During the next few sols, the rover put on her astronomer’s cap. With Lemmon’s guidance from Earth, the robot looked to the sky and focused Pancam, at the ready to capture images of Phobos as it transited the Sun. Just like on Earth, Mars has eclipse seasons. Once a year, or twice every Mars year, there is an eclipse season on the Red Planet and usually Opportunity has the chance to shoot one of the Martian moons.

Mars is the only one of the four rocky, terrestrial planets in our solar system that is endowed with two moons. They were discovered in 1877 by American astronomer Asaph Hall with what was then the largest refracting telescope in the world, a 26-inch telescope at the United States Naval Observatory.

The potato-shaped moons were promptly named after twins Phobos (Fear) and Deimos (Panic), who in Greek mythology, accompanied their father Ares, the god of war, into battle. [The Romans knew Ares as Mars and that’s the name that stuck for the planet that ‘burned’ red in the night sky.]

Phobos is the larger of the two moons, measuring about 22 kilometers (13.67 miles) across. It orbits just 6,000 kilometers (3,728 miles) from Mars and travels around the planet three times a day, zooming across the Martian sky approximately once every four hours. Deimos is just 13 kilometers (about 8.08 miles) across and orbits the Red Planet from a distance of about 20,069 kilometers (about 12,470 miles) about every 30 hours, which is a little more than a Martian day.

For Opportunity – and Spirit back in the day – capturing an image of Phobos or Deimos from the surface of Mars is, and always was, about more than just getting a cool shot of a moon orbiting by in the sky, but about collecting valuable scientific knowledge. The goal is to track the moons’ orbits. “The more precisely you know the orbits, the easier it is to constrain the long-term evolution of Phobos and Deimos, which tells you something about the interior of Mars and maybe the inside of the moons,” explained Lemmon. “It also makes it easier to “precisely point” the hi-resolution cameras on the orbiters so they can also freeze frame the small moons.

Scientists have been tracking the orbits of Phobos and Deimos from the ground since Hall discovered them and Lemmon, with the MER rovers, has been tracking the moons’ orbits since 2004. “We’re trying to come up with a very long baseline, long time period of having high-quality Phobos/Deimos observations, where we could say Phobos was orbiting exactly in this direction,” said Lemmon. “The Sun is the easiest backdrop for that.”

When Phobos is between the rover and the Sun, “you can pinpoint it,” he continued. “And that is more precise than almost any other observation that could be made and more precise than any other observation that is routinely made. That’s why we keep looking at Phobos.”

It’s no small feat. There’s a lot that goes into getting good moon shots with the rover. “It requires a lot more planning than most images and requires techniques that we don’t use very often,” said Lemmon. “Every time we attempt this, we have to review: ‘What did we do last time that worked?’”

The challenges are as intricate as they are intense. For starters, there is an error bar in terms of knowing precisely where the target moon will be, when exactly it will be there, and what, precisely, the rover’s attitude will be. “The trick here is that the Phobos eclipse is going to happen when Nature says it’s going to happen,” said Lemmon. And when it happens, it happens fast. “There is a specific 20-to-30 second window when you can take the pictures, so you have to arrange the rover’s day around the Phobos observation.”

The time of the Martian day or sol matters too. “It’s a very important part of the observation, not just from the standpoint of planning it, but a morning observation actually looks at a different part of Phobos’ orbit than an evening observation,” Lemmon explained. “So when we get more than one observation, the fact that they are a different time of day is a relevant distinction.”

Besides the challenge of aiming the Pancam at the Sun and making sure the rover is in the right spot and at the right attitude, there is the challenge of making sure the rover’s clock is on time. Just like your clock or watch on Earth, the robot’s clock drifts and can be off “by as much as 40 seconds,” said Lemmon. That alone, of course, can be the difference between getting the shot and not getting the shot.

Opportunity’s attitude and clock are updated periodically, and the clock is almost always updated close to a transit time, said Lemmon. Even so, getting the shot requires “a specific kind of communication” during the rover command window. But Lemmon and Opportunity have 13 years of experience going for them.

On the morning of 4719 (May 3, 2017), Opportunity succeeded in catching a partial eclipse at 10:20 hybrid local solar time (HLST). “We measured the clock error after the first event,” said Lemmon.