WASHINGTON — A private organization that recently selected finalists for one-way human missions to Mars in the mid-2020s has quietly suspended work on a pair of robotic missions, putting into question plans to launch those spacecraft in 2018.

Mars One, a Dutch-based nonprofit organization, announced in December 2013 it was starting work on two robotic missions it planned to launch in 2018 as precursors to its human expeditions to Mars. One spacecraft would orbit Mars and serve as a communications relay, while the other would be a lander to test technologies planned for later crewed missions.

At that time, Mars One announced it had selected Lockheed Martin to begin work on the lander mission and Surrey Satellite Technology Ltd. (SSTL) to start work on the orbiter. Mars One awarded contracts to each company to perform concept studies of the planned missions.

“These missions are the first step in Mars One’s overall plan of establishing a permanent human settlement on Mars,” Bas Landsorp, Mars One chief executive and co-founder, said in a December 2013 press conference here announcing the missions. “We believe we are in very good shape to make this happen.”

However, both companies confirmed with SpaceNews that, since the completion of those study contracts, they have not received additional contracts from Mars One to continue work on those missions.

“SSTL delivered the concept study for the Mars One communications system last year,” SSTL spokeswoman Joelle Sykes said Feb. 16. “There are no follow-on activities underway at the moment.”

“Lockheed Martin has concluded the initial contract with Mars One in which we performed mission formulation studies and developed payload interface specifications to support the selection of a payload suite for the 2018 Mars robotic lander,” the company said in a statement Feb. 17. “We continue to maintain an open channel of communications with Mars One and await initiation of the next phase of the program.”

Mars One has said little publicly about the status of its robotic missions since that December 2013 announcement. In January, it announced the winner of a university competition to develop an experiment that would fly on the lander. At that time, the organization said the lander was still scheduled to launch in 2018, but offered no details about the status of its development.

In a Feb. 17 email, Lansdorp said that Mars One was focused for the time being on addressing a study released last fall by a student-led group at the Massachusetts Institute of Technology (MIT). That report, which examined the life support requirements for Mars One’s planned crewed missions, concluded that flaws in Mars One’s designs could result in the deaths of crewmembers within months of landing.

“Especially with the recent MIT (student) report, we’re focusing on releasing the mission concepts study on life support systems by Paragon,” Lansdorp said, referring to Paragon Space Development Corp., a Tucson, Arizona-based developer of life support technologies. The Paragon study, he said, should be released in March.

Lansdorp offered no additional details about the status of Mars One’s robotic missions, declining to answer questions about the timeline for awarding follow-on contracts. “As soon as we are ready to announce follow-up contracts, we will do so by means of a press release,” he said.

In addition to the life support study, Mars One has also been working on selecting finalists for those first crewed missions. On Feb. 16, the organization revealed the group of 100 “Round Three” astronaut candidates, selected from a pool of 705 semi-finalists announced in May 2014. These 100 will participate in “group challenges” that will eventually lead to the selection of six groups of four people each to serve as initial crews for one-way missions to Mars that it plans to start launching in 2024.

Those plans, though, may depend on the progress Mars One makes on its robotic missions, and there time is of the essence. Lockheed Martin noted in its statement that its proposed Mars One lander is based on a “very mature” design used on NASA’s Phoenix Mars Lander mission launched in 2007 and the InSight lander it is building for launch in March 2016.

Despite that heritage, the company said, “we would have to start construction very soon to launch an InSight clone in the 2018 window.”