Martian astronomy: Percy sees a 'Phobos eclipse' from Mars — Phys.org

RESOLVING THE LEDGER

Martian astronomy: Percy sees a 'Phobos eclipse' from Mars — Phys.org | AIPROPX

edited by Swati Mestri , reviewed by Robert Egan

This article has been reviewed according to Science X's editorial process and policies . Editors have highlighted the following attributes while ensuring the content's credibility:

Add as preferred source What's so special about spotting Phobos crossing in front of the sun? Sure. It's just another image of Phobos crossing the disk of the sun, as seen from the surface of Mars. Like so much of the modern space era, these images, once amazing, are now almost routine. But there's some real science behind tasking the Mars rovers to conduct a little "Martian astronomy."

Opportunity was the first rover to directly spy a Mars moon transiting the sun in 2004. Viking 1 also caught the shadow of Phobos skimming its Chryse Planitia landing site in 1977. Both Curiosity and Perseverance have since made the effort to "look up" and monitor the celestial scene on occasion. The latest image was captured by Percy's main Mastcam-Z camera on Mars sol 1948 earlier this month, Aug. 12, 2026.

Do you say eclipse, occultation or transit? Certainly, Phobos' irregular, potato-shaped silhouette lends itself to a misshapen annular eclipse of the sun. Orbiting Mars at just under 6,000 kilometers (3,700 miles) above the planet's surface, speedy Phobos takes just 7 hours and 39 minutes to orbit the Red Planet, faster than the planet rotates. It covers just a quarter of the apparent disk of the sun at its maximum, and you would see it moving in real time, taking under a minute to cross the sun's disk.

I know what you're thinking: Is it safe for Percy to stare at the sun? "Mastcam-Z used solar filters, much like eclipse glasses, to take the images," Mark Lemmon (Space Science Institute) told Universe Today. "The filter change was done before aiming at the sun; the other order would not have resulted in damage, but that is a simple precaution. None of the instruments on the remote sensing mast can be damaged by the sun when they are aimed that way and not in use."

The rovers witness other strange sights, including dusty blue sunsets and transits of the lesser of the two Martian moons, Deimos. The rovers have even been tasked with trying to spot interstellar comet 3I/ATLAS on its close 29 million-kilometer (18 million-mile) pass near Mars on Oct. 3, 2025, and the close 140,000-kilometer (87,000-mile) pass of Comet C/2013 A1 Siding Spring past Mars on Oct. 19, 2014.

And just recently, Perseverance caught a bizarre occultation of Earth by Phobos. Mars is the only place in the solar system where you could stand on solid ground and witness an asteroid-shaped body crossing in front of the sun.

You'll note that Percy also caught a few sunspots on the solar disk. These sorts of observations come in handy when Mars is near solar conjunction on the far side of the sun relative to Earth. Percy noted a massive sunspot region in early May 2024, which later rotated into view for Earthbound observers and sparked a historic solar storm and auroral show about a week later.

Catching sight of Phobos crossing the sun also allows researchers to help refine the orbital positions of the two Martian moons.

"We image the transits because they give precise measurements, with accuracy better than 100 meters, of the location of Phobos at a specific instant," Lemmon says. "This constrains the Phobos orbit much better than can be done with the biggest telescope on Earth, since the rover is so much closer.

"The sun makes for a great target as a reference for Phobos' position. Better knowledge of the orbit is nice for planning: The first time transit imaging was done from Mars, Deimos was 40 kilometers off course using Viking-era ephemerides, but this problem is at the kilometer level now.

"However, the major purpose is to use the orbital evolution as a probe of the Martian (and Phobos') interior. Tides raised by Phobos on Mars cause the orbit to change; the changes probe the flexibility of the Martian mantle.

"This is why we keep doing it: We want a long-term baseline of these high-quality measurements. From the Spirit and Opportunity measurements to Curiosity and now Perseverance, we are in the third decade of this experiment. In addition, orbiter teams have been working on this with images of Phobos' shadow and occasional images of Phobos with stars from orbit."

This will prove handy when we want to approach, land on and explore the two moons. And that's coming right up, as the joint JAXA/NASA Mars Moons Explorer (MMX) is launching on an H3-24L rocket from the Tanegashima Space Center in Japan this year on Oct. 19. The mission will approach Phobos in 2027 and return samples to Earth starting sometime in 2031.

Finally, watch for more Mars-based astronomy now underway. "We are in the middle of a nighttime Phobos imaging campaign that will span 5–6 nights in the end," Lemmon says.

"Three are done, with image data on the ground for two. These were designed to complement the solar images by observing Phobos near background stars in different parts of its orbit. These are intended to add to the above measurements, since a complete description of an orbit at a moment in time requires 6 parameters, and the transit only measures two at high precision (of course, it measures the one that is by far most important compared to how well known it is, which is how far along the orbital path Phobos is). For this, we use short exposures to see where Phobos is and long exposures to measure the star trails for a position reference."

Phobos was discovered by American astronomer Asaph Hall using the U.S. Naval Observatory's new 26-inch (66-centimeter) refractor, the largest in the world at the time, during a favorable Mars opposition in 1877.

Modern astrophotographers have managed to tease out the elusive moons from the glare of Mars near opposition season. Currently high in the predawn sky in the astronomical constellation Gemini, Mars is once again approaching opposition on Feb. 19, 2027.

Certainly, i...