Correction: This story has been updated to clarify that while NASA was considering a mandatory laser communications payload for the next Discovery mission, the payload was never actually a requirement.

WASHINGTON — NASA on Nov. 5 released the final solicitation for its next Discovery-class planetary science mission, which must launch by Dec. 31, 2021, and cost no more than $450 million to build.

In an about-face, NASA decided not to require Discovery proposals for missions going beyond the Moon to carry an experimental laser communications payload that weighs roughly 25 kilograms and draws about 75 watts. In March, Michael New, the Discovery program’s chief scientist, told members of the NASA Advisory Council’s planetary science subcommittee the equipment would likely be mandatory.

Now, according to the final solicitation, “the use of any NASA-developed technology is strictly optional.” 

However, NASA is offering a $30 million bonus to missions that volunteer to carry the laser communications equipment, which NASA is now calling a Deep Space Optical Communications payload. The extra funding, provided on top of the $450 million mission-development cap, would pay for the hardware, integration and support personnel associated with adding the payload.

Meanwhile, mission planners are still barred from using nuclear batteries known as radioisotope power systems. The Mars rover Curiosity uses such a power source, which converts waste heat from decaying plutonium 238 into electricity, eliminating the need for solar panels. In the Discovery solicitation just released, NASA took care to block a possible loophole for nuclear-powered proposals by prohibiting technology demonstrations of radioisotope power systems.

The final solicitation also imposes strict limits on non-U.S. contributions to proposed missions. As NASA outlined in a draft solicitation released during the summer, non-U.S. contributions are limited to one-third of the total mission development cost. The same one-third limit also applies the chosen mission’s science payload.

Discovery’s $450 million cost cap does not include the price of launch, which NASA pays for separately. However, missions that require a bigger launch vehicle than the unspecified intermediate-class launcher NASA has built into its budget assumptions may be required to offset additional launch costs.

The solicitation also bars teams from proposing launching their spacecraft as secondary payloads. All missions must be the primary payload on their launch vehicle, NASA said.

Investigators also may not propose using Discovery funds to contribute to some larger mission by, for example, building an instrument for a non-NASA space probe. NASA calls such projects “missions of opportunity,” and expressly forbids them in the latest Discovery solicitation.

Scientists planning to submit proposals by the Feb. 16 deadline must first turn in a notice of intent by Dec. 5. NASA expects to narrow the field to at least two finalists around June. Final selection is expected in September, according to NASA’s announcement of opportunity.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.