WASHINGTON — U.S. Senate appropriators are attempting to breath new life into one of two deep-space mission proposals that were passed over in the most recent competition under NASA’s Discovery-series of cost-capped planetary probes.

In a proposed spending bill for 2014, the Senate Appropriations Committee directed NASA to resume design work on one of the Discovery finalists: a lander that would hop on and off a comet racing toward the sun; and a probe that would splash down in one of the large methane-ethane seas on Saturn’s largest moon, Titan.

In 2010, a total of 28 teams sent NASA proposals for a slew of robotic solar system exploration missions that could be ready to launch by the end of 2016 for no more than $425 million, not including the cost of an Atlas 5 rocket or comparable vehicle. The following year, NASA selected three finalists: the Jet Propulsion Laboratory’s Mars-bound Geophysical Monitoring Station, the Johns Hopkins University Applied Physics Laboratory’s Titan Mare Explorer (TiME) and the University of Maryland’s Comet Hopper.

Each team was awarded $3 million to refine their concepts over the next 12 months.

Last August, NASA chose the Mars mission proposal — an instrument-laden lander renamed InSight to avoid confusion with the since-canceled Gravity and Extreme Magnetism Small Explorer mission — to proceed toward a 2016 launch.

The two runners-up have strong Maryland pedigrees, a point almost certainly not lost on Sen. Barbara Mikulski (D-Md.), who chairs the appropriations panel and led the drafting of the 2014 Commerce, Justice, Science and Related Agencies Appropriations Act (S. 1329), which includes NASA. A report accompanying the bill, approved by the committee July 18, “directs NASA to provide additional funding in Discovery to initiate Phase B study activities on an additional Discovery mission from the most recent 2012 announcement of opportunity with the highest scientific value that meets the program’s cost cap.”

University of Maryland researcher Jessica Sunshine, the principal investigator behind the Comet Hopper proposal, quipped that she “had a heart attack” when she learned that Senate appropriators want to give Comet Hopper and TiME a second shot at becoming full-fledged missions.

“It was a surprise to me,” Sunshine said July 25. “Being as objective as I can be — and I realize I don’t have a lot of credibility here — I think it’s a great idea.”

Sunshine said Phase B funding, which typically amounts to 10-15 percent of total mission costs, would allow her team “to make sure everything you think will work is actually going to work.”

“Phase B is the time when you sit down and really define your technical specifications for everything,” she said. “No metal is bent; it’s still a study,” but “there are some long lead items that if you don’t procure them in Phase B you won’t make it.”

Comet Hopper, which would orbit and land multiple times on Comet Wirtanen as it approaches the sun, would be built by Lockheed Martin Corp. of Bethesda, Md., under the management of NASA’s Goddard Space Flight Center in Greenbelt, Md.

TiME, a floating lander that would be dropped onto the surface of one of the largest lakes on Titan, was proposed by Ellen Stofan of Gaithersburg, Md.-based Proxemy Research and would be built by the Applied Physics Laboratory in Laurel, Md.

TiME’s deputy principal investigator, Jonathan Lunine of Cornell University’s Center for Radiophysics and Space Research, told SpaceNews he and Stofan “are very much encouraged by the language in the Senate bill.”

“TiME is ready to go if and when the Senate language becomes law,” he wrote in a July 26 email. “Of course, it is all up to the political process, but the seas of Titan await us!”

Both Comet Hopper and TiME were designed to carry a government-furnished power source known as the Advanced Stirling Radioisotope Generator (ASRG). The plutonium-fueled device, still in development, is expected to be four times more efficient than the current-generation nuclear battery that powers the Mars Curiosity rover, for example.

InSight, which was selected at a time when NASA was taking flak for scaling back its contribution to Europe’s ExoMars missions, will rely on solar power when it lands near the martian equator in September 2016 to begin a two-year mission to study the red planet’s geological evolution.

While neither the Comet Hopper nor TiME team would have had to pay for the ASRG itself, NASA required them to set aside $20 million of their $425 million notional mission budgets to pay for environmental compliance, nuclear launch safety approval and related launch services. “This $20 million cost was not of course carried by the non-ASRG InSight team and $20 million greatly exceeds what missions typically pay for a power system,” Sunshine said. “Thus there really was a disincentive to use this technology that NASA needs tested for its future.”

Lockheed Martin Space Systems of Denver remains under contract to deliver two ASRGs in 2016. What NASA will do with the unfueled flight units is unclear. Len Dudzinski, NASA program executive for radioisotope power systems, told the NASA-chartered Outer Planets Assessment Group July 15 that a planned ground-based mock mission known internally as M1 is off the table for now.

Sunshine thinks that is for the best. “We have two groups of people that spent more than $3 million each to get up to speed and understand what needs to be done” to integrate an ASRG, she said. “To say you are going to go off and start a new study is throwing out a heck of a lot of expertise that you already paid for twice.”

Sunshine said her team was barred by Discovery program rules from working with the ASRG’s government-industry development team during the competition because the system was being offered to all comers as government-furnished equipment.

If NASA ultimately funds a Phase B effort for Comet Hopper or TiME, Sunshine said, the agency’s ASRG efforts would benefit. “Either one of us or both of us could accomplish a lot” in terms of working through the details of integrating an ASRG — which, unlike the radioisotope thermoelectric generators it would replace, has moving parts — with a science spacecraft.

Lunine agreed. “We did not see — nor do we now see — a need for a mock mission.”

There were questions about whether Lockheed could deliver a flight-ready ASRG in time for a 2016 liftoff, according to Sunshine. A Department of Energy-led design review held in June 2012 — just weeks before Discovery’s final down-select — raised “a lot of schedule concerns,” she said.

Those schedule concerns were rendered all but moot when NASA passed over the two ASRG-powered Discovery finalists. The ASRG team is still working toward a 2016 delivery, Sunshine said, but with no spacecraft headed to the launch pad that year, the team no longer needs to worry about delivering a fueled flight unit or leaving enough time for installation.

And given that the Phase B effort Senate appropriators want NASA to fund would not begin until 2014 and would run at least a year, a flight-ready ASRG should be available in plenty of time for a reinstated Comet Hopper or TiME mission, the launch of which would likewise slip.

With the Republican-controlled House and Democrat-led Senate billions of dollars apart on spending and the Oct. 1 start of the 2014 budget year looming, the odds of Congress enacting full-year appropriations in the weeks ahead are considered slim. But if either Comet Hopper or TiME eventually makes it to the launch pad, it would not be the first NASA mission Mikulski has raised from the dead, with or without sending an appropriations bill to the president’s desk.

After NASA canceled the Pluto-Kuiper Express mission in 2000 because of budgetary reasons, Mikulski stepped in with the funding and political pressure NASA needed to launch the Applied Physics Laboratory-built New Horizons mission in 2006. That same year, after months of cajoling from Mikulski, NASA finally reinstated the Hubble servicing mission it had canceled in the wake of the 2003 Space Shuttle Columbia accident.