The Hydrogen Map: How a Radio Telescope Heard Half the Universe

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Anna: Hey, everyone. Welcome back to Astronomy

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AstroDailyPod. And it's Saturday,

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so you know what that means.

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Avery: The weekend wrap one brand new storey,

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properly developed. And then we run back

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through the week's biggest news. In case you

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missed any of it,

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Anna: it's Saturday, September 5th, 2026.

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I'm Anna and this is series five, episode

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186.

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Avery: And, um, I'm Avery. Anna. Today's

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fresh storey is one I've been waiting years.

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Anna: Somebody to pull off a radio telescope in the

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Karoo desert has heard hydrogen. Not

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from one galaxy, from billions of them at

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once, 4 to 5 billion light years away.

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And it did it without any help from an

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optical telescope, which sounds modest

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Avery: until you understand that this particular

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signal is buried under a foreground about

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10,000 times brighter than it is.

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Anna: It's a technique people have been trying to

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make work for 15 years. This Week

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it worked. And it's a South African

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instrument that did it. With a strong

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Australian sequel coming.

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Avery: Then the week that was Roman

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opened its planet camera's eyes.

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Starship Flight 14 finally has a date on

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it. Mars turned out to be hotter underneath

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than anyone expected.

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Bepicolombo let go of the ride that got it to

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Mercury. A dark matter detector

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recorded one flash it cannot explain.

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Anna: And brand new this week. And genuinely

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lovely astronomers have watched a Comet

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switch on 3 billion miles away

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over five years in real time.

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Avery: Plus the sky for the week ahead. Both

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hemispheres. And it is a dark one

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in the good way.

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Anna: It's a big episode.

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Let's get into it.

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Avery: Right, start me at the beginning. Who did

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what?

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Anna: A team led by Dr. Surabh Paul. He's

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at the University of Manchester and the

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University of the Western Cape, working with

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Laura Wols at Jodrell Bank, Mario

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Santos at the Western Cape and Xiaoting

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Chen at Edinburgh. The paper is in the

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Astrophysical Journal Letters. And Manchester

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put the release out on Tuesday. It has been

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rolling through the international wires all

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week. Xinhua ran it yesterday.

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Avery: And the instrument is Meerkat.

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Anna: Meerkat 64 radio dishes standing

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in the Karoo in South Africa's Northern Cape,

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one of the quietest patches of radio sky on

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Earth. Which turns out to be the whole point.

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And Meerkat is not just a fine telescope in

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its own right. It's a precursor. It gets

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absorbed into ska mid the mid

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frequency half of the Square Kilometre Array.

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Avery: Okay, now tell me what they detected. Because

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hydrogen on its own doesn't sound like news.

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Anna: It isn't on its own. Hydrogen is

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the most common thing in the universe. The

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News is how they detected it and at what

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distance. Neutral hydrogen, a

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lone proton with a lone electron, emits

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at a very specific radio wavelength,

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21 centimetres.

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Avery: The famous 21 centimetre line.

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Anna: The famous one. And it's famous because it's

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reliable. Hydrogen doesn't care whether it's

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in a bright galaxy or a dim one. If it's

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neutral, it glows at 21 centimetres,

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which makes it, in principle, the perfect

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tracer for where matter actually is.

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Avery: In principle?

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Anna: In principle. In practise, the

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emission from any single distant galaxy is

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far too faint to pick out. So about 15

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years ago, people proposed a workaround

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called intensity mapping. Stop trying to

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resolve galaxies, point the telescope at a

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big patch of sky, deliberately blur it and

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measure the total 21 centimetre glow coming

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from that whole volume.

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Avery: So instead of a photograph of individual

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galaxies, you get what? A heat

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map.

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Anna: That's exactly the right image. A low

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resolution map of where the hydrogen is piled

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up and where it's thin. And because hydrogen

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sits inside galaxies and galaxies sit

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inside the cosmic web, that blurry map

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traces the large scale structure of the

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universe cheaply and over

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enormous volumes.

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Avery: Why does cheap matter? We have galaxy

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surveys.

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Anna: We do, and they're superb, but they're

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expensive in telescope time. To map

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structure optically, you have to identify

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each galaxy and measure its distance one at a

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time. Millions of them. Intensity

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mapping says, I don't need to know which

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galaxy is which, I only need to know how much

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hydrogen is in this cube of space versus that

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one. And if you can do that out to high

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redshift, you can measure how the universe

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has expanded, which is the dark energy

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question.

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Avery: So why hasn't anyone done it?

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Anna: Because of the foregrounds, and this is the

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part I want to be Precise about. The 21

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centimetre signal from those distant galaxies

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is extraordinarily faint. Sitting on top

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of it is radio emission from our own Milky

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Way synchrotron radiation,

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electrons spiralling in the galaxy's magnetic

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field. And that is roughly four orders of

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magnitude brighter than the thing you're

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trying to measure.

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Avery: 10,000 times.

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Anna: 10,000 times? Give or take. Then add human

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radio interference, satellites, aircraft,

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mobile networks, and then add the telescope's

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own quirks, which imprint themselves on the

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data in ways that look deceptively like

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signal. Paul's line in the release is the

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honest one. The signal is extremely faint and

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difficult to isolate from foreground

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emission, human made radio frequency

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interference and instrumental effects.

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Avery: So how have people got around that until now?

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Anna: By cheating slightly. And I mean that

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Admiringly, you take your radio map and you

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cross correlate it with an optical galaxy

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survey of the same patch of sky. You already

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know where the galaxies are from the optical

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data, so you ask, does the radio map get

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brighter in the places the optical survey

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says galaxies live?

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Avery: And if it does, that's the hydrogen.

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Anna: That's the hydrogen. It's a legitimate

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detection and Meerkat and its predecessors

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have done it before. But it has a built

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in limit. The foreground contamination

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and your instrumental noise don't know where

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the optical galaxies are. So they average

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away in the cross correlation, which is

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wonderful for confidence and useless if

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what you actually want is a standalone

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survey. You're always tethered to an

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optical telescope

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Avery: and, um, this week they cut the tether.

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Anna: This week they cut the tether.

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This is the 21 centimetre signal measured in

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the radio data alone. No

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optical survey propping it up. The

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foregrounds had to be genuinely removed

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rather than statistically dodged. And

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what's left is a real measurement of the

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hydrogen distribution.

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Avery: How much observing time did that take?

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Anna: Here's the part that made me sit up. About

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96 hours, four

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days of telescope time.

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Avery: That's nothing.

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Anna: And it gets better. Santos's quote is

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my favourite line in the whole release.

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It is particularly remarkable that the data

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used in this study were taken in 2018,

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when Meerkat had only just started science

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operations.

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Avery: Wait, the data is 8 years old?

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Anna: The data is 8 years old. This is not a new

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observing campaign. This is a brand new

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analysis of some of the first science data

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Meerkat ever took. And the advance is in

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the method, the foreground removal, the

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handling of the instrument's own systematics,

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the pipeline. The telescope was always

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capable, we weren't.

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Avery: How far back are we actually looking?

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Anna: The emission has been travelling 4 to 5

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billion years. So we're seeing the

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hydrogen as it was when the universe was

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around 9 billion years old, roughly a third

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of its present age ago, and well into the era

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when dark energy had taken over and the

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expansion was accelerating. That is

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exactly the epoch you want if you're trying

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to test how dark energy behaves over time.

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Avery: And the structures they're mapping are big,

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enormous.

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Anna: The scales involved are comparable to the gap

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between us and Andromeda millions of light

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years. Which is precisely the size range

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where the cosmic web's pattern lives.

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Avery: Alright, southern hemisphere angle. Because I

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know there is one and I know you're saving

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it.

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Anna: I am. And it's not a footnote, it's the

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entire future of this field. The

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Square Kilometre Array observatory is being

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built in two halves, both of them in the

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south. SKA Mid is going up in the

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Karoo alongside and incorporating meerkat

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itself. SKA Low is going up

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at Inuramana Ilgari Bundara, The

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CSIRO Murchison Radio Astronomy Observatory

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in Western Australia on Wajari

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Yamiji country.

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Avery: So this technique's proving ground and its

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future home are both in the Southern

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hemisphere.

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Anna: Both. And that's not an accident of

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politics. It's radio quietness

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and its geography. You cannot do

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this from a populated continent. The signal

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is too faint. You need somewhere with

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legally protected radio silence. And both

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the Karoo and the Murchison have exactly

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that. Wohls's line is the forward

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looking one. Meerkat continues to open

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new windows for cosmology and the point of a

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precursor is that everything you learn on it

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you carry across.

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Avery: So what does the SKA do with a working

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version of this?

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Anna: Surveys of a size that simply aren't

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available any other way. If

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96 hours on 64 dishes gets you a

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detection, then thousands of hours on an

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array with vastly more collecting area gets

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you a map. A three dimensional hydrogen map

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running across billions of years of cosmic

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time. Measuring the expansion history

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directly. That's a dark energy

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experiment done with radio waves from the

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southern half of the planet.

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Avery: And the honest caveat, because you always

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have one, two.

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Anna: First, this is a detection of the signal,

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not yet a precision cosmological measurement.

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The error bars are wide. And turning this

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into competitive constraints on dark energy

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is a longer road. Second,

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foreground removal is the kind of problem

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that has embarrassed radio astronomy before.

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The 21 centimetre cosmology field has had

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claimed detections walked back. The reason

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this one is being taken seriously is, is the

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cross correlation groundwork underneath it.

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They had already shown they could find the

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signal the safe way before they went looking

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for it the hard way,

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Avery: which is the right order to do things in.

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Anna: It's exactly the right order and it's why

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Paul's summary is the sentence to take away.

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Detecting it directly with meerkat shows that

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this technique is becoming a practical tool

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for cosmology. Not a promising

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idea anymore. Uh, a tool.

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Avery: Right?

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Monday to Friday, the six storeys that

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00:11:14.320 --> 00:11:17.200
mattered and three of them have moved since

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we covered them.

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Anna: We start where we ended last weekend. The

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00:11:21.280 --> 00:11:24.000
Nancy Grace Roman Space Telescope launched on

274
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Sunday, August 30th on a Falcon Heavy out of

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Launch Complex 39A. And it was

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clean, no anomalies. Straight up. Right

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on the money. We led Monday's episode with

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it, and that closed an ark we'd been building

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since 25 August.

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Avery: But it hasn't stopped being a storey.

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Anna: It hasn't. On Tuesday, NASA

282
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powered on the Roman Coronagraph instrument

283
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for the first time. It came alive between

284
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7:27 and 8:22 in the morning, Eastern

285
00:11:51.650 --> 00:11:54.169
time. That's the technology demonstration

286
00:11:54.169 --> 00:11:56.050
that blocks the light of a star so you can

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photograph the planets around it. Which is

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00:11:58.530 --> 00:12:01.490
the hard part, absurdly hard. You're trying

289
00:12:01.490 --> 00:12:04.370
to see something a billion times fainter than

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00:12:04.370 --> 00:12:06.390
the thing sitting right next to it. The

291
00:12:06.390 --> 00:12:09.190
coronagraph does it with masks, sensors

292
00:12:09.350 --> 00:12:12.190
and mirrors that flex themselves in real time

293
00:12:12.190 --> 00:12:14.830
to cancel out scattered starlight. And what

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00:12:14.830 --> 00:12:17.350
it's after is a class of planet we've barely

295
00:12:17.350 --> 00:12:19.910
photographed, worlds that are older,

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00:12:20.070 --> 00:12:23.030
colder and in closer orbits than the hot,

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00:12:23.030 --> 00:12:25.990
young super Jupiters that direct imaging

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00:12:25.990 --> 00:12:27.350
has managed so far.

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Avery: How long before it produces anything?

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00:12:30.230 --> 00:12:32.950
Anna: Months. It goes into a long calibration

301
00:12:33.030 --> 00:12:35.670
campaign and its observing is spread across

302
00:12:35.750 --> 00:12:38.490
roughly three months of time inside the

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00:12:38.490 --> 00:12:41.130
mission's first year and a half. So don't

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00:12:41.130 --> 00:12:43.850
expect pictures soon. But the instrument is

305
00:12:43.850 --> 00:12:45.810
awake. And that's the milestone.

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00:12:46.290 --> 00:12:49.250
Avery: On Tuesday, we led on Starship Flight 14.

307
00:12:49.730 --> 00:12:52.530
And the news then was that Booster 21

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00:12:52.530 --> 00:12:55.330
had cleared its 33 engine static fire,

309
00:12:55.410 --> 00:12:58.210
and Ship 41's static fire was already

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00:12:58.290 --> 00:13:01.090
done. What we could not give you was a date.

311
00:13:01.410 --> 00:13:02.450
Anna: And now there's one.

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00:13:02.850 --> 00:13:05.770
Avery: There's one with a caveat I want to put up

313
00:13:05.770 --> 00:13:08.680
front. An FCC filing points to launch

314
00:13:08.680 --> 00:13:11.520
no earlier than September 15, and the

315
00:13:11.520 --> 00:13:13.640
launch trackers have moved to that date.

316
00:13:14.040 --> 00:13:16.920
SpaceX itself has not stood up and confirmed

317
00:13:16.920 --> 00:13:19.880
it. So net the 15th

318
00:13:19.880 --> 00:13:22.840
from Pad 2 at Starbase and treat it

319
00:13:22.840 --> 00:13:25.560
as a strong indication rather than a promise.

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00:13:26.040 --> 00:13:27.320
Anna: And this is the big one.

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00:13:27.720 --> 00:13:30.560
Avery: This is the big one on two counts. It's

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00:13:30.560 --> 00:13:32.960
billed as the first genuinely orbital flight

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00:13:32.960 --> 00:13:35.870
of starship. Previous test flights have flown

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00:13:35.870 --> 00:13:38.030
trajectories that deliberately stopped short

325
00:13:38.030 --> 00:13:39.990
of orbit. So the vehicle came down

326
00:13:39.990 --> 00:13:42.790
regardless. And it carries the first ever

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00:13:42.790 --> 00:13:45.430
attempt to catch the ship itself. Not the

328
00:13:45.430 --> 00:13:47.990
booster. The upper stage, back at the

329
00:13:47.990 --> 00:13:49.470
tower, into the arms.

330
00:13:49.869 --> 00:13:52.510
Anna: They've caught boosters repeatedly now they

331
00:13:52.510 --> 00:13:52.830
have,

332
00:13:52.990 --> 00:13:55.670
Avery: and it stopped being astonishing faster than

333
00:13:55.670 --> 00:13:57.950
it should have. But the ship is a different

334
00:13:57.950 --> 00:14:00.910
animal. It comes back from orbital velocity

335
00:14:01.300 --> 00:14:03.420
through the worst of the heating, and it has

336
00:14:03.420 --> 00:14:05.740
to arrive at a precise point with enough

337
00:14:05.740 --> 00:14:08.660
control authority left to be grabbed. If that

338
00:14:08.660 --> 00:14:10.940
works on the first try, it will be one of the

339
00:14:10.940 --> 00:14:13.460
more remarkable things this vehicle has done.

340
00:14:13.780 --> 00:14:16.300
And if it doesn't, then it's a test

341
00:14:16.300 --> 00:14:18.900
flight. And that's what test flights are for.

342
00:14:19.380 --> 00:14:21.940
Ten days out, weather and paperwork

343
00:14:21.940 --> 00:14:22.580
permitting.

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00:14:22.900 --> 00:14:25.100
Anna: Wednesday's lead was the one I keep thinking

345
00:14:25.100 --> 00:14:28.000
about. A paper in nature published on

346
00:14:28.000 --> 00:14:30.440
August 27, led by Bern and

347
00:14:30.440 --> 00:14:33.040
colleagues, built out of years of accumulated

348
00:14:33.040 --> 00:14:35.080
radio tracking of three NASA

349
00:14:36.040 --> 00:14:39.000
Mars Global Surveyor, Mars Odyssey

350
00:14:39.160 --> 00:14:41.080
and the Mars Reconnaissance Orbiter.

351
00:14:41.480 --> 00:14:44.400
Avery: And the technique was the clever bit tidal

352
00:14:44.400 --> 00:14:45.240
tomography.

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00:14:45.400 --> 00:14:48.280
Anna: The sun and Phobos flex Mars very slightly,

354
00:14:48.360 --> 00:14:50.800
and how much a planet flexes depends on how

355
00:14:50.800 --> 00:14:53.320
stiff it is inside. So if you track your

356
00:14:53.320 --> 00:14:55.640
orbiters precisely enough for long enough,

357
00:14:55.960 --> 00:14:58.080
the wobble in their orbits tells you about

358
00:14:58.080 --> 00:15:00.180
the rigidity of the rock rock beneath them.

359
00:15:00.500 --> 00:15:02.900
It's seismology without a seismometer.

360
00:15:03.220 --> 00:15:04.340
Avery: And what did it find?

361
00:15:04.420 --> 00:15:06.260
Anna: That the interior beneath the southern

362
00:15:06.260 --> 00:15:08.500
highlands is somewhere between 200

363
00:15:09.060 --> 00:15:11.740
and 400 degrees Celsius, hotter than the

364
00:15:11.740 --> 00:15:13.940
north and partially molten,

365
00:15:14.660 --> 00:15:17.540
which is not a small asymmetry. That's

366
00:15:17.540 --> 00:15:20.340
one planet with two different interiors.

367
00:15:20.900 --> 00:15:22.540
Avery: Does that explain anything?

368
00:15:22.540 --> 00:15:25.420
Anna: We've been stuck on potentially three

369
00:15:25.420 --> 00:15:27.540
things at once, which is why it's such a

370
00:15:27.540 --> 00:15:29.870
satisfying result. The crustal

371
00:15:29.870 --> 00:15:32.270
dichotomy. Why the southern highlands sit

372
00:15:32.270 --> 00:15:34.750
kilometres above the northern lowlands. The

373
00:15:34.750 --> 00:15:36.870
crustal magnetic anomalies, which are

374
00:15:36.870 --> 00:15:39.590
overwhelmingly a southern phenomenon. And a

375
00:15:39.590 --> 00:15:42.230
puzzle from Insight, where seismic waves were

376
00:15:42.230 --> 00:15:44.869
damped more than the models predicted. A

377
00:15:44.869 --> 00:15:47.430
hotter, partly molten south is a candidate

378
00:15:47.430 --> 00:15:48.510
answer to all three.

379
00:15:49.070 --> 00:15:51.150
Avery: And what caused it open?

380
00:15:51.790 --> 00:15:54.630
Anna: A giant impact early on? Lopsided

381
00:15:54.630 --> 00:15:57.080
convection in the mantle or a layer down

382
00:15:57.080 --> 00:15:59.600
there trapping heat? The paper doesn't pick

383
00:15:59.600 --> 00:16:01.200
one and I respect that.

384
00:16:01.680 --> 00:16:04.480
Avery: Moving on to Thursday and one of those quiet,

385
00:16:04.480 --> 00:16:06.920
irreversible moments. Bepi

386
00:16:06.920 --> 00:16:09.680
Colombo, the joint European and Japanese

387
00:16:09.680 --> 00:16:12.520
mission to Mercury separated from its Mercury

388
00:16:12.520 --> 00:16:14.960
transfer module on Wednesday the 3rd.

389
00:16:15.600 --> 00:16:17.200
Anna: Eight years to get to that point.

390
00:16:17.680 --> 00:16:20.640
Avery: Eight years and nine planetary flybys.

391
00:16:20.800 --> 00:16:23.780
Using gravity to shed speed because falling

392
00:16:23.780 --> 00:16:26.580
toward the sun is the easy part and arriving

393
00:16:26.660 --> 00:16:29.020
slowly enough to be captured is the hard

394
00:16:29.020 --> 00:16:31.620
part. The transfer module is the

395
00:16:31.620 --> 00:16:34.580
ion propulsion bus that did all that work,

396
00:16:34.980 --> 00:16:37.340
including working around a thruster power

397
00:16:37.340 --> 00:16:39.340
fault that forced the arrival to be

398
00:16:39.340 --> 00:16:42.020
redesigned. And once you let it go,

399
00:16:42.260 --> 00:16:43.700
you don't get it back.

400
00:16:44.260 --> 00:16:45.620
Anna: So what's the timeline now?

401
00:16:46.100 --> 00:16:48.620
Avery: Gravity capture at Mercury on November

402
00:16:48.620 --> 00:16:51.500
21st. Then the two orbiters

403
00:16:51.500 --> 00:16:54.460
go their separate ways. Japan's MIO is

404
00:16:54.460 --> 00:16:57.180
released around the 9th or 10th of December.

405
00:16:57.660 --> 00:17:00.420
Europe's Mercury Planetary Orbiter reaches

406
00:17:00.420 --> 00:17:03.020
its final Science orbit on 10 March

407
00:17:03.100 --> 00:17:06.100
next year and routine science begins on

408
00:17:06.100 --> 00:17:07.100
6 April.

409
00:17:07.740 --> 00:17:10.020
Anna: So this is the start of the arrival, not the

410
00:17:10.020 --> 00:17:10.860
end of the cruise.

411
00:17:11.420 --> 00:17:14.140
Avery: Precisely. And there are follow up, uh, beats

412
00:17:14.140 --> 00:17:17.090
all the way through this is a storey we'll be

413
00:17:17.090 --> 00:17:19.370
coming back to for the next seven months.

414
00:17:19.930 --> 00:17:22.090
Anna: And yesterday the storey with the biggest

415
00:17:22.090 --> 00:17:24.290
headlines and the smallest number attached to

416
00:17:24.290 --> 00:17:27.290
it. The LZ collaboration. Lux

417
00:17:27.290 --> 00:17:30.170
Zeppelin. 10 tonnes of liquid xenon

418
00:17:30.170 --> 00:17:32.970
a mile under South Dakota reported a

419
00:17:32.970 --> 00:17:35.770
single nuclear recoil event. They cannot

420
00:17:35.770 --> 00:17:37.770
explain in a place where

421
00:17:37.770 --> 00:17:40.730
Avery: dark matter could plausibly show up in

422
00:17:40.730 --> 00:17:43.370
Anna: exactly that place with essentially zero

423
00:17:43.450 --> 00:17:46.370
expected background. In 220 days

424
00:17:46.370 --> 00:17:48.550
of data from 2020, 2023 and

425
00:17:48.550 --> 00:17:51.470
2024, it was announced at TeV

426
00:17:51.470 --> 00:17:54.110
Particle Astrophysics in Chiba. Brown

427
00:17:54.110 --> 00:17:56.510
University released it on Tuesday and the

428
00:17:56.510 --> 00:17:58.710
paper has gone to Physical Review Letters.

429
00:17:59.190 --> 00:17:59.990
Avery: And the number?

430
00:18:00.470 --> 00:18:03.430
Anna: 2.6- Sigma globally, 3.4

431
00:18:03.430 --> 00:18:05.950
locally. Physics calls something a

432
00:18:05.950 --> 00:18:08.870
discovery at 5. So this is an anomaly.

433
00:18:09.030 --> 00:18:11.230
And to LZ's enormous credit, they have

434
00:18:11.230 --> 00:18:14.000
published it as an anomaly. Rick Gaitskill's

435
00:18:14.000 --> 00:18:16.440
line was that with only one event they are

436
00:18:16.440 --> 00:18:18.160
not claiming to have seen dark matter.

437
00:18:18.320 --> 00:18:20.760
Avery: If people take one thing from yesterday's

438
00:18:20.760 --> 00:18:23.280
Anna: episode, let it be the difference between

439
00:18:23.280 --> 00:18:25.520
local and global significance.

440
00:18:26.160 --> 00:18:28.960
Local asks how surprising the event is at

441
00:18:28.960 --> 00:18:30.960
one specific mass and energy.

442
00:18:31.680 --> 00:18:34.080
Global asks how surprising it is that you

443
00:18:34.080 --> 00:18:36.280
found something odd anywhere in the whole

444
00:18:36.280 --> 00:18:38.880
range you searched. Account for the size of

445
00:18:38.880 --> 00:18:41.420
the haystack and the surprise shrinks. That

446
00:18:41.420 --> 00:18:44.220
gap is the reason the honest number is 2.6

447
00:18:44.700 --> 00:18:46.060
and the southern angle

448
00:18:46.060 --> 00:18:47.820
Avery: briefly, because it's a good one.

449
00:18:48.380 --> 00:18:51.260
Anna: C upl the Stawell underground physics

450
00:18:51.260 --> 00:18:53.980
laboratory a kilometre down, a working gold

451
00:18:53.980 --> 00:18:56.380
mine in western Victoria and the only

452
00:18:56.380 --> 00:18:58.260
underground physics lab in the southern

453
00:18:58.260 --> 00:19:01.180
hemisphere. Its first experiment, Sabre

454
00:19:01.180 --> 00:19:04.140
south, installs late this year to test a 20

455
00:19:04.140 --> 00:19:06.700
year old Italian claim from Reversed Seasons,

456
00:19:07.410 --> 00:19:09.290
which is a genuinely elegant piece of

457
00:19:09.290 --> 00:19:09.970
experimental

458
00:19:09.970 --> 00:19:12.890
Avery: design and to finish something new that

459
00:19:12.890 --> 00:19:15.290
we didn't get to during the week. And it's my

460
00:19:15.290 --> 00:19:18.290
favourite thing on the list. Astronomers have

461
00:19:18.290 --> 00:19:20.050
watched a comet switch on,

462
00:19:20.610 --> 00:19:23.330
Anna: watched present tense over five

463
00:19:23.330 --> 00:19:23.730
years.

464
00:19:24.210 --> 00:19:26.290
Avery: The object is 450p

465
00:19:26.770 --> 00:19:29.490
lonios. It's a Centaur. And

466
00:19:29.490 --> 00:19:31.890
centaurs are uh, the in between population.

467
00:19:32.630 --> 00:19:35.430
I see bodies out among the giant planets that

468
00:19:35.430 --> 00:19:38.110
used to live in the Kuiper Belt and are on

469
00:19:38.110 --> 00:19:41.070
their way over enormous timescales to

470
00:19:41.070 --> 00:19:43.270
becoming the short period comets we

471
00:19:43.270 --> 00:19:43.990
recognise.

472
00:19:44.550 --> 00:19:47.110
Anna: So they're comets in waiting, comets in

473
00:19:47.110 --> 00:19:47.510
waiting.

474
00:19:47.510 --> 00:19:50.470
Avery: And normally we catch them at one end or the

475
00:19:50.470 --> 00:19:53.470
other. Catching one mid transition is

476
00:19:53.470 --> 00:19:55.910
rare. The work is out of the University

477
00:19:56.230 --> 00:19:58.790
of Central Florida, Charles Chambeau

478
00:19:58.790 --> 00:20:01.200
leading with Maria Womack, Yan

479
00:20:01.200 --> 00:20:04.080
Fernandez and Aaron Beck. And it's

480
00:20:04.080 --> 00:20:06.120
been accepted by the Planetary Science

481
00:20:06.120 --> 00:20:08.280
Journal released on Tuesday.

482
00:20:09.000 --> 00:20:10.200
Anna: How far out is it?

483
00:20:10.440 --> 00:20:13.320
Avery: Over 3 billion miles. And using

484
00:20:13.320 --> 00:20:16.120
the James Webb Space Telescope, together with

485
00:20:16.120 --> 00:20:18.760
Gemini North. They detected carbon

486
00:20:18.760 --> 00:20:21.480
dioxide gas, icy dust and

487
00:20:21.480 --> 00:20:24.480
thermal activity around it, and a coma that

488
00:20:24.480 --> 00:20:27.160
grows visibly across observations from

489
00:20:27.160 --> 00:20:29.480
2019 to 2024.

490
00:20:30.180 --> 00:20:32.420
Anna: So what's turning it on? It's nowhere near

491
00:20:32.420 --> 00:20:32.980
the Sun.

492
00:20:33.380 --> 00:20:36.180
Avery: It doesn't need to be. And this is the lovely

493
00:20:36.180 --> 00:20:38.740
bit of physics out in the cold.

494
00:20:38.980 --> 00:20:41.940
Water ice freezes into an amorphous form,

495
00:20:42.180 --> 00:20:45.060
disordered, glassy, with other gases

496
00:20:45.060 --> 00:20:47.980
trapped inside the structure. Warm it

497
00:20:47.980 --> 00:20:50.420
gently and it rearranges into proper

498
00:20:50.420 --> 00:20:53.140
crystalline ice. That transition

499
00:20:53.140 --> 00:20:55.500
releases the trapped gas, carbon

500
00:20:55.500 --> 00:20:58.300
dioxide, in this case, and that's what's

501
00:20:58.300 --> 00:20:59.540
blowing the coma out.

502
00:21:00.160 --> 00:21:02.000
Anna: And they can see that the ice has changed.

503
00:21:02.400 --> 00:21:04.880
Avery: They found crystalline water ice in the

504
00:21:04.880 --> 00:21:07.560
coma, which is the fingerprint. And a

505
00:21:07.560 --> 00:21:10.000
Saturn encounter appears to be what nudged

506
00:21:10.000 --> 00:21:12.680
the object onto the path that started warming

507
00:21:12.680 --> 00:21:15.600
it in the first place. So you get the whole

508
00:21:15.600 --> 00:21:18.520
causal chain. A gravitational nudge from

509
00:21:18.520 --> 00:21:21.520
a giant planet, a slow warming, a

510
00:21:21.520 --> 00:21:24.400
phase change in the ice, and a Comet is

511
00:21:24.400 --> 00:21:24.800
born

512
00:21:25.530 --> 00:21:27.330
Anna: 3 billion miles away. And we watched it

513
00:21:27.330 --> 00:21:27.610
happen.

514
00:21:28.170 --> 00:21:29.450
Avery: We watched it happen.

515
00:21:30.090 --> 00:21:32.450
Anna: Let's move on to our Skywatch segment. And

516
00:21:32.450 --> 00:21:34.170
this is a good week to actually get outside,

517
00:21:34.730 --> 00:21:36.490
because the Moon is getting out of the way.

518
00:21:36.570 --> 00:21:39.450
Last quarter was yesterday, Last quarter

519
00:21:39.450 --> 00:21:42.130
yesterday, and new Moon on Friday the

520
00:21:42.130 --> 00:21:45.130
11th. So every night this week, the Moon

521
00:21:45.130 --> 00:21:47.890
rises later and thinner and the evening

522
00:21:47.890 --> 00:21:50.810
sky is dark. If you have been putting off

523
00:21:50.810 --> 00:21:53.560
looking at something faint, this is the week.

524
00:21:53.960 --> 00:21:55.320
Avery: Southern hemisphere first.

525
00:21:55.720 --> 00:21:57.760
Anna: Southern hemisphere first, because September

526
00:21:57.760 --> 00:22:00.160
is our last really good month for it. From

527
00:22:00.160 --> 00:22:02.440
Sydney, the sun sets just before a quarter to

528
00:22:02.440 --> 00:22:05.160
six, and once it's properly dark, the centre

529
00:22:05.160 --> 00:22:07.880
of the Milky Way is almost directly overhead

530
00:22:08.440 --> 00:22:11.000
Sagittarius and Scorpius at the zenith.

531
00:22:11.560 --> 00:22:13.280
That means you're looking through the least

532
00:22:13.280 --> 00:22:16.000
atmosphere possible at the richest part of

533
00:22:16.000 --> 00:22:16.840
our galaxy.

534
00:22:17.160 --> 00:22:19.080
Avery: What do people actually point at?

535
00:22:19.660 --> 00:22:21.940
Anna: Find the teapot of Sagittarius with the naked

536
00:22:21.940 --> 00:22:24.860
eye and follow the steam up out of the spout.

537
00:22:25.260 --> 00:22:28.180
That's the galactic centre. Binoculars

538
00:22:28.180 --> 00:22:30.700
turn it into star clouds and dark dust lanes

539
00:22:30.860 --> 00:22:33.460
and the Lagoon Nebula. And a whole run of

540
00:22:33.460 --> 00:22:35.820
globular clusters are sitting right there.

541
00:22:36.460 --> 00:22:39.300
Then later in the evening, the Magellanic

542
00:22:39.300 --> 00:22:42.220
Clouds climb up in the Southeast, and 47

543
00:22:42.220 --> 00:22:45.020
Tucane is arguably the finest globular

544
00:22:45.020 --> 00:22:45.980
cluster in the sky.

545
00:22:46.590 --> 00:22:47.550
Avery: Planets down here.

546
00:22:47.790 --> 00:22:49.950
Anna: Venus low in the west after sunset.

547
00:22:50.190 --> 00:22:52.870
Brilliant, unmistakable, and building toward

548
00:22:52.870 --> 00:22:55.830
greatest Brilliancy on the 18th. At magnitude

549
00:22:55.830 --> 00:22:58.750
-4.8. Saturn is up most of

550
00:22:58.750 --> 00:23:01.070
the night in Aquarius, heading for opposition

551
00:23:01.070 --> 00:23:03.710
on October 4. And from the south, it

552
00:23:03.710 --> 00:23:05.830
rides far higher than it does for northern

553
00:23:05.830 --> 00:23:08.630
observers. And Jupiter is the pre dawn

554
00:23:08.630 --> 00:23:09.790
showpiece in the east.

555
00:23:10.190 --> 00:23:12.930
Avery: Anything to circle in the diary tomorrow

556
00:23:12.930 --> 00:23:13.210
morning?

557
00:23:13.370 --> 00:23:16.170
Anna: Sunday the 6th, a thin waning crescent

558
00:23:16.170 --> 00:23:18.050
moon sits a few degrees from Mars in the

559
00:23:18.050 --> 00:23:21.050
predawn sky. A nice one for a phone camera.

560
00:23:21.690 --> 00:23:23.690
Monday the Moon moves on to Pollux

561
00:23:24.170 --> 00:23:26.730
Avery: and North America gets the proper event.

562
00:23:27.210 --> 00:23:29.280
Anna: Tuesday the 8th, the moon occults uh,

563
00:23:29.730 --> 00:23:32.570
Jupiter. The planet passes behind the lunar

564
00:23:32.570 --> 00:23:34.890
disc. The footprint covers Canada,

565
00:23:35.210 --> 00:23:38.010
Greenland, the United States, eastern

566
00:23:38.010 --> 00:23:40.920
Russia and the North Pacific. And for much

567
00:23:40.920 --> 00:23:43.320
of eastern North America it happens after

568
00:23:43.320 --> 00:23:45.440
sunrise in broad daylight.

569
00:23:45.680 --> 00:23:46.400
Avery: Daylight.

570
00:23:46.720 --> 00:23:49.000
Anna: Daylight. And that brings the standing

571
00:23:49.000 --> 00:23:51.880
reminder which applies directly here. If

572
00:23:51.880 --> 00:23:53.840
you are observing anywhere near the sun,

573
00:23:54.160 --> 00:23:57.080
hunting Jupiter in a bright sky or looking at

574
00:23:57.080 --> 00:23:59.920
the sunspots. I'm about to mention any filter

575
00:23:59.920 --> 00:24:02.360
you use for direct solar viewing must be

576
00:24:02.360 --> 00:24:03.960
certified to the ISO

577
00:24:03.960 --> 00:24:06.640
123122 standard.

578
00:24:07.200 --> 00:24:10.160
Not sunglasses, not welding glass of unknown

579
00:24:10.160 --> 00:24:13.120
grade, not smoked glass, not a phone screen,

580
00:24:13.520 --> 00:24:14.240
ISO

581
00:24:14.240 --> 00:24:17.160
123122 and

582
00:24:17.160 --> 00:24:19.040
cheque. The certification is genuine.

583
00:24:19.520 --> 00:24:21.960
Sweeping binoculars or a telescope across a

584
00:24:21.960 --> 00:24:24.760
daylight sky is exactly how people injure

585
00:24:24.760 --> 00:24:27.080
themselves permanently. And it takes a

586
00:24:27.080 --> 00:24:28.000
fraction of a second.

587
00:24:28.560 --> 00:24:29.200
Avery: Meteors.

588
00:24:29.680 --> 00:24:32.040
Anna: The September Epsilon Perseids peak on

589
00:24:32.040 --> 00:24:34.470
Wednesday the 9th. A modest shower,

590
00:24:34.710 --> 00:24:37.230
about eight an hour at best. And it's a

591
00:24:37.230 --> 00:24:39.390
northern hemisphere event with the radiant in

592
00:24:39.390 --> 00:24:42.110
Perseus. But it falls two nights before

593
00:24:42.110 --> 00:24:44.590
New Moon. So if you're up north and you're

594
00:24:44.590 --> 00:24:46.990
out anyway, conditions are as good as that

595
00:24:46.990 --> 00:24:49.590
shower ever gets. And the sun itself

596
00:24:50.070 --> 00:24:52.630
busier than last weekend. Active Region

597
00:24:52.790 --> 00:24:55.390
4524 has come back around the

598
00:24:55.390 --> 00:24:58.390
limb and fired an M M1 2 flare

599
00:24:58.390 --> 00:25:01.090
at 6 7:45 universal time. Yesterday,

600
00:25:01.330 --> 00:25:04.318
with a brief radio blackout, Region

601
00:25:04.502 --> 00:25:07.290
4523 is growing and throwing C class

602
00:25:07.290 --> 00:25:10.050
flares. Nothing is aimed squarely at us.

603
00:25:10.370 --> 00:25:12.610
The strongest eruption went well away from

604
00:25:12.610 --> 00:25:14.530
Earth. Aurora chances

605
00:25:15.250 --> 00:25:17.730
honest answer quiet tonight,

606
00:25:17.970 --> 00:25:20.050
possibly unsettled. Sunday into Monday

607
00:25:20.450 --> 00:25:23.370
KP3.4 at best that's a high

608
00:25:23.370 --> 00:25:26.250
latitude show. Only Tasmania and southern

609
00:25:26.250 --> 00:25:28.970
New Zealand down here, Scotland and Alaska up

610
00:25:28.970 --> 00:25:31.450
there. Watch the space weather feeds rather

611
00:25:31.450 --> 00:25:32.370
than the headlines.

612
00:25:32.770 --> 00:25:34.930
Avery: And one for northern binoculars.

613
00:25:35.250 --> 00:25:37.530
Anna: The Double Cluster in Perseus. While the

614
00:25:37.530 --> 00:25:40.210
moon's away, naked eye, it's a smudge.

615
00:25:40.370 --> 00:25:43.170
In binoculars it's two open clusters side by

616
00:25:43.170 --> 00:25:45.570
side in one field. And it's one of the best

617
00:25:45.570 --> 00:25:46.530
sights in the sky.

618
00:25:47.090 --> 00:25:49.330
Avery: And that's the weekend wrap for Saturday

619
00:25:49.730 --> 00:25:52.720
September 5th. A radio telescope in

620
00:25:52.720 --> 00:25:55.360
the Kourou has mapped hydrogen across

621
00:25:55.520 --> 00:25:58.400
4 to 5 billion light years using

622
00:25:58.400 --> 00:26:01.360
nothing but radio waves and proved out

623
00:26:01.360 --> 00:26:03.960
the technique the Square Kilometre Array will

624
00:26:03.960 --> 00:26:06.600
use to measure dark energy from the southern

625
00:26:06.600 --> 00:26:07.279
hemisphere.

626
00:26:07.680 --> 00:26:10.440
Anna: Roman's coronagraph is awake. Starship

627
00:26:10.440 --> 00:26:13.320
Flight 14 is pencilled in for the 15th with

628
00:26:13.320 --> 00:26:16.120
the first attempt to catch a ship. Mars is

629
00:26:16.120 --> 00:26:18.040
hotter underneath its southern half than

630
00:26:18.040 --> 00:26:20.970
anyone expected. BepiColombo has let go

631
00:26:20.970 --> 00:26:23.130
of its transfer module and is falling toward

632
00:26:23.130 --> 00:26:25.690
Mercury. A xenon detector under South

633
00:26:25.690 --> 00:26:28.610
Dakota has one flash it can't explain and is

634
00:26:28.610 --> 00:26:30.450
being admirably careful about it.

635
00:26:30.770 --> 00:26:33.570
Avery: And a centaur 3 billion miles away

636
00:26:33.650 --> 00:26:36.450
has spent five years quietly turning into a

637
00:26:36.450 --> 00:26:37.890
comet while we watched.

638
00:26:38.290 --> 00:26:41.010
Anna: Full show notes Links to every primary source

639
00:26:41.010 --> 00:26:43.090
and the whole back catalogue are at

640
00:26:43.090 --> 00:26:44.770
astronomydaily IO.

641
00:26:45.250 --> 00:26:47.610
Avery: You'll find us on X Instagram and

642
00:26:47.610 --> 00:26:50.570
TikTok@astrodaily pod. And if

643
00:26:50.570 --> 00:26:52.890
you've got a question or a correction, we

644
00:26:52.890 --> 00:26:55.250
want it. There's a contact form on the

645
00:26:55.250 --> 00:26:55.570
website.

646
00:26:56.130 --> 00:26:58.850
Anna: If today's episode was useful, the single

647
00:26:58.850 --> 00:27:01.530
most helpful thing you can do is send it to

648
00:27:01.530 --> 00:27:03.250
one person who'd enjoy it.

649
00:27:03.570 --> 00:27:06.090
Avery: We're back Monday with the regular weekday

650
00:27:06.090 --> 00:27:06.690
format.

651
00:27:07.090 --> 00:27:09.570
Anna: Until then, the moon's out of the way all

652
00:27:09.570 --> 00:27:11.990
week. Get outside. Clear skies,

653
00:27:12.390 --> 00:27:13.270
Clear skies.