As U.S. President Barack Obama prepares to send his 2013 budget proposal to Congress Feb. 13, the Planetary Society is urging the White House to devote a slightly larger percentage of NASA’s share to Earth-observing satellites, planetary probes, space telescopes and other such science-focused missions.

NASA’s Science Mission Directorate will get about 29 percent, or $5.1 billion, of the $17.8 billion Congress approved for the agency for 2012. That was up slightly from the 27 percent share the directorate received in 2011, and the Planetary Society would like to see that trend continue for 2013.

“A small increase to 30% of NASA’s overall budget would support the outstanding missions being completed and planned,” reads a Jan. 12 letter to Jacob Lew, the outgoing director of the White House Office of Management and Budget. The letter was signed by Planetary Society Executive Director Bill Nye and the Pasadena, Calif.-based organization’s president, Jim Bell, an Arizona State University professor who also chairs the NASA Advisory Council’s planetary science subcommittee.

NASA’s overall budget was cut by some $700 million between 2011 and 2012, and aerospace contractors anticipate further reductions for 2013.

The Planetary Society said it is concerned that if NASA’s budget continues to shrink ”the Science program will carry a disproportionate burden of any reduction.”