InSight HP3 Mole Update
German Aerospace Center (DLR)
In his logbook, Instrument Lead Tilman Spohn who is back in Berlin since April and communicating with JPL via the web, gives us the latest updates regarding the InSight mission and our HP3 instrument – the ‘Mole’ – which will hammer into the Martian surface.
Logbook entry 3 June 2020
More than three months have passed since my last blog post, when I had to report that the ‘Mole’ had unfortunately backed out again. Not as much as in October, but nevertheless, after going 1.5 centimetres into the surface, it reversed direction and backed out by 1.5 plus 3.5 centimetres, with the back cap ending a total of approximately five centimetres above the deepest position reached at the time and about seven centimetres above the surface. I described the situation in more detail in my previous post, in which I also detailed how the team attempted to explain the downward and then upward motion during one single hammering session (we had not seen this before).
As a consequence of the lack of success with the last attempt at pinning, the team decided to adopt an alternative strategy and try the ‘back-cap push’ technique instead. For this, the scoop is placed above the back cap and slowly lowered until it touches the cap. The arm is then further lowered and tensioned such that the scoop presses on the Mole with a force of about 50 newtons. When the Mole descends into the surface, the scoop follows its downward motion, but the load decreases as the Mole progresses. After approximately 1.5 centimetres, the pushing force reduces to zero and the push action has to be renewed.
Because of the orientation of the Mole in the pit and the limited reach of the arm, the scoop touches the back cap close to its edge at more or less a single point. The image below shows the situation as simulated in a laboratory at DLR. This simulation was done to assess how critical the placement was for the tether. As can be seen, an error in placement of just a few millimetres could have either caused the scoop to slip off the back cap or to damage the tether. In addition, as the Mole moves down, the scoop moves to the left relative to the Mole and towards the tether.
Therefore, the team proceeded very carefully. After each placement, the situation was checked through imaging and recordings of arm motor current data before a number of hammer strokes were commanded. We started the procedure with only a few (25) hammer strokes. Only after the team had gained some confidence in its ability to carefully place the scoop and in the rate of progress of the Mole did we increase the number of hammer strokes per session to, in the end, 150 strokes per session.
As I had reported in an earlier post, the present mode of operation of the InSight mission allows only one cycle of operations per week. (Remember, we are in a phase of the mission when the instruments should be ‘monitoring’ rather than ‘deploying’. In the deployment phase in early 2019, commanding was success oriented – that is, as needed. In the monitoring phase, we have far fewer team members; most have other project commitments.) Thus, placement of the scoop occurred only every other week (mostly on Saturdays), followed by what space engineers call a ‘ground-in-the-loop’, that is a checking of the placement of the scoop on Mondays to give the go-ahead for the next hammering, usually on the following Saturday.
We started about seven centimetres above the surface on Sol 458 (11 March) and we are now at the surface with the scoop on Sol 536 (30 May 30), after six cycles of hammering over 11 weeks. The movie shows the entire history of penetration through back-cap pushing.
It is possible that the hammering stopped when the left edge of the scoop was still one millimetre or so above the surface. Also, the scoop is obviously at an angle with respect to the regolith surface, such that the right edge of the scoop may still be a centimetre or so above the surface. In addition, we know that the surface is covered with about one centimetre of relatively loose sand that the scoop may compress.
Therefore, the next step will be another hammering with the scoop pushing on the back-cap. During that hammering, we expect the scoop to be stopped by the regolith (if it has not been stopped already at the end of the Sol 536 hammering) and we can see whether the Mole is able to dig on its own. We call this the ‘free-Mole’ test.
Clearly, the Mole was not stopped by a stone as has been suggested
You may recall that our leading theory was that the Mole did not move into the subsurface because the regolith did not provide enough friction to balance the recoil force of the Mole. Although this force is much smaller than the force that drives the Mole forward (five to seven newtons as compared to 900 newtons) it still needs to be provided by the overburden pressure. Calculations that I had discussed earlier in this blog suggest that the friction force will suffice if the Mole is fully buried. Some additional friction can be provided if we use the arm to load the surface, which we will do.
Should the Mole move into the subsurface on its own (albeit being helped somewhat by the regolith push), friction will increase and improve the situation as the Mole moves deeper. When the Mole back cap is at a depth of approximately 20 centimetres, loading the surface will have become ineffective and the regolith push should no longer be necessary. We will then do what we planned to do more than a year ago – command the Mole to hammer to depth.
So, you see, the next step, the free Mole test, will be very exciting. But what if the Mole is just not deep enough in for sufficient friction? We then have two options, either fill the pit to provide more friction and push on the regolith, or use the scoop to push at the back-cap again, but this time with its tip rather than with its flat bottom surface. This would be a somewhat more difficult operation but doable, as the Instrument Deployment Arm (IDA) team thinks.
In addition, winter is approaching on Mars’ northern hemisphere and dust storm season will begin soon. The atmosphere is already getting dustier and the power generated by the solar panels is decreasing. This may affect our ability to performing energy consuming operations with the arm in the near future. Stay tuned and keep your fingers crossed.
And, is it not wonderful how people can work together from home across large distances on Earth and to Mars?
Thank you very much team!