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The proposals are in for a new NASA robotic mission for launch in 2021 to explore the solar system, and scientists have submitted concepts for probes to the moon, planets, asteroids and comets for a chance to win $450 million in federal funding.

Scientists had to send in their proposals by Feb. 18 for consideration by NASA managers as the next mission in the space agency’s Discovery program, a series of relatively low-cost, focused science probes aimed at exploring the solar system.

Jim Green, director of NASA’s planetary science division, said Feb. 19 that the agency plans to select at least two finalists from the proposals in May to receive $3 million federal grants for detailed concept studies. NASA should pick a single winner by September 2016, he told a meeting of the Outer Planets Assessment Group.

“We’ll rapidly get down to making some announcements as soon as we can get through the evaluation,” Green said.

The mission must be ready for launch by the end of 2021, and must cost no more than $450 million, excluding the launcher, which NASA pays from a separate account.

Up to one-third of the mission’s cost can come from international partners without counting against the $450 million cost cap.

The concept selected by NASA will become the 13th mission in the agency’s Discovery program, which started in the early 1990s and had its first launch in 1996.

Discovery missions launched to date include the Mars Pathfinder rover mission, the NEAR Shoemaker probe that first orbited an asteroid, and the Stardust project, which returned samples of comet and interstellar dust to Earth.

NASA’s MESSENGER spacecraft currently orbiting Mercury, the planet-hunting Kepler telescope, and the Dawn mission now approaching the dwarf planet Ceres were also developed and launched under the auspices of the Discovery program.

The 12th Discovery mission, the InSight Mars lander, is due for launch in March 2016 to touch down on the red planet and measure its seismic activity.

The competition now underway will end with the selection of the 13th Discovery mission.

NASA hopes to infuse the next mission with new technologies, offering up government-furnished equipment with incentives to sweeten the deal for principal investigators leading each proposal.

The government has offered to supply a deep space optical communications system to test new high-speed data links with Earth, giving science teams an extra $30 million above their $450 million cost cap if they choose to use the laser telecom unit.

NASA’s LADEE mission demonstrated laser communications between the moon and Earth in 2013, but optical communications has not been tested from greater distances.

If science teams wish to send entry probes into the atmospheres of Venus or Saturn, they will need a new type of heat shield. NASA’s solicitation included a provision to furnish a newly-developed 3D-woven heat shield with a $10 million incentive.

A deep space atomic clock is available with a $5 million bonus, and NASA has offered to provide xenon ion thrusters and radioisotope heater units without incentives.

NASA stipulates the mission must use solar power, limiting mission possibilities beyond Jupiter and Saturn.

The Discovery solicitation released in November came four years after the last opportunity for a low-cost planetary mission in 2010, which ended with the selection of the InSight Mars lander.

Green said NASA has funding in the White House’s proposed budget to speed up the pace of Discovery missions, with another call for proposals as soon as late 2017.

“Our plan is to bring Discovery calls in closer,” Green said. “Right now, the president’s budget supports about every 36 months. It’s not the 24 months that we’d like, but this is far healthier than it had been projected in the past.”

The New Frontiers program, which covers medium-cost interplanetary missions between low-cost Discovery projects and multibillion-dollar flagship missions, should begin the process for the selection of its next mission in 2016, Green said.

The mission will have a cost cap around $1 billion, following up on the New Horizons probe approaching Pluto, the Juno mission to Jupiter and the OSIRIS-REx asteroid sample return mission set for launch in September 2016.

Unlike the Discovery program, which allows proposals for any solar system mission, the New Frontiers bids are limited to five missions:

Comet Surface Sample Return

Saturn Probes

Lunar South Pole-Aitken Basin Sample Return

Venus In-situ Explorer

Trojan Tour and Rendezvous

Follow Stephen Clark on Twitter: @StephenClark1.