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On 21 November 2026, a spacecraft that has been decelerating since 2018 will finally stop. Partway through the day, more than 200 million kilometres from the people flying it, the stacked BepiColombo orbiters will turn their chemical thrusters against their own motion and fire. If the burn behaves, the joint ESA-JAXA mission becomes the first to hold two spacecraft in orbit around Mercury simultaneously. If it does not, eight years of trajectory design has to be rebuilt from scratch around a rock that most spacecraft are barely engineered to survive.

That frame is among the last things the mission photographed with the hardware it no longer needs. M-CAM 1 is bolted to the Mercury Transfer Module, the ESA-built propulsion stage that carried both science orbiters across the inner Solar System and was discarded on 3 September 2026 at a point 63 million kilometres from the Sun (ESA, September 2026). What remains is roughly 1,500 kg of spacecraft still moving fast enough that Mercury cannot simply grab it.

The module carried three of those 1,024 by 1,024 pixel black-and-white cameras, which exist less for science than for triage: they let engineers check hardware health from a distance when the main science instruments are pointed the wrong way or blocked. They also became the mission’s accidental souvenir shop, recording Earth, both Venus encounters and all six Mercury passes. The final swing-by on 8 January 2025 came within about 295 km of the surface and was the last gravitational tug before the long approach.

Mercury’s reputation as a hostile destination is mostly a matter of velocity. It orbits at 0.387 astronomical units, where the Sun’s pull is roughly seven times stronger than at Earth, and a spacecraft launched from Earth arrives there carrying far too much speed. Run the numbers with the vis-viva equation, v^2 = GM(2/r - 1/a), and a direct transfer from Earth’s orbit to Mercury’s comes out near 57 km/s at the arrival point, against Mercury’s own orbital speed of about 48 km/s. That surplus has to go somewhere. Entering a low orbit around a small planet means surrendering most of it to the planet’s gravity, and the only cheap way to surrender velocity is to let a planet take it away for you.

That is what the nine flybys were for. BepiColombo launched from Kourou on an Ariane 5 on 20 October 2018 at 4,100 kg, comprising a 1,230 kg orbital module carrying 85 kg of instruments and a 255 kg Japanese magnetospheric orbiter carrying 45 kg (ESA fact sheet). It swung past Earth once, Venus twice and Mercury six times, shedding heliocentric energy in small instalments, and used solar-electric thrust between encounters to trim the orbit inward. It is named for the Italian engineer Giuseppe Colombo, whose 1970s work on gravity-assist trajectories let Mariner 10 visit Mercury three times. ESA notes that orbit insertion at Mercury demands more energy than sending a probe past Pluto, which is why a nearly $2 billion mission needed a nine-stop itinerary.

The itinerary almost did not survive 2024. On 26 April, while the module was mid-manoeuvre, its power system began drawing unexpected currents between the solar array and the unit that extracts and distributes power, starving the ion engines. Engineers at ESOC in Darmstadt restored thrust to about 90 percent within eleven days but could not recover full power, and the flight dynamics team concluded in September that the reduced thrust could no longer reach the December 2025 insertion date (ESA, September 2024). The hardware was unfixable, so the trajectory moved. The fourth Mercury flyby on 4 September 2024 was flown roughly 35 km lower than planned, a small shave of altitude that conserved propellant and let a redesigned path carry the spacecraft to Mercury a year late while keeping the science programme intact (ISAS).

September 2026 was the handover. When the transfer module let go, the remaining pair had to inherit power generation, propulsion, pointing and thermal control in one shot, at a distance where a radio round trip takes hours. Confirmation came through Doppler shift as the two halves changed velocity, read out from Cebreros in Spain and Malargüe in Argentina (ESA, 3 September 2026). By mid-September the instruments that had spent years staring at the back of a dead engine deck were working. The BepiColombo Laser Altimeter fired more than 700,000 pulses over 21.5 hours, MERTIS returned about a gigabit during a 22-hour look at empty space through its planet and calibration ports, and SIMBIO-SYS returned its first image in space (ESA, 6 October 2026). A thruster rehearsal on 24 September produced 11.4 m/s over 8 minutes 42 seconds, the first trajectory correction of the arrival sequence.

The propulsion that carried it here deserves the credit. ESA loaded 581 kg of xenon into the module’s three tanks at Kourou in June 2018, and that fuel went through four QinetiQ T6 gridded ion thrusters, 22 cm across and rated around 4.5 kW each, producing 75 to 145 mN of thrust at a specific impulse up to 4,300 seconds (Hofer et al., AIAA 2010-7114). Do that through Tsiolkovsky’s rocket equation, Δv = v_e × ln(m_0 / m_f), with an effective exhaust velocity near 39 km/s, and burning a bit over a fifth of the stack’s mass buys on the order of 10 km/s of velocity change. The same job on a chemical engine with a 3.3 km/s exhaust would demand a mass ratio near 20 to one, meaning a spacecraft more propellant than spacecraft. That arithmetic is the whole reason solar-electric propulsion exists.

The other constraint was sunlight. At 63 million kilometres the solar flux is roughly 5.6 times what Earth receives, and heat limits how squarely the array can face the Sun, so ESA canted the wings and accepted a deployed span near 30 metres to collect the power four thrusters needed (ESA fact sheet). The September 2024 power fault sat uncomfortably close to that limit, which is why the recovery plan was flown conservatively rather than pushed.

What happens on 21 November is closer to a chemical burn, because capture cannot be done slowly. Arrival commands go up days ahead and execute autonomously, because at Mercury’s distance there is no opportunity to watch a burn and correct it. After capture, the pair separates on 9-10 December, MPO settles into a 480 by 1,500 km polar orbit with a 2.3-hour period, and Mio stretches out to 590 by 11,640 km to study the magnetosphere from outside. Sixteen instruments begin formal operations in April 2027 (ESA fact sheet).

The paired-orbit arrangement matters more than it sounds. MPO works close in, repeatedly crossing the planet’s surface to build topography, map composition and weigh the interior from gravity and radio tracking. Mio works far out, crossing the magnetopause and sampling the solar wind that Mercury sits inside rather than outside. No single spacecraft can do both jobs at once, and building two cost an extra decade and an extra module to carry them.

What MESSENGER left open in 2015 is what these two orbiters are built to close, and the arrival itself is an argument for slowness in a field that rewards speed. BepiColombo spent five years producing thrust no desk lamp could outrun, because that was the cheapest way to buy the velocity change it needed, and the bill for that patience came due in a single burn that nobody will be able to touch.

On October 20, 2026, NASA named Sierra Lobo Inc. as the supplier of the closed Brayton-cycle converter for Space Reactor-1 Freedom, or SR-1. Electricity is the converter’s product: heat from the fission core will drive its machinery to generate current, while the reactor itself is being designed at the Department of Energy’s Idaho National Laboratory. The launch is set for late 2028. Success would mark the first departure of a fission-powered craft from Earth orbit. Startup is deliberately delayed: only after SR-1 has cleared Earth orbit will the reactor be allowed to run, keeping an operating reactor away from the launch pad (NASA, Oct. 20, 2026).

The assignment matters because it links two machines that have to become one spacecraft. The receiving hardware is listed on NASA’s SR-1 page: a power-and-propulsion bus and a 12-kilowatt Hall thruster (NASA SR-1 mission page). The converter’s job is to close that gap with reactor heat, turning an announced Mars goal into work that can be scheduled, reviewed and tested against flight hardware.

The machine belongs to an old idea that has had a long wait for flight hardware. The United States launched SNAP-10A in 1965, the first and only U.S. fission reactor flown in space, then spent decades developing reactors without sending one to another planet (NASA Space Nuclear Propulsion). NASA separates the field into two broad designs. Nuclear thermal propulsion heats a propellant directly and sends it through a nozzle, producing high thrust. Nuclear electric propulsion first makes electricity, then uses it to accelerate a gas. SR-1 Freedom belongs to the second class, so its defining feature is a long, frugal burn rather than a brief shove.

That choice changes what the spacecraft has to carry. A chemical engine carries both fuel and oxidizer; a fission reactor supplies energy but still needs a working gas for the thruster. NASA notes that nuclear systems can provide solar-independent power for years, along with electricity for instruments and communications. The design bet is that a years-long low-thrust cruise can consume less propellant than a faster chemical departure. As sunlight weakens with distance from the Sun, that trade becomes more attractive. Those choices also directly affect spacecraft mass, shielding, thermal design and mission timeline.

That bargain appears in NASA’s 12-kilowatt Advanced Electric Propulsion System thruster. A 2025 technical paper reports about 600 millinewtons of thrust and a specific impulse near 2,800 seconds at full power (NASA Glenn Research Center, 2025). The exhaust velocity implied by that specific impulse is v_e = Isp * g_0, roughly 27,500 metres per second. Yet the propellant flow is tiny: mass flow = F / (Isp * g_0) gives about 0.079 kilograms per hour. Applied to a 12,000-kilogram spacecraft, one thruster produces an acceleration near 0.00005 m/s2 and adds only about 4.3 m/s over a full day. The craft is not trying to leap. It is trying to spend almost nothing while it goes.

The vehicle inventory on NASA’s SR-1 page lists a 12,000-kilogram mass, high-assay low-enriched uranium fuel, a 20-kilowatt-electric closed Brayton converter, a 48-kilowatt power-and-propulsion bus, and a 12-kilowatt Hall thruster (NASA SR-1 mission page). Those power numbers describe different pieces and should not be added into one reactor output. One labels the converter; the other belongs to the bus. The electrical design therefore has to reconcile a 20-kilowatt fission source with a bus capable of 48 kilowatts. That gap makes power management, radiator sizing and the operating sequence central engineering questions before the first Mars departure.

The converter is where nuclear physics turns into machinery. In a closed Brayton cycle, a working gas is compressed, heated through a heat exchanger coupled to the reactor, expanded through a turbine to drive a generator, then cooled and returned to the compressor. “Closed” means the working fluid circulates. The reactor fuel and working gas need not touch, which keeps the power-conversion loop separate from the radioactive core. For an ideal Brayton engine, thermal efficiency follows eta = 1 - 1 / r_p^((gamma - 1) / gamma), where r_p is compressor pressure ratio and gamma is the gas ratio of specific heats. Real hardware loses efficiency to expansion, pressure drops and heat leakage.

Heat also has to leave. If reactor thermal input is Q_in and the converter efficiency is eta, the waste heat is Q_reject = Q_in * (1 - eta). At a useful 20 kilowatts, even a hypothetical 30 percent converter would have to shed roughly 47 kilowatts. That is why the artist concept shows radiator panels: the cycle cannot close without a way to dump heat in space. At 20 kilowatts of output, the converter is a heat-rejection problem as much as an electricity problem. Radiator area, fluid temperature and reactor thermal limits have to be solved as one system.

Safety has shaped the schedule as much as propulsion. NASA has ruled out critical reactor operation during launch; startup waits until the spacecraft is beyond Earth orbit. The agencies also apply interagency reviews, regulatory compliance and failure-tolerance analysis, while a NASA chief safety officer provides continuing oversight (NASA, Oct. 20, 2026). That separation reduces one class of accident: if the launcher fails, there is no active reactor to scatter. The remaining work is substantial. Fuel must be produced and qualified; the converter must survive vibration and years in space; the reactor must start remotely; and ground teams must rehearse commands across a communications delay that grows with distance.

The science payload gives the engineering test a reason to keep flying. SR-1 will carry SkyFall, three Mars helicopters derived from NASA’s Ingenuity design. They are to be released during atmospheric descent, fly to the surface and use ground-penetrating radar, imagery and atmospheric sensors to scout terrain and look for buried ice. NASA plans an initial Mars flyby in 2029 and a landing after a second approach in autumn 2030 (NASA SkyFall mission page). The helicopters can scout landing sites that a rover would take much longer to reach, and their data can be transmitted directly through an orbiter rather than a relay rover.

SR-1’s late-2028 launch also has a schedule built into it: NASA has targeted the 2028 Mars launch window, with arrival in 2029 before deployment (NASA SR-1 mission page). If the reactor, converter, bus and thrusters work together, the mission will establish an operating fission system in deep space. It will also test a supply chain that NASA wants to reuse for Lunar Reactor-1, the surface power system intended for its Moon Base (NASA SR-1 mission page).

Success will be a chain of systems behaving together: the reactor starts, the converter holds a stable electrical output, the radiators reject waste heat, the thrusters accelerate the craft and the payload separates cleanly. Each link can be tested on Earth. Their behavior together, after years of unattended exposure and a communications delay measured in minutes, cannot. If that chain holds, the useful outcome is larger than three helicopters. It is a flight-proven method for carrying watts instead of sunlight into places where the Sun is weak, absent or inconvenient.

Spaceflight systems tend to fail at their interfaces. A converter can meet its bench target and still strain the reactor; thrusters can work while the bus changes modes; the payload can separate but its radio link can fail. A useful sortie would give engineers measured limits, failure modes and workarounds for those seams. That knowledge is what a lunar reactor, cargo tug or outer-planet probe will need. The helicopters are visible; the operational inheritance is the point.

NASA’s Artemis III operations concept graphic has the Moon above the Earth and three rockets lined up beneath it: a New Glenn carrying a Blue Origin lander test article, the Space Launch System carrying Orion, and a Starship carrying a second test article. All three fly in low Earth orbit, and none goes to the Moon. That is the plan Administrator Jared Isaacman, Associate Administrator Amit Kshatriya and Artemis Program Manager Jeremy Parsons presented at Kennedy Space Center on October 23, and it reverses the promise that defined the program for a decade. Artemis III, the mission built to put astronauts back on the surface, is now a two-week low Earth orbit flight targeted for mid-2027, with four astronauts docking first with a Blue Origin test lander and then with a SpaceX Starship. The first landing since Apollo 17 moves to Artemis IV in early 2028.

Artemis II cleared the way, launching on April 1, 2026 and flying four astronauts around the Moon on a free-return trajectory, 406,771 kilometres from Earth at closest, before splashing down in the Pacific ten days later (Wikipedia, Artemis II). The mission worked, but the arithmetic behind the schedule never changed. The European Service Module, the Airbus-built element that powers Orion and pushes it around, lacks the delta-v to return from low lunar orbit; in Apollo the lunar module was the crew’s lifeboat, and in Artemis the lander must do that job and more (Wikipedia, European Service Module).

That lifeboat arrangement had a staging point, the Lunar Gateway, and NASA cancelled it in March 2026. A month before Artemis II flew, Isaacman had already restructured the flight sequence so Artemis III would stay in Earth orbit and Artemis IV would attempt the landing (Wikipedia, Artemis III). The pressure sits on the parts NASA does not build: both commercial landers are behind schedule, as is Axiom’s spacesuit, and the FY2026 budget request proposed ending SLS and Orion after Artemis III, citing a launch cost of about $4 billion (Wikipedia, Artemis III).

The flight plan is specific enough to be built. Blue Origin goes first, with a lander test article lifted off New Glenn from Launch Complex 36A carrying a crew module, avionics, partial life support and a reaction control system but no main propulsion. It can loiter in orbit for weeks, which is why it launches first, and Blue Origin would spend about two weeks checking it out before Orion flies (Wikipedia, Artemis III). Orion follows on an SLS Block 1 from Pad 39-B with an inert structural spacer in place of the Interim Cryogenic Propulsion Stage, since the mission never leaves low Earth orbit and does not need that stage’s RL10 engine (Wikipedia, Artemis III and Interim Cryogenic Propulsion Stage).

Orion is the chaser in both encounters, holding station while the passive target keeps its orientation, with time for another attempt if the first docking fails. Once mated, the crew stays about two days, equalises pressure in the Blue Moon vestibule with Orion’s atmosphere, opens the hatch and goes inside to check cabin habitability and the interfaces between the lander and Axiom’s AxEMU suit (Wikipedia, Artemis III). After undocking, a standard Starship v3 with a passive docking fitting on its nose cone launches from Pad 39-A, and Orion docks with it briefly without the crew leaving the capsule, because that vehicle has no crew cabin and no life support. The point is control: verifying that Starship can hold the attitude of a combined stack with Orion attached. Orion comes home on an upgraded heat shield and splashes down off San Diego for Navy recovery.

What the rehearsal is for became clearer in the briefing charts. Two landing architectures are on the table and NASA has not chosen between them. In the Starship version, Orion docks with a Starship Human Landing System in low Earth orbit and Starship pushes both vehicles to the Moon, which needs a propellant depot and several tanker flights to move cryogenic methane and oxygen in orbit, a manoeuvre never demonstrated at scale (SpacePolicyOnline, October 23, 2026). In the Blue Moon version, three transfer stages on New Glenn push the lander to lunar orbit while Orion flies straight to the Moon. Each company must land uncrewed first, and NASA intends “to wait as long as possible” before choosing the provider that carries the first crew back (CBS News, October 23, 2026). NASA will also accept a lighter “sortie” suit with about two hours on the surface (SpacePolicyOnline, October 23, 2026).

The destination is unsettled. Kshatriya conceded that NASA’s knowledge of the south pole is thin, while Isaacman called it “probably the most advantageous area” for ice in permanently shadowed terrain, and a National Academies report in mid-October identified four non-polar sites with scientific value (SpacePolicyOnline, October 23, 2026). NASA’s Aerospace Safety Advisory Panel objected a day earlier to requiring only one uncrewed landing per provider and to putting cryogenic propellant on the Moon for the first time (SpacePolicyOnline, October 22, 2026). China is aiming for a crewed landing by 2030 (CBS News, October 23, 2026).

The propulsion problem underneath is arithmetic. Orion’s service module carries 8,600 kg of usable MON-3/MMH propellant and flies one AJ10 main engine at 26.6 kN, eight R-4D thrusters and 24 smaller reaction control units (Wikipedia, European Service Module). A direct rendezvous profile asks that stack to leave Earth, arrive at the Moon and come home: roughly 3,100 m/s for the translunar injection, a few hundred for lunar orbit insertion, about 900 m/s to come back, close to 4,800 m/s in total. The rocket equation says what that costs: Δv = v_e * ln(m_0 / m_f), with v_e = Isp * 9.80665. At a generous 300 seconds of specific impulse, exhaust velocity is about 2,940 m/s, so 4,800 m/s demands a mass ratio near 5, and Orion would have to burn four-fifths of its own mass.

The cheap way to buy delta-v is not a bigger spacecraft but a cryogenic stage, because hydrogen’s chemical energy per kilogram is roughly three times that of the hypergolic mix and empty tanks are cheap. The Interim Cryogenic Propulsion Stage that flew Artemis II shows the arithmetic: a single RL10 at 110.1 kN, specific impulse 462 seconds, gross mass 32,748 kg, empty mass 3,800 kg, so Δv = 462 * 9.80665 * ln(32,748 / 3,800) comes to about 9,700 m/s, roughly double the whole cislunar budget (Wikipedia, Interim Cryogenic Propulsion Stage). NASA is now working with United Launch Alliance on an extended Centaur V to fly with the SLS core stage, so the launch vehicle can inject Orion to low lunar orbit and leave the service module’s small tank for the lunar leg and the return (Ars Technica, October 24, 2026).

The orbit where the vehicles meet matters as much as the propulsion. NASA’s charts show Orion and the Blue Moon lander joining in an elliptical lunar orbit with an apolune near 8,500 km, which lets a lander with limited propellant reach the surface; Parsons said a low lunar orbit a few hundred kilometres up would cut the lander’s propellant requirement substantially (Ars Technica, October 24, 2026; Wikipedia, Artemis IV). Docking adds its own problem: Orion is the active vehicle, the other side passive, and with Starship the mass difference is roughly an order of magnitude, so a closing speed of a few centimetres per second is the difference between a soft mate and a bent structure. Isaacman’s summary of the fix was blunt: reaching low lunar orbit means “solving propulsion and spacecraft integration problems that were left unaddressed for far too long” (Ars Technica, October 24, 2026).

The most useful thing NASA did on October 23 was turn a landing into a list of things that can be checked, in an orbit where a failed rendezvous ends in a re-entry within hours instead of a rescue measured in days. What Artemis III will not settle is the hardest part: the test articles are stripped of the systems that make the real flight hard, with no main engine on the Blue Origin vehicle and no life support aboard Starship. Those are the 2028 problems, and they will be solved, if at all, by the two companies racing each other to be ready. The long-lead hardware keeps renewing in the meantime: on October 21 an RS-25 fired for 500 seconds on the Fred Haise Test Stand at Stennis, reaching 111 percent of the thrust needed to launch SLS (NASA, October 21, 2026). Apollo was a state program with one rocket and one schedule. Artemis is a procurement program that has to fit itself around two private launch cadences, and the first way NASA tests that arrangement is to launch three rockets and see whether they can find each other.

Valles Marineris runs nearly 3,900 kilometres across Mars, reaches 600 kilometres wide in places, and drops more than nine kilometres from rim to floor. NASA calls it the largest canyon system in the solar system, and by the numbers it is not a close call: nearly nine times longer than the Grand Canyon, more than twenty times wider, roughly five times deeper (NASA, NASA Science). For most of the last two decades it has been treated as scenery, a feature that formed in the first billion years of Martian history and has been sitting quietly ever since. A paper published in Nature Communications on 15 October makes that assumption harder to hold. Jung-Hun Song of Imperial College London and colleagues report direct seismic evidence that the ground deep beneath the canyon is still moving, and that the structure controlling where it moves was inherited from the canyon’s own formation billions of years ago (Nature Communications).

The evidence comes from a single earthquake. On sol 976 of the InSight mission, 25 August 2021, the lander recorded an event its catalogue calls S0976a. It was unusual in two ways that made it useful. It arrived from the far side of the planet, roughly 146 degrees away, more than any other well-located marsquake InSight had found, and it was the only event in the mission’s catalogue that could be pinned to Mars’s western hemisphere. It was also puzzling. A seismic signal on Mars does not automatically mean a fault broke. Meteoroid impacts, landslides, and mass wasting all send vibrations through the crust, and the earlier analysis of S0976a had placed it near western Valles Marineris without settling on a cause. Nobody had shown that it was a marsquake at all, rather than something falling over.

Song’s team went back to the raw seismograms and asked what kind of motion would produce that signature. The answer turned on a problem of geometry. Working out how a fault ruptured normally requires many seismometers spread around the source, sampling the radiation pattern from many directions. InSight was one station, sitting roughly 8,000 kilometres away on the opposite side of Mars, so it sampled the pattern once, badly. The trick the team used is that seismic waves bouncing off the planet’s surface arrive as echoes, and each echo corresponds to a different angle between the wave and the surface it left. By jointly inverting the first-arriving P and S echoes, called PP and SS, the team recovered the fault geometry from a single station, validated first against a terrestrial event recorded the same way.

The solution is a predominantly double-couple strike-slip mechanism: blocks of crust sliding horizontally past each other on a steep fault. Across the acceptable solutions, 82.2 percent fall in the strike-slip family, with nodal plane strikes clustering around 48 and 143 degrees and dips steep enough to be nearly vertical. The best-fitting mechanism gives fault strikes near 60 and 330 degrees. The pressure axis sits roughly parallel to the canyon trough, the tension axis roughly perpendicular to it, which reads as horizontal extension across the canyon combined with shear along its length. Depth is the other half of the answer. The source lies in the middle to lower crust, above a Moho estimated at 55 to 65 kilometres beneath western Valles Marineris, with the allowed depth range running from about 20 to 40 kilometres depending on which interior velocity model is used. That is intact crust, not the loose material at the surface.

Rejecting the alternatives took as much work as finding the mechanism. The team ran impact-like explosions, single-force landslide models, and combinations of double-couple and non-double-couple sources, comparing each against the observed waveforms. None of the impact or landslide models reproduced the PP and SS arrivals. A landslide of the size needed to generate an event this big on Mars would have required a failure volume of at least 10 million cubic metres, which should have left a fresh deposit hundreds of metres to a kilometre across. The team checked every available post-event Context Camera image from the source region and found no fresh crater and no new collapse. The seismic data and the orbital imagery pointed the same way, which is what makes the result convincing rather than merely interesting: two kinds of evidence converging on one answer.

The geology says something the seismology alone cannot. Western Valles Marineris is crisscrossed by long-lived fault systems, and the stress orientation from the focal mechanism matches what those structures would experience today. Song’s team mapped faults, grabens, wrinkle ridges, and boulder-fall sites from high-resolution imagery and found a bimodal population of grabens with dominant strikes near 15 and 115 degrees, defining an orthogonal fault set. The P and T axes from the earthquake line up with those dominant trends. The best interpretation is slip on inherited fabric, not the creation of something new: the fault that broke in 2021 was already there, and the present-day stress field is now favourable for it to slip again.

What supplies that stress is a more open question, and the honest answer is a combination. Three sources are invoked. Long-term planetary cooling supplies a background horizontal compression, which no amount of waiting makes go away. Tharsis, the enormous volcanic rise west of the canyon, loads the lithosphere and flexes it, modifying both the magnitude and the orientation of the horizontal principal stresses. And the canyon itself, having been deeply incised, has unloaded the crust vertically, which reduces vertical stress and produces horizontal compression. At the source depth the basaltic crust carries lithostatic pressures of several hundred megapascals, a confining pressure that suppresses tensile opening and favours brittle shear failure instead. The paper is explicit that the relative contributions of flexural loading and dynamic support cannot be separated uniquely from this event alone.

Two results change what the data means beyond this canyon. The first is the aftershock question, and the answer is that InSight would not have seen one. The authors modelled aftershock production and wave propagation and found that ground displacements from a marsquake in western Valles Marineris would rarely exceed the ambient noise level at the lander, a consequence of geometric limits on long-distance propagation across Mars, including core shadowing. The probability of detecting even a single sizeable aftershock within 50 sols is 0.1 percent, which is a strange thing to have calculated, and I am glad they ran the numbers. The absence of follow-up events in the catalogue therefore says nothing about whether the fault is quiet, which is a limit of the instrument rather than of the planet. The second is the comparison across three regions. InSight’s seismograms now resolve three distinct styles of present-day deformation on Mars: strike-slip faulting beneath Valles Marineris from S0976a, compressional faulting in southern Elysium Planitia from S1222a, and normal faulting beneath Cerberus Fossae. Three different mechanisms in three different places means deformation is partitioned across a mechanically heterogeneous shell rather than driven by one planet-wide stress pattern.

The broader implication is about stagnant-lid planets, which is most rocky worlds. Mars has no plate tectonics; its outer shell behaves more like a single lid that cools slowly rather than recycling. Whether faults formed early in a planet’s history can stay weak enough to matter billions of years later has been argued about theoretically and never directly confirmed, because the geological record alone cannot distinguish a fault that reactivated today from one that has been dead since the Hesperian. A dated, located seismic event with a fault mechanism is exactly the missing measurement. Fault zones in a stagnant lid heal slowly, and stress fields evolve slowly, so the two together can keep old structures marginally close to failure indefinitely. The motion at the bottom of Valles Marineris is happening on the same fault that was already there when the canyon opened, under a stress field that has drifted slowly since.

Song put it plainly: “faults deep beneath the canyon can still slip and generate marsquakes” (Lamont-Doherty Earth Observatory, Columbia Climate School). The InSight mission ended in 2022 and its seismometer is off, but the recordings it returned are still producing results five years on. This one is the clearest evidence yet that a planet’s oldest fractures are still part of its present.

The day the captain left the bridge

I heard the news today, and I’m still sitting with it. Sir Patrick Stewart has died at 86. To the world he was many things: a Shakespearean actor, a knight, Professor X. To me, he will always be Captain Jean-Luc Picard first.

I first met him on the bridge of the USS Enterprise. I was 14 when I first watched The Next Generation. He didn’t swagger the way TV heroes usually did. He sat down, tugged his uniform straight, listened to his crew, and then made a decision.

Something about that calm made the stars feel reachable. Picard is the reason I started reading about real space exploration: rockets, orbits, probes, the people who actually build the ships. I know I’m not the only one. Today is a loss for a whole generation who looked up because a quiet man in a red uniform told us there was something worth finding out there.

More than a captain

Patrick Stewart was born in 1940 in Mirfield, a small mill town in West Yorkshire. His childhood was difficult, and he spoke openly about it later in life. He turned that pain into decades of public campaigning against domestic violence.

He found the stage young and never really left it. In 1966 he joined the Royal Shakespeare Company, and he spent years playing kings, villains and fools in front of live audiences. That training shows in everything he did afterwards: the voice, the stillness, the way a single pause could carry a whole scene.

In 1987 he was cast as Jean-Luc Picard in Star Trek: The Next Generation. Plenty of people doubted that a bald, middle-aged British stage actor could lead an American TV franchise. He proved them wrong across seven seasons and four feature films, and the show became one of the most loved in television history.

In 2000 a new generation met him as Professor Charles Xavier in X-Men, the patient teacher who believed people who were different deserved a place in the world. He played the role for nearly two decades, ending with a heartbreaking turn in Logan in 2017. In 2010 he was knighted for services to drama.

Then, at nearly 80, he did something few actors ever get to do. He came back to Picard in Star Trek: Picard, which ran for three seasons from 2020 to 2023. We got to see our captain grow old, doubt himself, and still choose to do the right thing.

What Picard stood for

Picard was a different kind of captain. He reached for diplomacy before phasers, and he treated every new species as something to understand rather than defeat. He read Shakespeare, loved archaeology, and listened to the people around him before he gave an order.

A few moments stay with me:
• The Measure of a Man. Picard defends the android Data in a hearing over whether he is property or a person. It is a courtroom drama about what makes a mind worthy of rights, and it feels more relevant every year.
• The Inner Light. A probe lets Picard live an entire lifetime on a dying planet in what is only minutes on the Enterprise. He comes back carrying a family, a community and a flute. It may be the best hour Star Trek ever made.
• Chain of Command. Captured and tortured, Picard refuses to say he sees five lights when there are four. It is a lesson in holding on to the truth when everything pushes you to give it up.

Even his catchphrases said something about leadership. “Make it so” meant he trusted his crew to carry out the plan. “Engage” meant the thinking was done, and now we go.

Above all, Picard showed that exploration could be a mature, curious, peaceful act. The point of going out there was never conquest. It was to learn, to meet others, and to come back a little wiser.

From the bridge to the launch pad

Star Trek has always had a habit of turning viewers into scientists. Engineers, astronomers and astronauts have said for decades that the show is why they first looked up. Mae Jemison, the first African American woman in space, credited Star Trek as an early inspiration, and she later appeared in an episode of The Next Generation, sharing the screen with Picard’s crew.

That loop between fiction and reality is the part of his legacy I care about most. Picard made the future look like a place you’d want to live: a place where curiosity was a career and science was something you did together. For a lot of kids, and a lot of adults, that was enough to start asking real questions.

For me it started small. I wanted to know how a ship actually gets to orbit, why some missions take years to reach their target, and who decides where we go next. One question led to the next, and space exploration became a lasting interest rather than a TV backdrop.

The real version is slower and messier than warp drive, but it is happening. Space telescopes are looking back to the earliest galaxies, robots are roaming Mars, and crewed missions are aiming at the Moon again. None of it is the Enterprise. All of it carries a little of the spirit Picard stood for: go carefully, go together, and go to learn.

Engage

It’s strange to grieve someone you never met. But Patrick Stewart spent decades in our living rooms, and for many of us he helped shape who we became. He gave us a captain who led with his mind and his conscience, and he showed us what a decent person in charge could look like.

The best way I know to honour him is to keep doing what Picard taught me to do. Stay curious. Listen before deciding. Treat the unknown as something to understand, not something to fear. And keep looking up.

Thank you, Sir Patrick. The bridge is quieter tonight, but the course you set is still laid in.

Engage.

Image: Patrick Stewart at the 2019 San Diego Comic-Con International. Credit: Gage Skidmore, CC BY-SA 3.0, via Wikimedia Commons.

A moon usually has a simple job description: it goes around a planet. The candidate announced this week in the CD-35 2722 system breaks that shortcut. It appears to orbit a brown dwarf, while the brown dwarf itself circles a small star. The outer object has at least the mass of Jupiter. The middle object is about 37 times as massive as Jupiter, too heavy to fit the usual planet category but too light to sustain hydrogen fusion like a star, according to the European Southern Observatory’s release.

That makes the system a three-level orbiting arrangement, but “exomoon” is not quite the right label. The authors of the Nature study call it a planetary-mass exosatellite and describe the result as evidence, not a final, uncontested discovery. That caution matters. The object was not photographed as a separate dot. Researchers inferred its pull from the motion of its brown-dwarf host.

Artist’s impression of CD-35 2722. The star is at left, the brown dwarf is in the foreground, and the newly found object is near the centre. Credit: ESO/M. Kornmesser. ESO image page.

Astronomers have found more than 6,000 exoplanets, but no moon orbiting one of those planets has been confirmed without serious dispute. A planet’s moon is smaller and fainter than the planet, and the planet is already lost in the glare of its star. The geometry makes the signal hard to isolate. In CD-35 2722, the host is a brown dwarf, which is dimmer than an ordinary star and can be observed directly with instruments built for high-contrast infrared work. That changes the observing problem, even if it does not make the interpretation easy.

The result also tests how useful familiar categories are. A moon-like object is defined by the body it orbits, while a planet is often defined by its mass, formation history, or the way it dominates its orbit. Those definitions work tolerably well in the Solar System. They become awkward when a Jupiter-mass body circles a brown dwarf. Calling it a moon describes its orbit. Calling it a planet describes its scale. Calling it an exosatellite avoids pretending that one word settles the physics.

There is a practical reason to care about the label debate. If this detection holds up, it gives observers a new class of target. They can search for satellites around directly imaged brown dwarfs, where the host’s spectrum is accessible and the companion’s gravitational effect may be measurable. The sample will be small, but the method is concrete enough to test.

CD-35 2722 is a star roughly half the Sun’s mass, located about 73 light-years away in the constellation Columba. Its companion, CD-35 2722 B, was directly imaged years ago and is a brown dwarf. The ESO team describes the brown dwarf as orbiting the star on a timescale of about 5,000 years, which means that the star and brown dwarf barely move across a human observing campaign. The newly reported object is the fast-moving part of the system: it circles the brown dwarf.

The researchers used the Very Large Telescope’s CRIRES+ instrument, a high-resolution infrared spectrograph. The observing programme monitored the brown dwarf from October 2023 through January 2025, according to the open version of the paper. The aim was to measure tiny changes in the brown dwarf’s radial velocity, the component of its motion toward or away from Earth.

A satellite changes that velocity because the brown dwarf does not sit perfectly still. Both bodies orbit their common centre of mass. If the satellite is on one side of that balance point, the brown dwarf moves slightly toward us; half an orbit later it moves slightly away. The resulting pattern is periodic. In principle, it is the same basic idea used to discover the first exoplanets around Sun-like stars, applied one level down to a brown dwarf and its companion.

The analysis found a periodic signal consistent with at least one planetary-mass body orbiting CD-35 2722 B. The Nature paper reports a Jupiter-mass exosatellite candidate. “Candidate” is doing real work here. A radial-velocity curve can be distorted by instrumental systematics, activity in the brown dwarf’s atmosphere, or an incomplete model of the system. The team therefore presents the finding as strong evidence rather than an official first exomoon announcement.

The system’s arrangement is unusual even before the terminology begins. The star is the largest body. The brown dwarf is a companion that formed or evolved in a regime between planets and stars. The outer body has the mass of a gas giant but occupies the satellite role. In our Solar System, Jupiter’s moons are far less massive than Jupiter. Here, the supposed moon may be a substantial fraction of its host’s mass, so the two-body orbit is better understood as a binary pair nested inside a wider stellar orbit.

Follow-up observations will have to answer several questions. Does the signal repeat with the same period? Can independent instruments recover it? Does the inferred mass remain stable when the orbit and brown-dwarf properties are fitted together? And can the system’s long-term dynamics keep the outer body bound while the brown dwarf travels around the star? A few clean observing seasons could do more for the claim than a louder headline would.

Radial velocity does not measure an object’s mass directly. It measures the line-of-sight speed of the body being observed. The mass estimate comes from fitting an orbit to the velocity curve and combining that fit with the host mass and orbital geometry. If the orbit is viewed nearly face-on, the true companion mass can be larger than the minimum value inferred from the measured wobble. That is one reason the paper uses language such as “at least as massive as Jupiter.”

CRIRES+ helps by spreading infrared light into a detailed spectrum. Molecules in the brown dwarf’s atmosphere absorb particular wavelengths. As the dwarf moves, those absorption lines shift through the Doppler effect. The instrument tracks the shifts against a wavelength reference, while the analysis separates the brown dwarf’s motion from the Earth’s motion and from the instrument’s behaviour. The expected signal is small, so many individual spectral lines can be combined to improve the velocity estimate.

The nested orbit adds another layer. The brown dwarf’s broad, slow motion around the star is nearly constant over the observing window, while the satellite’s motion should be much faster. A useful model must account for both. It must also distinguish orbital motion from changing atmospheric patterns. Brown dwarfs are not quiet, featureless balls of gas: clouds and temperature variations can alter their spectra. A signal that survives those tests is much more persuasive than a visually pleasing curve.

Stability is a separate engineering-style constraint. The satellite must remain inside the brown dwarf’s gravitational sphere of influence, often approximated by its Hill sphere, while avoiding an orbit so close that tides or disruption become dominant. The star’s distant pull sets the outer boundary. A long-lived configuration needs room between those limits. The reported architecture remains useful even if the object is eventually reclassified: it gives dynamical modelers a real system to stress-test.

The CD-35 2722 result is best read as a careful detection claim, not as the end of the exomoon hunt. Researchers have found a periodic radial-velocity signal in the directly imaged brown dwarf CD-35 2722 B, and the signal is consistent with a companion at least as massive as Jupiter. That is enough to make the system worth following. It is not enough to erase every alternative explanation.

The interesting part is the mismatch between orbit and category. The object behaves like a moon because it circles a brown dwarf. Its mass makes it look like a planet. The brown dwarf itself occupies the middle ground between planet and star. Nature has supplied a system that forces the vocabulary to catch up with the observations.

For now, “planetary-mass exosatellite” is the honest name. More observations will decide whether it becomes the first broadly accepted exomoon-like detection, or a well-measured example of how messy planetary systems can be.