It’s true. At this moment there is a rare coin on Mars. Specifically, the coin is a 1909 ”V.D.B.” Lincoln-head American penny.
The question is, why is this penny on Mars?
OK, yes, this is another post about the Curiosity rover on Mars. The 1909 penny is attached to Curiosity, and there’s a perfectly good reason for this.
Curiosity’s proper name is the “Mars Science Laboratory.” It is designed to act like a field geologist, carefully examining rocks and geological features to learn about Mars’ past.
Like any good geologist, Curiosity is equipped with the tools of the trade. Sure, they are a lot more hi-tech than the average geologist would carry around, but then again we are talking about exploring another planet with a nuclear-powered robot working tens of millions of miles from the nearest human.
One of a geologist’s most commonly used tools is called a “hand lens.” A hand lens is small but powerful magifying glass that allows the geologist to see fine detail.
Curiosity is equipped with many cameras, and one of them is high-power, focusable camera designated the “Mars Hand Lens Imager,” or MAHLI for short. It does for Curiosity on Mars what a field geologist’s hand lens does on Earth.
When Curiosity is drilling into rocks or otherwise investigating samples within reach of its robot arm or rock-blasting laser (I did mention Curiosity was high-tech, didn’t I?) it uses the MAHLI camera at the end of its robot arm for a close visual inspection of items of interest.
This presents Curiosity and the engineers here on Earth with a challenge. For MAHLI’s close-up pictures to be useful, you have to make sure the color and brightness values are correct, and you need to be confident that you’ve got everything in focus. So how do you calibrate your camera for the correct color and size if you’re not there to see things with your own eyes?
Easy – you take a calibration “target” image and take pictures of it from time to time to use as a reference.
Curiosity’s calibration target for the MAHLI camera is at the “shoulder” of the robotic arm to which MAHLI is attached. Below is a picture of Curiosity being prepped for launch, with arrow pointing at the calibration plate:
Now here is a closer view of that same image, zooming in on the MAHLI calibration target:
And here’s the plate itself in close-up:
And there it is… the 1909 Lincoln-head penny. Why is that there?
The Curiosity engineers at the Jet Propulsion Laboratory are giving a tip-of-the-hat to a time-honored geologist’s method for providing a sense of scale size to their close-up photographs. When a geologist photographs rocks they traditionally place a coin in the image so the viewer can get a sense of the photograph’s scale size. Without it, you don’t know if you’re looking at a close-up of a pebble or a boulder or the side of a mountain. Take for example this image:
See how the penny in this image gives you a sense of the size of the rock features? Geologists have been putting coins in their photographs for scale since they first started taking pictures of rocks. It’s kind of a tradition.
The penny affixed to Curiosity cost the engineer who worked on the calibration target about $5. He bought it with his own money. He chose the 1909 penny because it was the first year of the Lincoln-head penny.
Now then, for your coin collectors out there, what do you think that penny, the first coin ever sent to Mars, will be worth in fifty or so years when humans go to Mars, find Curiosity, and are deciding what to do with it?