels, according to sources and documents.

NASA had hoped to spend about $5 billion on Earth- and space-science missions in 2011, but the funding available under the current stopgap spending measure, or continuing resolution, falls $450 million short of that total, the documents show. U.S. President Barack Obama requested $19 billion for NASA in 2011, but the agency currently is constrained to spending rates commensurate with the $18.72 billion appropriated for 2010.

According to a March 9 laundry list detailing potential program impacts to NASA’s Science Mission Directorate (SMD), the shortfall could delay the Ice, Cloud and Land Elevation Satellite 2 (ICESat-2) and Soil Moisture Active and Passive (SMAP) missions while increasing their cost. A copy of the list was obtained by Space News.

Although NASA requested $75 million and $132 million this year for ICESat-2 and SMAP, respectively, SMD could be forced to reduce planned spending on ICESat-2 by $22 million in order to stay within the $1.4 billion spending ceiling Congress appropriated for Earth science initiatives last year. SMAP funding would have to be reduced by $30 million, according to the document.

Both missions topped the National Research Council’s list of large-scale climate-monitoring priorities in its 2007 Earth science decadal survey. ICESat-2, led by NASA’s Goddard Space Flight Center in Greenbelt, Md., is a follow-on mission to the ICESat spacecraft currently in orbit. It is designed to continue measurements of changes in polar ice-sheet mass to anticipate changes to global sea levels.

Although NASA does not expect to nail down a cost estimate for ICESat-2 until it undergoes a key design review in November, budget documents show the agency projects spending $500 million-$600 million between 2008 and 2016. NASA is budgeting roughly $100 million for ICESat-2’s launch but has yet to settle on a rocket to send it into a 450 kilometer polar orbit in January 2016. The one-year launch delay could eliminate a potential dual-launch opportunity, adding to the overall cost of the mission, according to the document.

SMAP, designed to improve weather forecasts and flood and drought predictions, is expected to cost between $780 million and $900 million. Led by the Jet Propulsion Laboratory in Pasadena, Calif., it is slated for launch in November 2014.

Budget woes have already forced NASA to shelve a pair of big-ticket environmental monitoring missions — the Climate Absolute Radiance and Refractivity Observatory and the Deformation, Ecosystem Structure and Dynamics of Ice satellite — that just last year were planned for launch 2017. The president’s latest budget request includes future-year funding projections that are considerably lower than those released last year.

Another mission likely to be affected by the continuing resolution is the Global Precipitation Measurement satellite, a rain- and snow-monitoring mission NASA is developing with the Japanese Aerospace Exploration Agency. The mission’s planned July 2013 launch could be delayed if NASA is forced to trim its budget by $15 million, the March 9 document shows.

NASA has already scaled back the precipitation measuring mission by abandoning plans to fly a carbon copy of its microwave-imaging instrument aboard a spacecraft operating in a low-inclination orbit to focus on the Earth’s equatorial regions.

Meanwhile, House and Senate lawmakers are continuing to negotiate a continuing resolution that would fund the federal government for the remainder of the current fiscal year. The current stopgap funding measure expires March 18.

The House in February passed a measure that provides just $18.1 billion for NASA. Companion legislation introduced by the Senate’s Democratic majority would have provided $18.5 billion for NASA, but that measure was defeated in a floor vote.

The House bill, which likewise failed to pass the Senate during a March 9 floor vote, would leave NASA’s Science Mission Directorate funded at the $4.4 billion Congress appropriated last year. The Senate bill would have increased that by roughly $350 million.

Although lawmakers are unlikely to reach agreement before the current continuing resolution expires, congressional aides said they expect House and Senate leaders to approve yet another short-term continuing resolution that will keep the government running for an additional three weeks.