WASHINGTON — Engineers plan to use the robotic arm on its InSight Mars lander to push a heat flow probe into the surface, acknowledging that they have “few alternatives” if that effort fails.

The Heat Flow and Physical Properties Package instrument team has spent nearly a year trying to get the instrument’s probe, or “mole,” to burrow into the surface. The mole has an internal hammering mechanism that is designed to drive the probe as deep as five meters into the surface in order to measure the heat flow from the planet’s interior.

The mole, though, stopped only about 30 centimeters below the surface. The mission has tried a number of ways to get the mole moving again, including removing the instrument housing on the surface to allow the lander’s robotic arm to try and fill in the hole created by the mole, as well as pin the mole to one side of that hole, increasing the friction needed for the mole to work its way into the surface.

In October, that use of the arm to pin the mole worked briefly, allowing the mole to burrow into the surface, only for it to rebound partially out of the hole. A second attempt led to the mole again rebounding partially out of the hole in January.

In a Feb. 21 statement, NASA’s Jet Propulsion Laboratory said it will instead use the robotic arm to push on the top, or “back cap,” of the mole in an effort to force the mole into the ground. Engineers had avoided using that approach before out of concerns it could damage the tether extending from that back cap to the main body of the instrument on the surface.

“After the team’s experience of operating the scoop, we all became more confident that the risk of accidental damage to the tether (with its power and data lines) was small enough to be worth taking,” Tilman Spohn, principal investigator for the instrument at the German space agency DLR, wrote in a Feb. 21 blog post.

Scientists believe that InSight landed in a region with a thick “duricrust” of regolith not seen elsewhere on the surface. The mole was designed to use friction from looser regolith to move into the surface, and the duricrust may be causing the mole to rebound when it hammers.

This latest effort to get the mole into the surface will take place in late February and early March. Engineers may also use the robotic arm to put more regolith into the hole around the mole.

It’s not clear what the next steps would be if this attempt to push against the mole’s back cap fails. The JPL statement noted that engineers decided on this approach because there were “few alternatives left” to try to the get the mole into the surface.