Each flip of Curiosity’s six aluminium wheels stamps the letters JPL into Mars in Morse code — a signature hidden in dots and dashes that isn’t merely ornamental, as a result of the rover images these tracks to measure how far its wheels slipped on sand and rock

Curiosity has been urgent the identical brief message into Mars since its first take a look at drive in August 2012. Each of the rover’s six aluminium wheels carries three teams of cut-outs: .— .–. .-... Read as Morse code, they spell JPL, for the Jet Propulsion Laboratory that constructed the rover and manages its mission.

The element sounds just like the type of personal joke engineers add to a machine earlier than sending it to a different planet. It is that, partly. But NASA’s description of Curiosity’s first wheel tracks explains that the sample additionally offers the rover a recognisable visible reference. On tremendous, moderately featureless floor, Curiosity can manufacture the landmarks it must verify its personal motion.

This just isn’t a brand new discovery or a scientific outcome. It is a bit of engineering that has been seen in mission images from the start. What me was the financial system of it: a signature, a ruler and a security verify occupy the identical holes in a wheel.

What one wheel revolution leaves behind

Each wheel is about 50 centimetres in diameter and has a tread constructed round raised chevron-shaped grousers. Across one a part of the wheel are straight bands carrying the Morse-code cut-outs. When that part meets soil tender sufficient to maintain an impression, the standard zigzag tread is interrupted by a sequence of distinct bars.

The code is exact. J is dot-dash-dash-dash, P is dot-dash-dash-dot and L is dot-dash-dot-dot. Because the identical function seems on all six wheels, a readable model can recur with every full revolution on appropriate floor. Scientific American’s contemporary account described these bands as reference factors that seem as soon as per wheel revolution.

The title’s “every turn” wants that qualification. Bare rock can not protect a crisp stamp, and never each piece of a drive is photographed as a person observe. The function is on the market every time the floor and the rover’s navigation routine make it helpful.

Wheel rotation just isn’t the identical as distance travelled

On agency, degree floor, distance may be estimated from the variety of wheel rotations. Mars doesn’t reliably provide agency, degree floor. A wheel can rotate whereas slipping in sand, or one a part of the rover can transfer in another way because the suspension crosses a rock or a slope. The commanded motion and the actual motion then diverge.

That is the place visible odometry enters. According to JPL’s account of Curiosity’s navigation software, the rover combines wheel-rotation measurements with info from gyroscopes and accelerometers. It additionally compares picture pairs taken earlier than and after a drive step, monitoring how options within the scene shift between frames. From that change, the software program estimates how the rover really moved in six levels of freedom.

If the wheels report one quantity of journey however the pictures present much less progress, the rover has proof of slip. The mismatch issues on steep floor and in sand, the place persevering with to show the wheels with out making the anticipated progress can worsen a troublesome state of affairs.

Why write a landmark into the soil?

Visual odometry works nicely when the scene comprises rocks, shadows and different steady particulars for the software program to observe. A comparatively clean mattress of tremendous materials might provide fewer distinct factors. The observe sample offers the cameras a synthetic function whose scale and spacing are already identified.

The Morse code just isn’t a separate sensor and it doesn’t change the remainder of the navigation system. It is one useful form among the many terrain options the software program can observe. NASA flight director Bobak Ferdowsi defined the essential concept quickly after touchdown: the staff knew what number of wheel cycles had been commanded, whereas the marks on the bottom helped reveal whether or not the rover had really superior by the corresponding quantity.

I’ve written earlier than about JPL groups engaged on Mars time. The wheel code is one other reminder that working a rover from Earth is stuffed with oblique measurements. Nobody watches Curiosity transfer in actual time. The machine has to go away proof of its movement, study that proof and report again.

The wheels are lighter and thinner than they give the impression of being

Curiosity is a car-sized rover, however mass was unforgiving throughout launch and touchdown. A 2022 engineering paper on its wheel damage describes wheels 50 centimetres throughout and 40 centimetres large, with aluminium pores and skin solely 0.75 millimetres thick. The paper labels the Morse sample plainly as an odometry function.

Those skinny skins later gathered punctures and tears from sharp Martian rocks, prompting adjustments to route planning, traction management and wheel monitoring. That historical past makes the cheerful hidden signature look barely completely different. The holes weren’t ornament minimize right into a heavy, expendable tyre. They have been built-in right into a construction designed below extreme mass limits, then requested to function touchdown gear and survive years of rock, sand and slopes.

A joke that also needed to earn its place

The Planetary Society describes the code as a playful approach of placing JPL’s mark on Mars. That interpretation is tough to withstand. The first clear tracks did, in any case, write the builders’ initials on one other world.

Yet the half I like is that the joke was made helpful. Spacecraft carry strict limits on mass, energy, complexity and threat. A flourish is less complicated to defend when it additionally offers navigation software program a identified sample on in any other case bland floor.

Curiosity doesn’t want Morse code to know what JPL means. It wants distinction, spacing and a function that may be discovered once more in a picture. Humans equipped the letters. The rover makes use of the geometry. In the identical marks, one aspect of the mission left a signature and the opposite discovered a technique to measure whether or not six turning wheels have been actually carrying it ahead.

Standards

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