The Smallest Things Remember

0:00Hello and welcome to Astronomy Daily. It's Wednesday, the 9th of September, 2026. This is Series 5, Episode 189, and I'm Anna. And I'm Avery. Anna, what's the smallest thing anyone has ever seen out beyond Neptune? As of yesterday, about 5 kilometers across, roughly the size of a decent suburb, 4.5 billion kilometers away, in permanent twilight. And we can see that. Hubble and Webb can see that, working the same patch of sky at the same moment. One invisible light, one in the infrared, 27 brand new objects, the faintest ever directly detected out there, and the thing that makes it a lead story isn't that we found them. It's what they're wearing. Meaning what? Meaning the smallest objects in the Kuiper Belt have the same colors as the big ones. Which, if you know how those little ones are supposed to have been made, is not what you'd expect at all.

1:01They're remembering something they shouldn't be able to remember. That's our lead. After that, a volcano that grounded nearly 3,000 flights and the satellites that watched it happen. Two European spacecraft deliberately flown into the atmosphere over the South Pacific so that a team in a chartered jet could film them burning up and find out what a satellite turns into on the way down. And a galaxy that doubled in size in less than a billion years, caught by a one meter telescope. Plus the sky for both hemispheres. New Moon on Friday, which means the next few nights are as dark as September gets. Let's start out past Neptune. All right. Before the new result, set the scene. What actually is the Kuiper Belt? And why do we care so much about it? Start with what it isn't. It isn't a belt in the sense of a neat ring. And it isn't crowded. If you were standing on one object, you would almost certainly not be able to see another one.

2:02What it is is the leftovers beyond Neptune out past 30 astronomical units. There's a population of icy bodies that never got assembled into a planet. Never got the chance. Never got the chance. And that's exactly why they matter. Everywhere else in the solar system, the raw material got processed. Earth melted, Mars melted. The asteroid belt has been ground down and cooked by four and a half billion years of collisions and sunlight. Out past Neptune, it is dark. It is cold. We're talking 40, 50 degrees above absolute zero. And it is empty enough that things mostly leave each other alone. Those objects are the closest thing we have to the original building blocks, still sitting where they were made. A fossil record. A fossil record that's still in the ground. And there's a structure to it that's worth having, because the whole result turns on it. There are two broad populations out there. The first are called the dynamically cold objects.

3:05Cold meaning their orbits are calm, nearly circular, barely tilted. Those ones almost certainly formed roughly where they are now and have never been disturbed. And they're distinctive? Very. They're red. Deeply uniformly red. That's a radiated organic material on the surface, built up over billions of years. The second population is the dynamically hot objects. Elliptical orbits tilted, scattered. Those didn't form where we find them. They formed closer in and were flung outward when the giant planets migrated early on. And they're a mixed bag of colors, because they came from a range of starting distances. So color is a birth certificate. Color is roughly a birth certificate. That's the premise. Now, here's the problem we've had for 30 years. Everything I've just described, we learned from the big ones. Objects 100 kilometers across and up.

4:05Ground-based telescopes bottom out at about 25 kilometers and below that, it's guesswork. So what did they do? They pointed Hubble and Webb at the same patch of sky simultaneously. Same field, same time, one working invisible light, and one in the infrared. And the simultaneity is not a detail. It's the whole trick. These are faint objects moving against the background stars. And if you want to color, you need the two measurements taken at the same moment. Otherwise, you're comparing an object to a slightly different version of itself. And how faint are we talking? NASA's own description is the best one I've read. One of these objects is the equivalent of standing on the earth and picking out a small swarm of fireflies on the moon. That's absurd. It's absurd, and they found 27 of them. 27 previously unknown transneptunian objects, the faintest ever directly detected, and the smallest is about 5 kilometers across.

5:08That's five times smaller than anything a ground-based telescope can reach. And two papers came out of it. Two papers, both published yesterday, the 8th of September, in the Astronomical Journal. One on color, led by Anastasia Morgan, a PhD candidate at Northern Arizona University. One on the size distribution, led by Maryelle Eduardo, a PhD candidate at the University of Victoria in Canada, with the National Research Council of Canada and NASA Goddard involved across both. Start with the colors. What was the expectation? The expectation was that the small ones would look different. And there's a good reason for that expectation. Nobody thinks a 5 kilometer object out there is a pristine original. The standard assumption is that objects that small are fragments. Shrapnel. The debris left over when bigger things hit each other. And if you smash something open, you're seeing the inside. Exactly. That red surface is a rind. It's a few meters of irradiated material built up over billions of years.

6:12And underneath it, the ice is fresh and much less red. So if the small objects are collision fragments, a good fraction of them should be showing us their interiors. And the population as a whole should look bluer and messier than the big ones. And they don't. They don't. The small ones match the big ones. Within each population, the little objects carry the same color relationship as their large counterparts. Morgan's line is that the smallest objects are somehow remembering and preserving the history of how they were made. And that holds for both populations, the calm ones, and the scattered ones? That's the part that got my attention. It holds for the hot population too. The ones that were thrown outward from somewhere else. David Trilling at Northern Arizona puts it this way. These dynamically hot objects retain a signature of where they were born, even though they've been orbitally scrambled since. So you've got a 5 kilometer lump of ice on a wild tilted orbit,

7:13which has been kicked halfway across the solar system by Neptune. And it is still visibly wearing the color of the neighborhood it grew up in. Now the second paper, sizes. The size distribution is the other half of the argument. And honestly, it might be the stronger half. If you count objects by size, you get a curve. How many small ones for every big one? And the shape of that curve is a fingerprint of how the population was made. Gradual accretion. Pebbles sticking to pebbles, grinding and colliding gives you one shape. Direct rapid formation gives you another. And what did they find? The same shape in both populations. Eduardo's finding is that planetesimal formation ends up producing the same distribution of sizes for the cold objects and the hot objects, despite the fact that they formed in different regions of the disc. Different neighborhoods, same recipe. So the process doesn't care where you are. The process appears not to care where you are.

8:13And put the two papers together and you get something quite specific. These small objects are not primarily collisional rubble. They look like they were made small and have stayed that way. Which lines up with a model that's been gaining ground for about 15 years. The idea that planetesimals don't grind their way up from dust grain to boulder to mountain, but form quickly at large sizes when a cloud of pebbles collapses under its own gravity. And we've actually seen one of these up close. We have. And it's the best supporting evidence there is. New Horizons flew past Arakoth on New Year's Day 2019. A cold, classical object, two lobes resting against each other like a snowman. Nothing about it looked violent. The two halves came together at walking pace. That is what gentle in-place formation looks like. And this new work says Arakoth probably isn't a curiosity. It's the type specimen.

9:13What are the caveats? There are always caveats. Three, and they're the honest kind. Twenty-seven objects is a real detection, but it is a small sample. And everything here is a statement about populations. Not a measurement of any individual rock. Second, these are broad colors from a handful of filters, not spectra. And third, all of this is one patch of sky. A very deep patch, but one line of sight. So what fixes that? Volume. And that's the part that lands closest to home for a lot of our listeners. Because the machine built to deliver volume is in the southern hemisphere. The Vera Rubin Observatory sits on Sarah Pachone in Chile. And its whole design premise is repeatedly imaging the entire southern sky. It is expected to find trans-neptunian objects in numbers that make our current catalog look like a pilot study. Tens of thousands of them.

10:15Different job to web, though. Completely different job, and they need each other. Rubin finds them and gives you orbits. Hubble and web are what you point at the interesting ones. And there's a third piece that Australia and New Zealand happen to be very good at. Stellar occultations. You work out when a tiny object will pass in front of a background star. You put telescopes along the shadow path, and you time the blink. That's how you get a real size and shape for something you can't resolve. A lot of those shadow paths cross the southern oceans. And a lot of that work gets done by people with portable gear, standing in a paddock at three in the morning. Which is a nice place to leave it? It's a nice place to leave it. 27 new objects. The smallest ones, the size of a suburb. And they're still wearing the colors of a solar system that hasn't existed for four and a half billion years. Story 2. And it's a change of altitude. A knock crack a towel.

11:16The volcano in the Sun District between Java and Sumatra has spent the last five days doing serious damage to the aviation map of Southeast Asia. And the reason it's on this show is that the entire response ran through satellites. Give me the event first. It escalated on the 4th of September and went into its major explosive phase on the 5th, which ran for more than 24 hours before settling back into the strombolian pattern it's been in for years. At the peak, Indonesia's meteorological agency had ash going up to about 6,000 meters to the east of the volcano, and 15,000 meters to the west. That's 50,000 feet. That is well above the cruising altitude of everything flying that day. And the disruption? Eight airports closed across Java and Sumatra. Ashfall across five provinces into Jakarta and West Java.

12:182961 flights grounded and something like 170,000 travelers stranded. A decent number of them Australians, because that corridor is on the way to and from a lot of places we fly. Operations were essentially back by yesterday. So where does the space part come in? It's the whole nervous system of the response. There is a global arrangement for this. Nine volcanic ash advisory centers, each responsible for a slice of the planet. The one that covers Indonesia is the Darwin Center, run by Australia's Bureau of Meteorology. And the advisories they were issuing through the week, Plume height, direction, forecast drift, are built primarily on geostationary satellite imagery, which for that part of the world means Japan's Himawari 9, sitting over the equator and imaging the full disk of the Earth every 10 minutes.

13:1910 minutes is fast. It has to be. An ash cloud at 50,000 feet moves and it doesn't show up on aircraft weather radar. Radar is built to see water droplets and dry ash is close to invisible to it. So the only warning a crew gets is the one that comes up from the ground and the ground gets it from orbit. And there's a reason everyone treats this so seriously. There is. And it's a British airways flight in 1982 over Java as it happens. A 747 flew through an ash cloud nobody knew was there and lost all four engines. Ash melts in the hot section of a jet engine, then resilitifies as glass on the turbine blades and chokes it. That aircraft glided for 16 minutes before they got the engines restarted. Nobody was killed and the entire modern advisory system exists because of flights like that one.

14:21And NASA published imagery. This morning, our time. NASA's Earth Observatory ran it as their image of the day, the operational land imager on Landsat 8 and Veers on SwomeNPP, showing the plume and the ash fall. Different job from Himowari. The geostationary satellites give you speed, the polar orbiters give you resolution. And Anak Krakato itself has history. The name means child of Krakato. The parent volcano is the one that destroyed itself in 1883 in the loudest event in recorded history. The child grew out of the Caldera and in December 2018, one flank of it collapsed into the sea and generated a tsunami that killed more than 400 people with essentially no warning. So this is a well-instrumented, closely watched mountain. And even so, the useful early day to this week came from 360,000 kilometers of sightline, not from the summit.

15:29Story 3. On the 31st of August and the 1st of September, the European Space Agency deliberately flew two of its own satellites into the atmosphere over the South Pacific, and then chartered a jet and flew a team underneath them to watch it happen. On purpose. Entirely on purpose. And it's one of the more quietly impressive things Esa has done. The satellites were Samba and Tango, two of the four cluster spacecraft. Cluster launched in 2000, four identical satellites flying information so that they could measure Earth's magnetosphere in three dimensions rather than one line at a time. 26 years of operations. It is one of the great unglamorous missions. And rather than just letting them come down. And rather than letting them come down whenever and wherever, Esa has been doing targeted re-entries, steering each one into a specific window over open ocean. Salsa went first in September 2024. Samba came down on the 31st of August this year and Tango, the last one, at 2330 and 31 seconds Central European summertime on the 1st of September, over the South Pacific a few hundred kilometers from Tonga.

16:44And that precision buys you something. It buys you the ability to put an airplane in the right place. The campaign is called Rosie, led by an international team under Yerzy Shilha, who runs a Slovak company called Astro Solutions. They flew a business jet fitted with 30 instruments, cameras and spectrometers with filters chosen for specific elements, 29 of the 30 worked. About about 50 seconds on each satellite from something like 120 kilometers away. And the description from onboard was that there was a sudden explosion as the satellites came apart. What are they actually measuring because it isn't the light show? It isn't. Two things. The first is engineering, what breaks up, when, at what altitude and what survives. That feeds directly into the models used to decide whether a spacecraft can be allowed to re-enter uncontrolled at all. The second is the one that's becoming urgent, chemistry. A satellite doesn't disappear when it burns. It becomes vapor. And that vapor stays in the upper atmosphere.

17:47Their filters were tuned for titanium, sodium, potassium, and aluminium. And the compound they care most about is aluminium oxide. Because there is a real open question about what it does to ozone chemistry at those altitudes. And the reason that's urgent is arithmetic. It's pure arithmetic. We are launching constellations of tens of thousands of satellites with design lives of about five years. Which means that from here on, satellites re-entering the atmosphere is not an occasional event. It's a continuous process. And we are running that experiment without knowing the answer. Stain lemons, Esa's acting head of space debris, frame this week's data as being about improving re-entry models and building better satellites. Which is the polite version of saying we have been guessing. And there's a follow-up mission. There is. And it's a lovely idea. Draco, launching in 2027, is a spacecraft built for the sole purpose of destroying itself while taking notes.

18:52Over 200 sensors, four cameras, and a capsule designed to survive the breakup and transmit the recording afterwards. So instead of watching from a jet 100 kilometers away, we get the view from inside. How did Esa mark the end of it? Philippe Escubei, who has managed cluster, said that once you build something like this, you imbue it with a soul. 26 years, four spacecraft, and the last thing they did was come home in a controlled way over the emptiest ocean on Earth and teach us something on the way down. Story 4, and this one is about a galaxy you have almost certainly seen a picture of. Messier 74, the Phantom Galaxy. About 32 million light years away in Pisces perfectly face on two beautifully clean spiral arms. Webs infrared image of it when everywhere a few years back. So what's changed? It's size. A study out of the Instytuto de Astrophesica de Canadias, published in Astronomy and Astrophysics and released on the 4th of September, finds that M74 is more than twice as big as the catalog say.

20:05The galaxy we've been looking at is about 45,000 light years across. They're tracing stars out to roughly a hundred thousand. How did everyone miss half a galaxy? Because it's faint. This is the low surface brightness problem and it's one of the great quiet biases in astronomy. A galaxy doesn't have an edge. It has a point where the light drops below whatever your survey can detect and we have spent a century calling that point the edge. Ignacio Ruiz with Ignacio Trujillo and Miguel Cera Ricart went about 10 times deeper than the Sloan survey. And here's the part I like. They did it with a 1 meter telescope. 1 meter. The transient survey telescope, a 1 meter aperture and enough patience. Deep imaging isn't only about how big your mirror is. It's about how carefully you handle the sky background and the scattered light. This is a result that a very large telescope chasing very distant things was never going to go looking for.

21:11And what's actually out there? A disc of young stars. The average age in that outer region is 640 million years, which for a galaxy that's been around for billions is essentially yesterday. So M74 didn't slowly ooze outward. It grew a new outer disc fast. Triggered by what? A neighbor, UGC 1176, about 400,000 light years away, appears to have passed close about a billion years ago. Gravitational that's a stir, not a collision. It doesn't wreck the spiral. It drags gas outward and lights up star formation where there wasn't any. And the broader claim? That this is probably common and we simply haven't been able to see it. If galaxies routinely double in size in under a billion years through encounters like this one, then galaxy growth is a lot lumpier and a lot faster than the smooth picture we teach. And the evidence has been sitting in the outskirts the whole time, just below where anyone was looking.

22:16Okay, moving on to Skywatch and the headline is Darkness. New Moon falls on Friday 11th of September at 427 in the morning, Universal Time. That's 227 on Friday afternoon in Sydney. So tonight, tomorrow night and right through the weekend, you have about as dark as Sky as September gives you, wherever you are. Southern hemisphere first. From Sydney and anywhere at similar latitudes, this is the last really good month for the center of the galaxy. As darkness falls, Scorpius and Sagittarius are high, close to overhead and the core of the Milky Way runs right through them. Under a dark sky, away from town, that band is not subtle. It has texture. It has dark lanes. And the dark lanes are dust clouds between us and the center. You do not need a telescope. You need 40 minutes with no phone screen. And after that, it starts sliding west. It starts sliding west through October. So this is the window. Planets. Venus is low in the west just after sunset and it's building.

23:22Greatest brilliancee comes on the 18th of September at magnitude minus 4.8. You may see the 22nd quoted elsewhere. That's a different definition and we're going with the 18th. Saturn is up in the east through the evening, climbing towards opposition on the 4th of October, with the rings about 7 degrees open. North America. Same dark window and it's the better half of the year for you in one specific way. The teapot. Sagittarius sits low in the south after dusk and the teapot asterism is genuinely easy once you've seen it. NASA's own guidance for next week, the 14th to the 20th, is to use the returning crescent moon to find and terries in Scorpius first, then step across to the teapot. Follow the steam from the spout to the thickest part and you are looking at the center of the Milky Way. Lower for you than it is for us so you want a clear southern horizon, but that's the direction. Both hemispheres, morning sky. Jupiter and Mars before dawn, both. Jupiter is unmistakable and closing on regulus through the month. Mars is fainter and lower and takes a bit more work.

24:33And a date for the calendar in both hemispheres. The 19th is international observe the moon night, which is a good excuse to point anything you own at the terminator. The line between lunar day and night, where the shadows are long and the craters look three-dimensional. And the equinox. The 22nd. Spring here, autumn there, and day and night close to equal everywhere. Looking further ahead, the 6th of October brings a pre-dawn lunar occultation of Jupiter. And Sky and Telescope have been billing that one as the spectacular event of the year. We'll build to it properly closer to the time. Safety line before we go. It matters this fortnight because Venus is brilliant and low in the west, and every year around now people get the idea of trying to find it in daylight. It is genuinely possible, and it is one of the easiest ways to hurt yourself in this hobby. Never sweep the sky near the sun with binoculars or a telescope.

25:34You can be on the sun before you know your near it, and unfiltered, that is permanent damage in less than a second. If you're going to look anywhere near the sun, the standard is ISO 12312-2. That's the specification for certified solar viewers and eclipse glasses. Sunglasses are not that, stacked sunglasses are not that, exposed film and smoked glass are not that. Check the certification, check the filter for scratches and pinholes, and if it's a telescope filter, it goes on the front of the instrument, never the eyepiece end. Wait for it to get properly dark and let Venus come to you. Wait for it to get properly dark. It'll be the brightest thing in the western sky, and it will not be hard. And that's astronomy daily for Wednesday 9 September. 27 of the faintest objects ever seen beyond Neptune, still wearing the colors they were born with. A volcano tracked from orbit while 3000 flights waited. Two European spacecraft flown home over the Pacific and filmed on the way down.

26:39And a galaxy that turns out to be twice the size we thought. Every paper and release we've mentioned is linked in the show notes, along with the full episode transcript. That's astronomy daily.io, the whole back catalog is there, the newsletter if you'd rather read than listen, and the contact form, which is where a lot of our best questions come from. You'll find us on X, Facebook, Tumblr, Instagram, TikTok and YouTube at AstroDailyPod, and of course, wherever you get your podcasts. A rating genuinely helps other people find us. We're back tomorrow. Until then, keep looking up. And if you're anywhere dark this week, go and look at the middle of the galaxy while it's still overhead. Clear skies everyone. The astronomy day, the star is the toe.