The Smallest Things Remember

0:00 Anna: Hello and welcome to Astronomy daily. It's Wednesday the 9th of September 2026. This is series five, episode 189. And I'm Anna. Avery: And I'm Avery. Anna, uh, what's the smallest thing anyone has ever seen out beyond Neptune? Anna: As of yesterday, about five kilometres across, roughly the size of a decent suburb four and a half billion kilometres away in permanent twilight. Avery: And we can see that. Anna: Hubble and Webb can see that working the same patch of sky at the same moment. One invisible light, one in the infrared.

0:38 27 brand new objects, the faintest ever directly detected out there. And the thing that makes it a lead storey isn't that we found them, it's what they're wearing. Avery: Meaning what? Anna: Meaning the smallest objects in the Kuiper Belt have the same colours 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. They're remembering something they shouldn't be able to remember. Avery: That's our lead. After that, a volcano that grounded nearly 3,000 flights.

1:10 And the satellites that watched it happen. Anna: 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. Avery: And a galaxy that doubled in size in less than a billion years, caught by a 1 metre telescope plus the Anna: 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.

1:42 Avery: Alright? Before the new result set the scene, what actually is the Kuiper Belt and why do we care so much about it? Anna: Start with what it isn't. It isn't a belt in the sense of a neat ring, and it isn't crowded. It. If you were standing on one object, you would almost certainly not be able to see another one. What it is, is the leftovers. Beyond Neptune, out past, uh, 30 astronomical units, there's a population of icy bodies that never got assembled into a planet. Avery: Never got the chance.

2:15 Anna: 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.

2:48 Avery: A fossil record A fossil record that's Anna: 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. Cold meaning their orbits are calm, nearly circular, barely tilted. Those ones almost certainly formed roughly where they are now and have never been disturbed. Avery: And they're distinctive. Anna: Very. They're red, deeply, uniformly red. That's irradiated organic material on the surface, built up over billions of years.

3:28 The second population is the dynamically hot objectselliptical 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 colours because they came from a range of starting distances. Avery: So colour is a birth certificate. Anna: Colour 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.

4:02 Objects a hundred kilometres across and up. Ground based telescopes bottom out at about 25 kilometres. And below that, it's guesswork. Avery: So what did they do? Anna: They pointed Hubble and Webb at the same patch of sky simultaneously. Same field, same time. One working in visible 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 a colour, you need the two measurements taken at the same moment, otherwise you're comparing an object to a slightly different version of itself.

4:40 Avery: And, um, how faint are we talking? Anna: 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. Avery: That's absurd. Anna: It's absurd. And they found 27 of them. 27 previously unknown trans Neptunian objects, the faintest ever directly detected and the smallest is about five kilometres across. That's five times smaller than anything a ground based telescope can reach. Avery: And, um, two papers came out of it.

5:15 Anna: Two papers, both published yesterday, 8th September in the Astronomical Journal. One on colour, led by Anastasia Morgan, a PhD candidate at Northern Arizona University. One on the size distribution, led by Marielle Eduardo, a PhD candidate at the University of Victoria in Canada, with the National Research Council of Canada and NASA Goddard involved, Across both. Avery: Start with the colours. What was the expectation? Anna: The expectation was that the small ones would look different. And there's a good reason for that expectation.

5:49 Nobody thinks a five kilometre 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. Avery: And if you smash something open, you're seeing the inside. Anna: Exactly. That red surface is a rind. It's a few metres of irradiated material built up over billions of years, and 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.

6:23 And the population as a whole should look bluer and messier than the big ones. Avery: And, um, they don't. Anna: They don't. The small ones match the big ones. Within each population, the little objects carry the same colour 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. Avery: And that holds for both populations, the calm ones and the scattered ones. Anna: That's the part that got my attention.

6:53 It holds for the hot population too, the ones that were thrown outward from somewhere else. David Trilling at Northern Arizona puts it this 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 five kilometre lump of ice on a wild tilted orbit which has been kicked halfway across the solar system by Neptune, and. And it is still visibly wearing the colour of the neighbourhood it grew up in. Avery: Now, the second paper sizes.

7:24 Anna: 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. Avery: And what did they find? Anna: The same shape in both populations. Eduardo's finding is that planetesimal M 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.

8:05 Different neighbourhoods, same recipe. Avery: So the process doesn't care where you are. Anna: The process appears not to care where you are. And 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.

8:40 Avery: And we've actually seen one of these up, uh, close. We have. Anna: 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. Avery: What are the caveats? There are always caveats.

9:18 Anna: Three, and they're the honest kind. 27 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 colours 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. Avery: So what fixes that? Anna: 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.

9:57 The Vera Rubin Observatory sits on Cerro Pachon 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 catalogue look like a pilot study. Tens of thousands of them. Avery: Different job to Webb, though, completely different job. Anna: And they need each other. Rubin finds them and gives you orbits. Hubble and Webb are what you point at, the interesting ones. And there's a third piece that Australia and New Zealand happen to be very good stellar occultations.

10:34 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. Avery: Which is a nice place to leave it. Anna: It's a nice place to leave it. 27 new objects, the smallest ones the size of a suburb.

11:05 And they're still wearing the colours of a solar system that hasn't existed for four and a half billion years. Avery: Storey2 and it's a change of altitude. A knock. Krakatow, the volcano in the Sunda Strait 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. Anna: Give me the event first. Avery: It escalated on 4 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.

11:49 At the peak, Indonesia's Meteorological Agency had ash going up to about 6,000 metres to the east of the volcano and 15,000 metres to the west. That's 50,000ft. That is well above the cruising altitude of everything flying that day. Anna: And the disruption. Avery: Eight airports closed across Java and Sumatra. Ash fall across five provinces, into Jakarta and West Java. 2,961 flights grounded and something like a hundred and seventy thousand travellers stranded, a decent number of them Australians, because that corridor is on the way to and from a lot of places we fly.

12:34 Operations were essentially back by yesterday. Anna: So m. Where does the space part come in? Avery: It's the whole nervous system of the response. There is a global arrangement for this. 9 volcanic ash advisory centres, each responsible for a slice of the planet. The one that covers Indonesia is the Darwin Centre, 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.

13:10 Which for that part of the world means Japan's Himawari nine sitting over the equator and imaging the full disc of the earth every 10 minutes. Anna: 10 minutes is fast. Avery: It has to be. An ash cloud at 50,000ft 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. Anna: And there's a reason everyone treats this so seriously.

13:47 Avery: There is, and it's a British airways flight in 1982 over Java. As it happens, a, uh, 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 resolidifies 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. Anna: And NASA published imagery this morning our time.

14:27 Avery: NASA's Earth Observatory ran it as their image of the day. The operational land imager on Landsat 8 and veers on Suomi NPP showing the plume and the ashfall. Different job from Himawari. The geostationary satellites give you speed, the polar orbiters give you Resolution and Anak Anna: Krakatau itself has history. Avery: The name means child of Krakatau. 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.

15:16 So this is a well instrumented, closely watched mountain. And even so, the useful early data this week came from 360,000 kilometres of sight line, not from the summit storey 3. Anna: On 31 August and 1 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.

15:52 The satellites were Samba and Tango, two of the four Cluster spacecraft. Cluster launched in 2004. Identical satellites flying in formation 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. Avery: And rather than just letting them come Anna: down, rather than letting them come down whenever and wherever, ESA has been doing targeted reentries, steering each one into a specific window over open ocean.

16:26 Salsa went first in September 2024. Samba came down on 31 August this year, and tango, the last one at 23, 30 and 31 seconds, Central European Summer Time on 1 September over the South Pacific, a few hundred kilometres from Tonga. Avery: And that precision buys you something. Anna: It buys you the ability to put an aeroplane in the right place. The campaign is called Rosie, led by an international team under Jirzi 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.

17:06 29 of the 30 worked. They got about 50 seconds on each satellite from something like 120 kilometres away. And the description from onboard was that there was a sudden explosion as the satellites came apart. Avery: What are they actually measuring? Because it isn't the light show, it isn't two things. Anna: The first is engineering what breaks up when at, uh, 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.

17:37 The second is the one that's becoming urgent chemistry. A satellite doesn't disappear when it burns, it's. It becomes vapour. And that vapour stays in the upper atmosphere. Their 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. Avery: And the reason that's urgent is arithmetic. Anna: It's pure arithmetic. We are launching constellations of tens of thousands of satellites with design lives of about five years.

18:14 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. Stane Lemons, ESA's acting head of Space debris, framed 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, uh, there's Avery: a follow up mission. Anna: 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.

18:53 Over 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 kilometres away, we get the view from inside. Avery: How did isa mark the end of it? Anna: Philippe Escoube, 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.

19:27 Avery: Storey four. And, um, 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. Webb's infrared image of it went everywhere a few years back. Anna: So what's changed its size? Avery: A study out of the Instituto de Astrophica de Canarias, published in Astronomy and Astrophysics and released on 4th September, finds that M M74 is more than twice as big as the catalogues say.

20:06 The galaxy we've been looking at is about 45,000 light years across. They're tracing stars out to roughly a hundred thousand. Anna: How did everyone miss half a galaxy? Avery: Because it's faint? This is the low surface brightness problem and it's one of the great quiet biases in astronomy. A, uh, galaxy doesn't have an edge, it has a point where the light drops below whatever your survey can detect. And, and we have spent a century calling that point the edge. Ignacio Ruiz with Ignacio Trujillo and Michele Sarah riccart went about 10 times deeper than the Sloan survey.

20:43 And here's the part I like. They did it with a one metre telescope. Anna: One metre. Avery: The Transient Survey telescope. A, uh, one metre 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, and the scattered light. This is a result that a very large telescope chasing very distant things was never going to go looking for. Anna: And what's actually out there? Avery: A, uh, 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.

21:25 So M, M74 didn't slowly ooze outward. It grew a new outer disc, fast Anna: triggered by what a neighbour. Avery: UGC 1176, about 400,000 light years away, appears to have passed close about a billion years ago. Gravitationally, 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. Anna: And the broader claim that this is Avery: 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.

22:10 And the evidence has been sitting in the outskirts the whole time, just below where anyone was looking. Anna: Okay, moving on to Skywatch. And the headline is Darkness. New Moon falls on Friday the 11th of September at 4:27 in the morning, Universal Time. That's 2:27 on Friday afternoon in Sydney. So tonight, tomorrow night and right through the weekend, you have about as dark a 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 centre of the galaxy as darkness falls.

22:48 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 centre. You do not need a telescope. You need 40 minutes with no phone screen. Avery: And after that it starts sliding west, Anna: it starts sliding west through October. So this is the window. Venus is low in the west just after sunset and it's building.

23:22 Greatest Brilliancy comes on the 18th of September at magnitude -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. Avery: North America, same dark window. Anna: 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.

23:56 NASA's own guidance for next week, the 14th to the 20th, is to use the returning crescent Moon to find Antares in Scorpius first, then step across to the teapot, follow the steam from the spout to the thickest part and. And you are looking at the centre of the Milky Way lower for you than it is for us. So you want a clear southern horizon, Avery: but that's the direction both hemispheres morning Anna: sky, Jupiter and Mars before dawn both. Jupiter is unmistakable and closing on Regulus through the month.

24:29 Mars is fainter and lower and takes a bit more work and 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. Avery: And um, the equinox the 22nd. Anna: Spring here, autumn there and day and night close to equal everywhere. Looking further ahead, 6 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.

25:12 We'll build to it properly closer to Avery: the time safety line before we go. Anna: 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. You can be on the sun before you know you're near it and unfiltered, that is permanent damage in less than a second if you're going to look anywhere near the sun.

25:44 The standard is ISO 123122. 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 cheque the certification cheque, 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. Avery: Wait for it to get properly dark and let Venus come to you. Anna: Wait for it to get properly dark. It'll be the brightest thing in the western sky.

26:17 And it will not be hard. Avery: And that's Astronomy daily for Wednesday 9th September. Anna: 27 of the faintest objects ever seen beyond Neptune, still wearing the colours they were born with. A volcano tracked from orbit while 3,000 flights waited. Two European spacecraft flown home over the Pacific and filmed on the way down. And a galaxy that turns out to be twice the size we thought. Avery: Every paper and release we've mentioned is linked in the show notes, along with the full episode transcript.

26:50 Anna: That's astronomydaily IO, the whole back catalogue 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. Avery: You'll find us on X, Facebook, Tumblr, Instagram, TikTok and YouTube @astrodaily pod, and of course, wherever you get your podcasts, A, uh, rating genuinely helps other people find us. Anna: 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.

27:25 Avery: Clear skies, everyone.