It's taken almost 18 months and the hard work of many clever people but the Heat Flow and Physical Properties Package (HP³) aka "the Mole" has finally dug its way completely into the Martian soil. This sophisticated experiment, part of NASA’s InSight Mission, is a self-hammering probe, designed to dig its way deep into the Martian ground and take its temperature. Unfortunately, it has encountered several snags over the last year, so scientists and engineers have had to come up with several out-of-the-box solutions.

Now it seems that efforts have paid off. The mole is completely "in" the soil and the "finishing touches" are "in sight" say the team after the latest work, which involved not only the little probe but also InSight’s robotic arm. The arm was able to provide support to the Mole by gently and carefully pushing on its back. Since the last update, the arm was used to fill in the wider than expected hole created by HP³.

The soil at InSight’s location was surprisingly unlike any seen in other places on Mars. In particular, a layer of a formation known as duricrust caused a lot of problems for the Mole. The probe works by using soil for friction to self-hammer deeper and deeper, but it couldn’t get enough grip to properly pierce the duricrust.

In June it was announced that the Mole was fully underground, at last level with the Martian surface and no longer sticking out. This was a breakthrough, but the team took some time to decide on the next critical move. Some thought to fill in the pit, while others preferred to use the robotic arm to push the probe further down before filling it in.

The final solution was a hybrid. They did a “scrape test” to better understand what was the best move forward. The scoop scraped some of the soil into the hole to give the probe some friction and it turns out the Mole was deeper than expected. The test covered it completely.

“I think, at the latest after filling the pit, we should be able to counter the recoil with sufficient force and the Mole will hopefully 'dig' deeper into the Martian soil on its own. Keep your fingers crossed!” Dr Tilman Spohn, Principal Investigator of HP³, wrote in a blog post updating the situation.

The team will use the arm to press on the ground now and give the Mole a bit more support for its upcoming dig. They are very optimistic as measurements of the soil shows better contact with the Mole, so unaided digging might soon begin in earnest.