WASHINGTON — A heat flow instrument on NASA’s InSight Mars lander suffered a setback Oct. 26 in its efforts to penetrate into the Martian surface.

Images returned late Oct. 26 by the lander showed that the probe, or mole, emerging from a hole onto the surface. The most recent images show most of the mole now above ground.

The mole has an internal hammer that was designed to make thousands of strokes, slowly pushing it deeper into the surface. The mole became stuck early this year, though, after penetrating only about 30 centimeters below the surface, far short of its goal of five meters.

Engineers spent months developing plans to try to get the mole moving again. Earlier this month, they reported success with using the scoop on the end of the lander’s robotic arm to pin the mole to one side of the hole, increasing its friction and allowing the mole to move deeper into the surface.

That progress continued in recent days even after moving the arm so that it was no longer pinning the mole. “The images that we got down on 23 and 24 October clearly show that the Mole had digged further!” wrote Tilman Spohn, principal investigator for the Heat Flow and Physical Properties Package instrument at the German space agency DLR, in an Oct. 25 blog post. The mole, he added, appeared to be penetrating into the surface at a slightly faster rate than when it was pinned by the arm.

In that blog post, Spohn said his team had commanded the mole to perform two sets of 150 strokes of its hammer on sol, or Martian day, 326 for the mission. “We will then increase the number of strokes per sol and hope to see it disappear from our view into the Martian regolith soon,” he wrote.

But images returned after that hammering showed that the mole had bounced mostly out of the hole. “While digging this weekend the mole backed about halfway out of the ground,” the mission announced via a pair of tweets Oct. 27. “Preliminary assessment points to unexpected soil properties as the main reason.”

Mars continues to surprise us. While digging this weekend the mole backed about halfway out of the ground. Preliminary assessment points to unexpected soil properties as the main reason. Team looking at next steps. #SaveTheMole #Teamwork pic.twitter.com/UURvU8VTwZ — NASA InSight (@NASAInSight) October 27, 2019

The mission added that one possibility is soil is falling in front of the mole, filling the hole. “Team continues to look over the data and will have a plan in the next few days.”

“We’re analyzing the problem and will share what we have tonight and provide more detailed information tomorrow after engineers have analyzed the data,” Thomas Zurbuchen, NASA associate administrator for science, said in a separate series of tweets Oct. 27.

We are aware that there is a problem with the mole on @NASAInSight that we have been trying to get into the ground. We’re analyzing the problem and will share what we have tonight and provide more detailed information tomorrow after engineers have analyzed the data. — Thomas Zurbuchen (@Dr_ThomasZ) October 27, 2019

He added that despite the problem with the mole, the mission is “functioning very well” overall and that the mole was not a “so-called Level 1 [requirement] for mission success.” However, the overall instrument is one of two major payloads on the mission, along with a seismometer that is working normally.