On March 9, 2017, NASA’s Curiosity rover took this picture with its turret-mounted MAHLI camera of the calibration target installed near the “shoulder” of its robotic arm. In addition to color chips and a metric line graph, the target also includes a U.S. coin: a 1909 Lincoln penny, adhered heads-up.

Curiosity’s coin isn’t just for good luck though; it’s also a nod to geologists who typically use familiar objects in field photos to help determine scale. Curiosity is after all our only working robot geologist on the surface of Mars—that is until Perseverance arrives on Feb. 18, 2021!

“When a geologist takes pictures of rock outcrops she is studying, she wants an object of known scale in the photographs,” said MAHLI Principal Investigator Ken Edgett of Malin Space Science Systems in San Diego, which designed and built the rover’s cameras. (Source) “If it is a whole cliff face, she’ll ask a person to stand in the shot. If it is a view from a meter or so away, she might use a rock hammer. If it is a close-up, as the MAHLI can take, she might pull something small out of her pocket. Like a penny. Of course, this penny can’t be moved around and placed in MAHLI images; it stays affixed to the rover.”

The penny on Curiosity is a 1909 “VDB” wheat cent. 1909 is the first year Abraham Lincoln was featured on the coin, in honor of the 100-year anniversary of his birth, and VDB are the initials of the designer—Victor David Brenner—which are engraved on the opposite side in small letters below two curving shafts of wheat.

The absence of an “S” below the date indicates this coin was made at the Philadelphia mint (rather than San Francisco.) Incidentally the Philadelphia 1909 coins are much more common collector-wise than the S versions, which are considered rare today and can be worth in uncirculated quality for up to $2,000 each. (The non-S ones are worth about $30 uncirculated.)

Curiously enough (see what I did there?) there was a bit of a controversy around the coins themselves when they were released over a century ago. Brenner, a talented engraver and Lithuanian immigrant, had designed the now-famous sculpted portrait of Lincoln two years prior to his commission to create the new one-cent design. On the production die he included his initials, which some people criticized as self-serving and conspicuous.

In addition some people didn’t find the stylized wheat motifs on the “tail” of the coin scientifically accurate enough…and many Southerners simply disliked a coin commemorating Lincoln.

The U.S. Mint subsequently removed the initials on later 1909 coins but eventually replaced them in smaller letters in 1918. Brenner’s initials remain on U.S. pennies today, just below Lincoln’s right shoulder. Read more here.

These days the use of pennies (and coins in general) may be in decline but still many people are familiar with the size and color of a one-cent coin, and that’s also part of the reason why it was included on Curiosity.

“Everyone in the United States can recognize the penny and immediately know how big it is, and can compare that with the rover hardware and Mars materials in the same image,” Edgett said. “The public can watch for changes in the penny over the long term on Mars. Will it change color? Will it corrode? Will it get pitted by windblown sand?”

Now there’s a penny for your thoughts!

Learn more about Curiosity’s historic one-cent coin here.

Note: this article has been updated in January 2021.