Moon & Mars Mondays · your week on the Moon and Mars

From The Decisional Pass

A quick note before we start. This one is reaching you on a Tuesday, and it was supposed to reach you on Monday morning. I'm rebuilding the machinery that sends these letters — the goal is every edition out at 9 a.m. Pacific on its own day — and this week the machinery and I disagreed about what day it was. That's mine to fix, not yours to work around, and it should settle over the next couple of weeks.

I'm sending it again because the thing this letter is about happens tonight, and a heads-up that arrives after the event isn't much of a heads-up. Thank you for your patience while I sort out the plumbing.

Tonight, at 11:35 p.m. Pacific, a piece of a Moon mission is going to hit the Moon.

Not a lander. Not a probe. The second stage of the Falcon 9 that launched on January 15, 2025 carrying two lunar landers — Firefly's Blue Ghost Mission 1, which made it down intact, and ispace's Resilience, which did not. The stage did its job, released both, and has been drifting in a chaotic Earth-Moon orbit ever since under a catalog designation almost nobody would recognize: 2025-010D.

Eighteen months later, its orbit has run out of places to go.

Three tons of TNT, arriving at 5,400 mph

The numbers come from Bill Gray, who runs the orbital-computation software that a lot of the amateur and professional tracking community uses, and who has been refining this prediction since observers picked the object back up in September 2025. As of his most recent update:

• Impact: 2026 August 5, 06:35:37.5 UTC — that's 11:35 p.m. PDT tonight, 2:35 a.m. EDT Wednesday
• Where: lunar latitude 19.461° N, longitude 93.293° W — near the crater Einstein, on the near side but right at the edge of the disk we see from Earth, on sunlit ground
• Speed: 2.43 km/s. About 5,400 mph, roughly seven times the speed of sound in air
• Mass: about 4,900 kg
• Energy released: 14.5 billion joules, which Gray puts at "the energy about three tons of TNT"

What I love about this is Gray's own error bar. His software says it can place the impact to about half a second and a third of a kilometer. He then declines to believe his own software: "I'm not really trusting the above to better than a few seconds and a few kilometers."

The reason is beautifully mundane. This is a big, light, empty metal tube. Sunlight pushes on it. Solar radiation pressure on an object with that much area and that little mass is a real force, and it depends on things nobody can measure from here: exactly how the stage is tumbling, how reflective it still is, whether anything has vented. You can model gravity to absurd precision. You cannot model a photon nudge against a tumbling tank you last saw as a smudge on a survey frame.

That gap — between what the arithmetic says and what the person who wrote the arithmetic will stand behind — is the most honest thing in this entire letter.

Why a junk impact is worth a telescope

There's a paper about this. Benjamin Fernando and a couple dozen co-authors — including Gray himself — posted an observing plan on July 16 inviting professional and amateur astronomers to try to catch it. The abstract is unusually plain: "On 2026 August 5, at approximately 06:35 UT, a spent Falcon 9 upper stage will impact the lunar surface near Einstein Crater."

The scientific case is that we almost never get this. Natural lunar impacts are frequent but unannounced — you catch a flash if you happen to be staring at the right patch. Here we have a known mass, a known velocity, a known geometry, and advance notice. The campaign wants to use it to test how well you can localize a lunar impact from the flash alone (which matters for seismology), to watch how the ejecta plume develops, and to check assumptions about what happens when spacecraft debris hits something.

The paper is refreshingly honest that the observable brightness is "imprecisely predicted at present." Nobody is promising a show. They're promising a rare controlled input into a system we normally only get to observe by accident.

If you have a telescope and a clear sky tonight, the target is the sunlit limb near Einstein. If you don't — and most of us don't — the thing worth watching is what the observations do to the prediction. Retiring uncertainty on a hard-to-track object, in public, against a deadline nobody can move: that's the interesting engineering.

Curiosity photographed its own tires again

Meanwhile — and it turns 14 years on the surface this Thursday — Curiosity spent part of last week doing something that sounds like housekeeping and isn't: taking close-up pictures of its own wheels with MAHLI, the camera on the end of the arm.

The wheels are thin aluminum, about 20 inches across, and they have been getting chewed up by Martian bedrock since 2013. The team's framework for it is one of my favorite pieces of operations thinking, because it converts a scary-looking photo into a schedule input. The raised zigzag treads are called grousers, and NASA's testing established the rule of thumb years ago: when three grousers on a given wheel have broken, that wheel has reached about 60 percent of its useful mileage.

That's it. That's the whole trick. You don't panic at holes; you count grousers, and you re-plan the route.

It works because the wheels are only half the system. The other half is route choice, and route choice is only as good as your last look at the tires. Wheel imaging isn't a health scare; it's the measurement that closes that loop. Lucy Thompson's July 23 mission update was explicitly a thank-you to the rover engineers who do this work, and she put the odometer in it: more than 23 miles (37 kilometers) driven, and more than 4,400 feet — about 1.35 kilometers — of climb up Mount Sharp.

I spent a fair amount of time with OPTIMISM, Perseverance's Earth twin, in JPL's Mars Yard. Nothing teaches you respect for wheel wear like watching a rover cross rock you personally chose because it looked "representative."

Missing time in the rock record

The science half of Curiosity's week is, if anything, better. In the July 31 mission update, deputy project scientist Abigail Fraeman wrote that the rover is closing in on a possible erosional supersurface — light-colored rocks sitting directly beneath dark ones, with no gentle transition in between.

A supersurface is a regional-scale unconformity: wind or water stripped away layers of sediment before new sediment piled on top. What you're looking at, when you look at that contact, is missing time. The rock record on Mount Sharp is a strip chart of Martian history, and here somebody tore out a stretch of the paper.

The team spotted it months ago from orbital data plus images of the buttes above, and has been driving toward it since. That week's work was the setup: two large Mastcam mosaics covering the whole layer, ChemCam long-distance imaging of the most interesting stretches, LIBS measurements on targets named Patacamaya, Tarucachi and Mojoncasa, and MAHLI plus APXS on a target called Monte Darwin. The team even backed the rover up to get a better perspective before going in.

Fraeman's framing is the right one: orbital data can tell you a break is probably there. Only a rover on the ground can tell you its composition and its centimeter-scale geometry — whether this is one clean erosional event or a messy stack of them.

Also on the board

• Psyche's Mars pictures got their victory lap. Psyche was NASA's Astronomy Picture of the Day on July 29, for the time-lapse assembled from its Mars gravity assist. Worth being precise about the timeline: the flyby itself was May 15, at 2,864 miles (4,609 km) altitude, and JPL released the data and video on July 17. The flyby doubled as an instrument checkout — imager, magnetometer, and gamma-ray/neutron spectrometer all exercised against a well-characterized target before the real one in 2029. Principal investigator Lindy Elkins-Tanton: "All instruments delivered great results. We didn't anticipate big discoveries, given how extensively the planet has been studied." Calibrating on a planet you already understand is exactly how you learn to trust an instrument at a planet you don't.
• A test stand for New Glenn. On July 24 NASA and Blue Origin announced an agreement to run second-stage hot fire testing for New Glenn on the B-2 stand at Stennis Space Center, starting later this year. That's Artemis-relevant plumbing: Blue Moon MK1 needs a ride, and its ride needs a second stage everyone trusts.

Clear skies tonight, wherever you are. And thanks again for bearing with the schedule while I get it steady.

— Elio

References

• Upper stage impacting the moon on 2026 August 5 — Project Pluto (Bill Gray)
• Observational planning for the 2026 August 5 Falcon 9 Upper Stage lunar impact — Fernando et al., arXiv:2607.14625
• Curiosity Blog, Sols 4961-4967: Approaching a Break in the Rock Record? — NASA Science, July 31, 2026
• Curiosity Blog, Sols 4954–4960: Celebrating Our Rover Engineers Past and Present — NASA Science, July 23, 2026
• Routine Inspection of Rover Wheel Wear and Tear — NASA Photojournal
• NASA's Psyche Mission Delivers Mars Flyby Data, Time-lapse Video — NASA Science, July 17, 2026
• APOD: Psyche Receives Gravity Assist from Mars — NASA, July 29, 2026
• NASA to Support Blue Origin New Glenn Rocket Testing, Advance Artemis — NASA, July 24, 2026

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