Updated 8:48 p.m.

WASHINGTON — U.S. President Barack Obama will ask Congress for $17.7 billion for NASA for 2013, an amount that would leave the agency funded at its lowest level in four years, according to sources familiar with the forthcoming budget proposal.

NASA’s planetary science division would shoulder a heavy share of the cut. Under the president’s proposal, its budget would drop from $1.5 billion to $1.2 billion, a 20 percent reduction.

NASA’s Earth sciences division fared considerably better; the $1.77 billion allocated in 2012 for building and operating climate-monitoring satellites would rise slightly under the president’s proposal. Astrophysics and heliophysics funding, however, would remain flat.

The president’s budget proposal also shields the congressionally mandated Space Launch System from cuts. The heavy-lift rocket development effort would get $1.88 billion next year, a slight increase that includes funding for related ground systems and infrastructure work. The rocket’s companion spacecraft, the Orion Multi-Purpose Crew Vehicle, would receive $1 billion, or about $200 million less than Congress appropriated for 2012.

Due on Capitol Hill Feb. 13, the $17.7 billion NASA budget proposal represents only a slight reduction from the $17.8 billion Congress approved in November for 2012. But compared to the $18.7 billion Obama penciled in for 2013 in the five-year budget he sent Congress this time last year, it represents a 5 percent cut.

Things could have been worse. According to a source familiar with the Obama administration’s internal budget deliberations, the White House Office of Management and Budget asked the agency last fall to submit budget proposals for three scenarios: a 5 percent cut, a 10 percent cut and a 15 percent cut — all relative to the outyear spending plan submitted last year.

With no top line budget relief, the cost overruns on NASA’s James Webb Space Telescope weighed heavily on the agency’s planetary science division, sources said. News of the 20-percent cut was first reported Feb. 8 by the Washington Post. A spate of news stories about the deep cuts in store for NASA’s popular robotic Mars exploration program followed.

Rep. Adam Schiff (D-Calif.), a House Appropriations Committee member whose Pasadena, Calif., district is home to NASA’s Jet Propulsion Laboratory, which specializes in executing planetary missions, issued the following statement Feb. 9 following a meeting with NASA Administrator Charles Bolden: “As I told the Administrator during our meeting, I oppose these ill-considered cuts and I will do everything in my power to restore the Mars budget and to ensure American leadership in space exploration.”

Meanwhile, late Feb. 9, the White House directed federal agencies, including NASA and the U.S. Air Force, to cancel embargoed budget briefings with reporters that had been scheduled for Feb. 10. Administration sources said the White House did not give agencies a reason for canceling the briefings.

Budget briefings planned for Feb. 13 — many of them to be televised or webcast — are still going forward.

NASA is scheduled to roll out its budget in a televised briefing from the agency’s Washington headquarters at 2 p.m., the same time the Air Force is scheduled to brief reporters at the Pentagon on its 2013 spending proposal.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...