The two Voyager spacecraft, launched in 1977, each carry a gold-plated copper phonograph record holding sounds and images of Earth. On the aluminium cover of each record is a detail that gets less attention than the music: a small sample of uranium, put there so that whoever finds it can work out how long it has been travelling.
The claim that it functions as a built-in clock is true.
What needs correcting is the timescale, which is far longer than the figure usually attached to it.
How the clock works
The cover carries an ultra-pure source of uranium-238, electroplated onto a patch of the surface about two centimetres across rather than pressed in, with a radioactivity NASA puts at about 0.00026 microcuries. Uranium-238 is radioactive, and it decays at a fixed, well-measured rate. Half of any quantity of it turns into other elements over about 4.51 billion years, a span known as its half-life.
That steady decay is what makes it a clock. A finder who measures how much uranium-238 is left, against how much of the decay product has built up beside it, can calculate how long the process has been running. That figure is the time since the sample was placed on the spacecraft, which is to say the age of the record.
It is an elegant choice, because it needs no shared language and no working knowledge of human history. Any civilisation that understands radioactive decay arrives at the same number.
Why “a billion years” undersells it
The figure commonly quoted for the record is roughly a billion years, and it is worth being clear about what that number actually describes.
A billion years is the rough estimate for how long the physical disc should survive in the near-vacuum of interstellar space, where there is little to erode it. The uranium clock is a separate matter. With a half-life of about 4.51 billion years, the sample remains readable as a clock for far longer than the disc is expected to last, and in principle for several billion years. The limit on dating the record is not the uranium running out. It is the disc itself wearing away.
So the clock does not keep time for “roughly a billion years”. It keeps time for as long as there is measurable uranium left, which is a much longer window than the object it is attached to will physically endure.
It is not the only clock on the cover
The uranium was never meant to be trusted on its own. The same cover carries a second, independent way of fixing the date, in the form of the pulsar map.
That diagram, the starburst of lines also etched on the earlier Pioneer plaques, shows the position of the Sun relative to 14 pulsars, rapidly spinning stars that flash with very regular periods. Those periods are encoded on the cover in binary. Because pulsars slow down slightly and predictably over time, a finder could compare the periods as recorded against the periods as measured, and read the gap as elapsed time.
Two clocks, built to check each other rather than to be taken on faith. If the uranium and the pulsars disagreed, that disagreement would itself be informative.
The catch is being found at all
For all the care in the dating, the harder problem is the one the record cannot solve, which is reaching anyone.
The Voyagers are not aimed at any particular star, and they are in no hurry. Voyager 1 will not pass within a couple of light-years of another star for roughly 40,000 years, and even then “pass” means an enormous distance rather than an encounter. The odds that either record is ever recovered and read are, on any honest accounting, very small.
That is the part the uranium clock quietly acknowledges. The people who built it were not betting on a finder. They engineered a way to answer a question that may never be asked, and built it to stay accurate long after the civilisation that made it is gone. The clock will keep good time whether or not anyone is ever there to read it.
On 3 February 1966, a 58-centimetre Soviet sphere wrapped in airbags struck the dust of Oceanus Procellarum, bounced across the Moon, rolled to a stop, opened four petals for stability, and began sending the first photographs ever taken from the surface of another world.
The pictures did not first reach most readers through Pravda, TASS, or Soviet television. They reached them through a British newspaper scoop, because the signal from Luna 9 was unencrypted, Jodrell Bank Observatory in Cheshire was listening, and the image format looked close enough to the Radiofax technology used by newspaper picture desks.
The landing that had to bounce
Luna 9 was the Soviet Union’s first successful lunar soft-landing after a string of failed attempts. Earlier spacecraft had missed the Moon, struck it too hard, or failed before they could complete the descent. This one used a blunt solution: slow the main spacecraft, eject the small landing capsule just above the surface, let airbags absorb the impact, and allow the sphere to bounce until the Moon stopped it.
It worked. NASA’s archive describes Luna 9 as the first spacecraft to make a survivable landing on the Moon and return photographs from its surface, in the Oceanus Procellarum region in February 1966. The capsule was small enough to disappear in a landscape of rocks and shadows, but for a few days it became the most important camera in the Solar System.
The landing solved a technical problem and a psychological one. For years, some scientists had worried that a spacecraft might sink into a deep layer of fine lunar dust. Luna 9’s pictures showed a firmer, rock-strewn plain: rough, cratered, and hostile, but solid enough to hold a machine.
Why Jodrell Bank was already listening
Jodrell Bank was not a passive bystander in the Space Race. The University of Manchester observatory had tracked Sputnik, followed Soviet and American spacecraft, and built a reputation for listening to missions whose governments did not always announce what they were doing in real time.
When Luna 9 reached the Moon, Sir Bernard Lovell’s team was tracking it with the great radio telescope in Cheshire. Jodrell Bank’s own history notes that the observatory received the first images transmitted from the surface of the Moon by the Soviet probe in 1966.
The crucial detail was the shape of the signal. The images were coming down line by line in a form that resembled the Radiofax technology newspapers used to transmit photographs between offices. The Soviets had built a lunar imaging system, but the receiving problem suddenly looked like something a newspaper picture desk already knew how to solve.
The newspaper machine that read the Moon
The Daily Express sent a portable Muirhead picture receiver to Jodrell Bank. The Science Museum Group, which holds the D-700 photographic facsimile receiver associated with the episode, describes it as the machine used to publish the first photographs taken from the surface of the Moon in 1966.
The process was almost absurdly earthly. A signal recorded from a Soviet lunar lander was played through a newspaper facsimile receiver in England, and a picture of the Moon slowly appeared as lines on paper.
The Daily Express published the images on 4 February 1966 before the Soviet authorities made their own official release. The National Air and Space Museum notes that Jodrell Bank’s intercepted version beat the Soviets to publication, although the quality was not ideal.
There was a technical reason for that imperfection. The British receiver was close enough to decode the image, but not perfectly matched to the Soviet geometry. The first newspaper readers to see the Moon from ground level saw a slightly distorted version of it.
What the first surface pictures showed
The pictures were not spectacular in the modern sense. They were grainy black-and-white panoramas, built from scan lines, with nearby rocks, a low horizon, and the hard geometry of sunlight on an airless plain.
But they carried information no telescope on Earth could provide. The rocks had scale. The shadows had shape. The horizon looked close because the Moon is smaller than Earth, and there was no atmosphere to soften the blackness behind the stones.
Luna 9 operated on batteries and lasted only a few days. The National Air and Space Museum says the lander survived long enough to transmit a panorama from very close to the surface, while other mission summaries describe several image transmissions during its brief surface life.
That was enough. Four months later, NASA’s Surveyor 1 made the first American soft landing on the Moon, touching down on 2 June 1966. By then, Luna 9 had already shown that the surface could hold a spacecraft.
The older Soviet camera trick behind it
Luna 9 was not the first time Soviet engineers had turned an impossible-looking imaging problem into a working machine. In October 1959, Luna 3 became the first spacecraft to return pictures of the Moon’s far side, a story that belongs to the same lineage of capture, scan, and radio transmission.
That earlier probe used film, developed it onboard, scanned the result, and transmitted it back to Earth. As Space Daily has covered in its Luna 3 feature, the pictures were crude, but they redrew the map of the Moon.
Luna 9 brought that tradition down to the surface. Instead of showing a hidden hemisphere from space, it showed the ground itself: the place future landers, rovers, and astronauts would have to touch.
The lander that is still missing
After the batteries died, Luna 9 became a silent object somewhere west of Reiner and Marius craters in Oceanus Procellarum. Its reported coordinates were approximate, and the lander was only about the size of a beach ball. From orbit, it could hide among natural rocks, shadows, and small craters.
That uncertainty is the reason the recent search should be handled carefully. A 2026 paper in npj Space Exploration by Lewis J. Pinault, Ian A. Crawford, and Hajime Yano reported several candidate artificial-looking features near the historically uncertain Luna 9 landing area, using a machine-learning system trained on Lunar Reconnaissance Orbiter Camera imagery of known Apollo landing sites.
The paper did not prove the lander has been found. It identified promising candidates near 7.03 degrees north and 64.33 degrees west, and said confirmation would require targeted follow-up imaging under better conditions. Other search efforts have proposed different candidate sites, which means the honest version is still unresolved.
Luna 9 remains different because of what it sent back before it vanished. It was not a rover trail, a reflector, or a flag. It was the first low horizon from another world, stretched through a newspaper fax machine, printed in ink before the country that landed it had finished controlling the story.
The lander is still somewhere in the Ocean of Storms, its batteries dead since February 1966, its petals open to vacuum. If one of the candidate sites is confirmed, the object will not have changed much. There is no rain on the Moon, no wind, no weather to fold the petals shut. The first camera station on another world is still sitting in the dust that made it famous.
A small space telescope concept has been proposed to settle one of the most stubborn arguments in exoplanet science: why the galaxy seems to skip over worlds that are about 1.8 times the size of Earth. The proposed mission, called the Early eVolution Explorer (EVE), would stare at young star clusters for 2.5 years and try to catch planets while they are still being assembled — before the physics that carves the so-called radius valley has finished its work.
The mission concept, detailed in a recent arXiv preprint led by George Zhou and collaborators, has been pitched as a NASA Small Explorers (SMEX) candidate. It remains unfunded.
The valley that shouldn’t be empty
Of the thousands of exoplanets confirmed since the early 1990s, a strange gap shows up in the size distribution. Planets a little larger than Earth are common. Planets a little smaller than Neptune are common. Worlds right in between — about 1.6 to 1.8 Earth radii — are oddly rare. Astronomers have called this the radius valley, and they have spent more than a decade arguing about what creates it.
Two explanations dominate. The first, the shrinking gas-dwarf hypothesis, says these planets all start out puffy, with thick hydrogen and helium envelopes. Intense radiation from their young host stars strips those envelopes away. Planets that lose their atmospheres shrink into the super-Earth pile. Planets that hang on stay as sub-Neptunes. The valley is what’s left in the middle.
The second is the water-world hypothesis. It argues that these are fundamentally two different kinds of planet from birth — small rocky worlds on one side, and dense, water-rich worlds on the other, with no migration between them. A 2022 study led by University of Chicago astronomer Rafael Luque, reported in Popular Science, found that density — not radius — separated planets around red dwarf stars into three families: rocky, gaseous, and water-rich worlds that may be roughly half H2O by mass.
Both hypotheses fit the data the field currently has. That’s the problem.
Why young planets are the answer
To distinguish the two, astronomers need to look at planets before atmospheric stripping has run its course. If the gas-dwarf model is right, young planetary systems should be teeming with puffy sub-Neptunes that haven’t yet lost their envelopes. If the water-world model is right, those puffy planets shouldn’t be there at all — the population should look much like it does today, just younger.
The catch: young planets are scarce in the catalog. Young stars are noisy. They flare. They spot. They rotate fast. All of that makes it hard to pick out the tiny dip in starlight that signals a transiting planet.
EVE is designed around that noise problem.
Three sensors, one job
The telescope would carry three separate detectors covering near-ultraviolet, optical, and near-infrared wavelengths simultaneously. Stellar flares tend to brighten dramatically in the UV while barely registering in the infrared. A real transiting planet does the opposite — it produces a roughly wavelength-independent dimming. By comparing all three channels in real time, EVE’s pipeline can flag stellar activity and isolate the planetary signal.
The observing plan calls for monitoring 30 fields of young star clusters, each for 30 days, capturing light from about 20,000 newly formed stars across the 2.5-year mission. That cadence is long enough to catch multiple transits of close-in planets and short enough to fit dozens of clusters into one mission lifetime.
The numbers that will settle it
The Zhou team’s modeling produces two starkly different predictions, which is what makes the mission valuable. If the gas-dwarf hypothesis is correct and young sub-Neptunes really are puffy and abundant, EVE should find roughly 100 small young planets. If the water-world hypothesis is correct, EVE should find only about five.
That’s a 20-to-1 ratio. Few exoplanet experiments offer a discriminator that clean.
A null-ish result — five planets — would not be a failure. It would be a strong constraint, and a hint that the planets we see around mature stars were largely built the way they look, rather than carved by radiation over hundreds of millions of years. A planet-rich result would close the door on water worlds as the dominant explanation for the valley.
The funding question
EVE has been submitted into NASA’s Small Explorers line, the same program that has produced compact astrophysics missions like IXPE and SPHEREx. NASA’s 2026 Astrophysics Small Explorer Announcement of Opportunity set the framework for the current competition, with selected concepts moving through phased study before any flight decision.
The budget environment is tight. A House appropriations subcommittee in April advanced a fiscal year 2027 spending bill that held NASA’s overall funding flat at $24.438 billion, rejecting a White House proposal to cut the agency by 23%. Within that flat top line, science took a $1.25 billion reduction from 2026 levels, though the figure was well above the administration’s $3.9 billion request.
Flat funding is a softer landing than the proposed cuts, but it still squeezes the science portfolio. SMEX missions are relatively cheap by NASA standards — typically capped around $150 million plus launch — which is part of what makes them politically survivable in lean years. EVE’s modest scope and sharp scientific question are the kind of pitch the Explorer program was designed to reward.
Why this matters beyond one valley
The radius valley is a small feature on a histogram. The argument behind it is not. Whether sub-Neptunes are stripped gas dwarfs or born water-rich changes the inventory of potentially habitable real estate in the galaxy. It changes how astronomers interpret atmospheric measurements from JWST and future observatories. It changes which target stars are worth pointing at for biosignature searches.
It also changes the assumed history of every planet we’ve already found. A mature super-Earth that was once a puffy sub-Neptune is a very different object than one that has always been rocky — different interior chemistry, different volatile inventory, different odds of holding onto an atmosphere long enough for chemistry to get interesting.
Recent work on individual planets has hinted at how varied these worlds can be. JWST thermal-emission spectroscopy of the ultra-hot super-Earth 55 Cancri e, for instance, suggests the planet may have a volatile atmosphere sustained by a magma ocean — a configuration that doesn’t fit neatly into either side of the radius valley debate.
The mission’s logic
EVE is small, narrow, and built around a single question with a binary answer. That focus is a feature. Big flagship telescopes get justified by their flexibility — they can do many things passably. Explorer-class missions earn their slots by doing one thing better than anything else can.
The Zhou team’s bet is that catching planets young, in UV through near-infrared, around the noisiest stars in the sky, is something only a dedicated instrument can do well. If NASA agrees, the next several years could finally produce data that ends a debate the field has been having since the Kepler era.
If not, the radius valley keeps its secret a little longer.
On the morning of January 16, 1966, readers of Pravda opened the state newspaper to find a name they had never seen in print before: Sergei Pavlovich Korolev. Below it ran an obituary, and a photograph of a broad-faced, heavy-jawed man in a dark suit covered in medals. He had died two days earlier, on January 14, on an operating table at a Moscow hospital, age 59. For nearly a decade, the man who had launched Sputnik, who had put Yuri Gagarin into orbit, who had aimed probes at the Moon and Venus and Mars, had been referred to in Soviet communiqués only as Glavny Konstruktor — the Chief Designer. The world learned who he was after he was already in a coffin.
The secrecy had been deliberate. The Kremlin worried about Western assassination. It also disapproved, officially, of what it called the cult of personalities. So the architect of every Soviet space first remained an unnamed bureaucratic title, even as Khrushchev waved his rockets at the world.
The man behind the title
Korolev was born on January 12, 1907, in Zhitomir, in what is now Ukraine. He trained under the aircraft designer Andrei Tupolev in Moscow, qualified as a pilot, and by 1933 had launched the first liquid-fuelled rocket in the USSR, according to a Guardian profile drawing on James Harford’s biography.
Then, in 1938, secret service agents arrested him as a spy. He was beaten. He confessed, under torture, to crimes of treason and sabotage he had not committed, and was sentenced to ten years’ hard labour at Kolyma, the most notorious of the Gulag camps.
He lost all his teeth there. His jaw was broken. He may have suffered a heart attack. He survived months at the gold mine before being transferred to a prison design bureau in Moscow, where he spent the rest of the war building aircraft and rockets from behind bars.
From Kolyma to the R-7
In 1945, the Red Army made him a colonel and sent him to Germany. The Soviets had seized stores of V-2 components from the Nazis, and they needed someone who could understand them. Wernher von Braun and most of his team had already defected to the Americans with several complete rockets — a head start that would shape the entire Cold War in space.
Korolev studied what was left. He slept a few hours a night. He shouted at engineers and forgave them an hour later. In 1957, he launched the Soviet R-7, the world’s first intercontinental ballistic missile, on a flight from Baikonur to the Kamchatka peninsula. He had beaten the United States to an operational ICBM.
Weeks later, on October 4, 1957, a modified R-7 placed Sputnik 1 in orbit. Korolev had insisted on the satellite being audible — the engineers bolted on transmitters loud enough that ham radio operators around the world could pick up the beep on cheap shortwave receivers, a deliberate choice we’ve written about in the story of Sputnik’s transmitter. The point was for ordinary people to hear it. They did.
The team that built Sputnik, as NPR documented on the satellite’s 50th anniversary, did not fully grasp what they had done. The political shockwave in Washington was larger than anything they had planned for. The Chief Designer kept working.
A name held back on purpose
What followed was the most concentrated run of firsts in the history of spaceflight, all directed by a man whose existence was a state secret. Korolev’s bureau put the first dog in orbit, the first woman, the first multi-person crew. It directed the first spacewalk. It built the first Soviet spy satellite and the first Soviet communications satellite. It flew probes at the Moon, Venus, and Mars — all on a budget that Western analysts would later describe as a shoestring.
Each triumph was announced in the name of Soviet science, the Soviet people, the Soviet system. The man responsible was referred to, when referred to at all, as the Chief Designer. As National Review noted in a 2016 retrospective, the achievement was considered to belong to the collective.
The Kremlin had practical reasons too. A named chief designer was a target. He could be approached by Western intelligence. He could be assassinated. He could be poached. Anonymity was protection, even if it cost him every form of public recognition a scientist could earn.
The morning of April 12, 1961
On the dawn of April 12, 1961, at Baikonur, a peasant farmer’s son named Yuri Gagarin rose on a 20-storey rocket into low Earth orbit. The capsule was small. The pilot was one of the few selected cosmonauts who actually fit inside it.
One orbit. One hundred and eight minutes. Gagarin shouted Poyekhali! — let’s go — and a few minutes later was looking down at clouds and oceans no human eye had ever seen from that altitude. From orbit he radioed back what he saw — the Earth, the clouds, the curve of the horizon — and called the view beautiful.
Gagarin became an instant global celebrity. He toured capitals around the world. The Soviet system had its perfect ambassador, a short, smiling peasant’s son who could disarm hostile crowds in any capital.
Korolev, meanwhile, watched it all from behind the curtain. The cosmonaut he affectionately called his little eagle was a household name on six continents. The man who had designed the rocket, selected the pilot, and directed the mission was, officially, no one.
A body that wouldn’t hold
Five years of relentless work followed Gagarin’s flight. Korolev was already a damaged man — Kolyma had left him with a broken jaw, missing teeth, a weak heart, and what colleagues described as periodic bouts of exhaustion he refused to treat. He slept little. He lived frugally. He carried the entire human spaceflight program in his head and his shouted directives.
In January 1966, he checked into a Moscow hospital for what was meant to be routine surgery. During the procedure, complications arose. Korolev, whose jaw had been broken in the Gulag, could not be properly intubated. His weakened heart failed on the table. He died on January 14, 1966, at the age of 59.
The Politburo decided, this time, that the man should be named.
The Pravda of January 16 carried the announcement. The text identified Sergei Pavlovich Korolev as the academician who had led the design of Sputnik, of Vostok, of Voskhod, of the lunar and planetary probes. His photograph appeared above the fold. For most Soviet citizens, and for everyone outside the USSR, it was the first time they had ever seen his face or read his name in connection with the program.
His ashes were interred in the Kremlin wall, the highest honour the Soviet state could give. He received, in death, the public identity the system had denied him in life. Cosmonauts who had trained under him for years were finally permitted to acknowledge, on the record, who had run their program.
Wernher von Braun, his American counterpart, would learn the name along with the rest of the world. The two men had been racing each other for nearly two decades without von Braun knowing who he was racing.
What the anonymity cost
Some historians argue the secrecy may have cost the Soviet Union the Moon. Korolev had been working on the N-1, a massive lunar rocket meant to carry cosmonauts to the surface, when he died. His successor, Vasily Mishin, was a capable engineer but lacked Korolev’s authority with the political leadership and the rival design bureaus. The N-1 failed in all four of its test launches between 1969 and 1972, each time spectacularly. The program was quietly cancelled.
By then, Armstrong and Aldrin had already walked on the Moon, atop a Saturn V designed by a publicly celebrated Wernher von Braun, who had spent the 1960s on the cover of magazines and as a fixture on American television. Whether a named, internationally lionised Korolev could have held the Soviet lunar program together is one of the great counterfactuals of the space age.
Gagarin, the face the system had built up to the world, did not outlive his mentor by long. He died on March 27, 1968, in a MiG-15 training crash near the village of Novosyolovo, age 34. He had been grounded from spaceflight after the Soyuz 1 disaster killed his friend Vladimir Komarov in 1967 — the state had decided it could not risk its biggest legend. The legend of Gagarin, as Space Daily has reported on the cosmonaut’s enduring grip on Russian memory, only grew after his death.
Both men ended up in the Kremlin wall, urns of ashes set into the brick on Red Square. The peasant’s son who had become the most photographed man in the world, and the engineer whose first photograph in any newspaper was the one printed with his obituary. They lie within a few metres of each other.
The story of how the Soviet system organised its space program — secrets, hidden designers, named cosmonauts, suppressed parachute details, invented Gagarin quotes about not seeing God in orbit — has been the subject of decades of declassification and revision, including museum exhibitions that brought Soviet space artefacts to Western audiences for the first time. Pieces of Korolev’s machines now sit behind glass in cities he was never allowed to visit while alive.
His name today is on a city — Korolyov, just outside Moscow, home to the mission control centre that still flies Russian crews to the International Space Station. It is on craters, on streets, on schools. The Chief Designer has, in the end, become one of the most recognised names in Russian science.
He has been dead for sixty years. He spent fewer than nine of his 59 years as a publicly named man, and all of those nine years he was in prison, in a design bureau, or in a Kolyma mine. The face on the front page of Pravda on January 16, 1966, belonged to someone the world had been watching for a decade without knowing it. The beeps from Sputnik. The voice from Vostok. The probes drifting toward Venus. All of it had a single signature on it, and the signature was a job title.
The world finally learned the name on a winter morning in Moscow, and by then the man it belonged to was already cooling in a hospital room.
Astronomers have found a bright knot of X-rays close to the centre of the Milky Way that may be the remains of a star that exploded around seventeen centuries ago. The candidate sits inside Sagittarius C, a star-forming region about 26,000 light-years from Earth, and the case for it rests on data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton. The findings were published in The Astrophysical Journal by Zhenlin Zhu and Mark Morris of the University of California, Los Angeles, with Gabriele Ponti of Italy’s National Institute for Astrophysics and Ping Zhou of Nanjing University; a record of the paper is available via arXiv.
The word doing the heavy lifting is “possible”.
This is a supernova remnant candidate, not a confirmed one, and that distinction shapes how the whole result should be read.
What the data actually shows
The evidence is a “blob” of X-ray emission, in NASA’s own phrasing, embedded in a larger bubble of expanding gas around a young, massive star. That bubble, where radiation has stripped electrons from hydrogen, is the bright radio source known as Sagittarius C. The X-ray blob is what the team argues could be the wreckage of a massive star that collapsed and exploded.
One detail is easy to miss in the coverage. The X-ray data here is not new. It comes from five Chandra observations taken between July 2001 and August 2014, roughly 57 hours in total, combined with XMM-Newton. What is new is the analysis, not the sighting. That may be the more interesting part of the story: a result pulled from archived data that had been sitting in the record for a decade.
Why ten times brighter matters
The most concrete number in the paper is a brightness comparison. The X-ray emission from the blob is more than ten times brighter than the X-rays from large, known clusters of bright, massive stars. That figure is the team’s main reason for discounting the obvious alternative, which is that the glow comes from a crowd of hot young stars rather than an explosion. On the available evidence, the authors do not think the star-cluster explanation fits.
There is a complication they report plainly, and it cuts against the supernova reading rather than for it. A real remnant should show enhanced amounts of elements forged in the blast, things like iron, oxygen, and silicon. The team searched for that signature and did not find it. Their suggested explanation is that the debris has already mixed into the surrounding gas, which is reasonable for an older remnant.
A non-detection is still a non-detection.
The seventeen-century question
If the blob is a supernova remnant, the team estimates it is expanding at about two million miles per hour, or roughly 3.2 million kilometres per hour, and is at least about 1,700 years old. Counting back from now, “at least 1,700 years” points to an explosion around 300 AD or earlier. The “at least” is worth holding onto, because it is a floor rather than a fixed date, and the event could be considerably older.
This is not the first hint that something detonated here. Observations with NASA’s now-retired SOFIA mission had already shown an expanding shell of gas around Sagittarius C, which suggested a star had exploded in the same spot. The X-ray candidate fits that earlier clue, though fitting a prior hint is not the same as confirmation.
What confirmation would mean
The reason this is worth attention is location. If it holds up, this would be one of the closest supernova remnants ever identified to Sagittarius A*, the supermassive black hole at the galactic centre. That region is dense with massive stars, strong magnetic fields, and fast-moving clouds of gas, all of which make a clean detection harder and a confirmed remnant there especially useful for studying how stars live and die in the most crowded part of the galaxy.
What would settle it is the chemistry. Deeper X-ray observations that either find the missing elemental signature, or rule it out, would do more than another brightness measurement. For now the honest summary is the one the researchers themselves use: a strong candidate, an old prior hint from SOFIA, and a heavy-element search that came back empty. Whether the blob graduates from possible to confirmed is the thing to watch.
In March 1979, NASA’s Voyager 1 probe swept past Jupiter and, in an image taken not for science but for navigation, captured the first active volcano ever seen on a world other than Earth. The volcano was on Io, the innermost of Jupiter’s four Galilean moons. The person who found it was Linda Morabito, an optical navigation engineer at the Jet Propulsion Laboratory, who was using the picture to work out exactly where the spacecraft was.
She was not looking for a volcano. She was looking at the stars behind Io to fix the position of the probe.
The plume was in the way.
How a navigation image found a volcano
Spacecraft far from Earth can be located partly by photographing a known moon against the background stars and measuring the geometry. Voyager did this routinely. After its closest approach to Jupiter on 5 March, the probe took a set of long-exposure images of Io for exactly this purpose, the lowest-priority frames of the encounter.
On the morning of 9 March, Morabito processed one of them, taken the day before. To bring out the faint background stars she stretched the contrast of the image, and a shape appeared off the edge of Io: a large, umbrella-shaped cloud rising some 270 kilometres above the surface and catching the sunlight. Her own account describes working through the rest of the day, with colleagues, to rule out the ordinary explanations, an imaging artifact or an unknown moon behind Io. By evening the answer was clear. It was a plume from an erupting volcano, later named Pele.
When the team went back through the images Voyager had already taken, they found more. The Voyager 1 images held eight active volcanic plumes in all, on a moon no one had expected to be doing anything at all. Across the two Voyager flybys that year, the tally rose to nine erupting volcanoes.
The prediction that arrived first
Except that someone had expected it, just days earlier, and almost no one had noticed yet.
On 2 March 1979, three days before Voyager 1’s close pass of Io, the journal Science published a paper by Stan Peale, Patrick Cassen and Ray Reynolds titled “Melting of Io by tidal dissipation.” They argued that Jupiter’s gravity should flex Io enough to heat its interior and melt much of it, and they ended with a specific forecast: the consequences of a largely molten interior might be visible in the pictures Voyager was about to take.
The timing is hard to improve on. A theoretical paper predicted volcanism on Io, and within a week a navigation image confirmed it. Morabito has said she was unaware of the paper at the time, though others on the mission, including the project scientist Edward Stone, were already watching for anything that might bear it out. It is one of the cleaner examples in planetary science of a prediction and its confirmation arriving almost together.
Why Io melts
The mechanism the paper described is tidal heating. Io’s orbit is not quite circular, and it is kept that way by a steady gravitational tug-of-war with two of Jupiter’s other large moons, Europa and Ganymede, whose orbits are locked in a simple ratio with Io’s. Because the orbit is slightly eccentric, the strength of Jupiter’s pull on Io rises and falls as it goes round, and the moon is squeezed and released over and over. That constant flexing generates heat through friction inside the body, enough to keep much of Io’s interior molten.
The result is the most volcanically active body in the solar system. Io is only a little larger than Earth’s Moon, yet it carries an estimated several hundred volcanoes, with plumes that can reach hundreds of kilometres into space. Its surface is constantly remade by eruptions.
What it changed
Before Io, internal heat in a planet or moon was mostly explained by two things: leftover warmth from formation, and the slow decay of radioactive elements. Both favour large bodies, which hold their heat and have more radioactive material to begin with. A moon the size of Io should, by that reasoning, be cold and dead.
Tidal heating added a third source, one that does not depend on size but on orbit. That idea has travelled well beyond Io. It is now central to why scientists think Jupiter’s moon Europa and Saturn’s moon Enceladus may hold liquid water oceans beneath their ice, kept liquid by the same kind of flexing. The search for habitable environments in the outer solar system rests, in part, on the heat source that a navigation engineer stumbled onto in 1979.
What endures
Io is still erupting now, more than four decades on, and later missions have watched its volcanoes come and go. The first sighting still stands apart. It was not a planned observation by a dedicated instrument. It was a side effect of checking where the spacecraft was, found by someone careful enough to ask what the smudge on the edge of a moon actually was, three days after a paper had said it should be there.
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