The Largest Galaxy in the Universe, Measured at Last

WEBVTT

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Anna: Here's a question to start your week. How big

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can a single thing actually get?

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Not a country, not an ocean. One

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object held together as one.

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Avery: And I'm guessing you don't mean my inbox.

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Anna: Bigger. Today, astronomers have

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finally put a tape measure around the largest

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galaxy we know of and found its edge

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for the very first time.

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Avery: Plus a Japanese spacecraft that, huh, just

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set the record for the closest asteroid flyby

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ever flown. A British launch put on hold at

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the last minute. And news that our nearest

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big neighbor galaxy is quietly powering

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down.

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Anna: It's Monday 3rd August. This is

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Astronomy Daily. I'm Anna.

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Avery: And I'm, um, Avery. Let's get into it.

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Anna: So let's start with that tape measure. If I

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asked you to name the biggest galaxy in the

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universe, an astronomer would very likely say

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IC1101. And they've

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been saying it for decades. But here's the

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strange until now, nobody could

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actually tell you where it ends.

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Avery: How do you not know where a galaxy ends? It's

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right there in the pictures.

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Anna: You'd think so. IC

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1101 sits about

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1.15 billion light years away

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at the heart of a cluster of galaxies called

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Abel 2029. William

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Herschel first logged it back in 1790.

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Though of course he had no idea it was a

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galaxy. Nobody did until the 1920s.

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It's what we call a giant elliptical, or

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more precisely a, uh, brightest cluster

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galaxy a, uh, BCG. And

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BCGs are the great white sharks of the

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galaxy world. They sit at the center of a

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cluster and they grow by swallowing their

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neighbors one merger at a time over

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billions of years.

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Avery: So it's basically been eating the other

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Anna: galaxies in its cluster for most of

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cosmic history. Yes, and that's exactly

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why its edge is so hard to find. When

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a galaxy grows by cannibalizing others,

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it ends up wrapped in an enormous faint

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halo of starsstars flung out into

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the space between galaxies. That glow

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is so dim, it blurs into the background of

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the cluster itself. So the honest answer

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to how big is

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IC1101? Has for

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years been a, uh, shrug. You'll see figures

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online anywhere from about 400,000

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light years across to a wild 6

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million light years. Nobody could pin it

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down.

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Avery: That's not a rounding error. That's a factor

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of 15. So what changed?

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Anna: A team led by Carlos Marrero de la

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Rosa took the deepest images ever

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captured of this galaxy. Ultra deep

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exposures in two colors using the Isaacman

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Newton's Wide Field Camera. And when I say

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deep I mean, they were sensitive enough to

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detect a glow 30 magnitudes per

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square arcsecond. Faint in plain

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terms, they could pick out starlight roughly

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a billion times fainter than the night sky

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you'd see with your own eyes.

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Avery: And the trick is separating that whisper of

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light from everything else.

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Anna: That's the whole game. Bright foreground

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stars smear light across the image. The

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galaxy scatters its own light, too. So they

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carefully modeled all that stray light and

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subtracted it using a detailed map of

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how the telescope spreads out a point of

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light. Only then could they trace the

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galaxy's true profile all the way out

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until it finally melts into the cluster.

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And here's what they IC

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1101 reaches an edge about

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263,000 light years from

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its center, which gives it a diameter of

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roughly 520

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kiloparsecs. That's about

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1.7 million light years from one side

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to the other, holding around 3.4

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trillion suns worth of stars.

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Avery: Okay, I need a yardstick for that. How does

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it compare to us? To the Milky Way?

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Anna: Our galaxy is about 100,000 light years

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across. So IC1101

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is something like 17 times wider

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than the Milky Way. And en masse,

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our entire galaxy, including all its dark

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matter, comes in around one to one and a

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half trillion solar masses.

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IC1101 has more

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than three trillion solar masses in its

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stars alone. It is, by any

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measure we can now defend with data, the

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largest galaxy known.

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Avery: You said now defend with data. That word

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now is doing some work.

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Anna: It is, because the second finding is

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almost better than the first. This

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galaxy isn't finished. Beyond that

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main edge, the team spotted a scatter

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of faint, lopsided structures. Eight

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of them and two line up neatly with

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disturbances astronomers had already seen

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in the hot X ray gas of the cluster.

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That's the fingerprint of ongoing feeding.

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IC1101 is still pulling

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material in, still assembling,

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probably shaped by a collision with another

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group of galaxies 2 to 3 billion years

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ago that's still settling down today.

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Avery: So it holds the record, and it's still

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growing. That feels almost greedy.

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Anna: Which brings us to the real question in the

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paper's title. How large can

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galaxies actually get? Because

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IC1101 now sits right at

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the extreme upper end of what we call the

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mass size relation, the biggest

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edge yet measured for any galaxy.

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It's not just a trivia record, it's a data

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point that tells us the ceiling for how big

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a galaxy can grow in the present day

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universe. And IC1101 is

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sitting up there, brushing against it,

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still reaching.

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Avery: Can, um, anyone actually see this monster for

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themselves?

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Anna: You'd want a good telescope. It's a faint

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smudge in the constellation Serpents near

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the Virgo border. Well placed in the evening

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for both hemispheres right now. But

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honestly, the wonder here isn't in the

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eyepiece. It's the idea.

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Somewhere out there is an object nearly

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2 million light years wide.

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3 trillion stars strong, and it's

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still hungry.

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Avery: From the largest thing we know to a very

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small, very fast one. Japan's space

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agency JAXA held a press conference last

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Thursday to confirm a result that had

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asteroid scientists grinning. Their

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Hayabusa2 spacecraft has just flown the

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closest asteroid flyby ever attempted

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by any mission.

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Anna: This is the same spacecraft that brought

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samples of asteroid Ryugu back to Earth,

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isn't it?

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Avery: The very same Hayabusa2, launched back

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in 2014, dropped those precious Ryuko

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samples in the Australian outback in 2020,

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and then, with fuel still in the tank,

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kept going. It's now traveled something like

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10.7 billion kilometers on

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an extended mission. And on 5 July,

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it threaded past a near Earth asteroid called

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Torifune.

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Anna: How close is closest ever?

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Avery: 400 meters from the surface. 400.

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That's 744 meters from the asteroid's

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center. And they did it. While both objects

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were screaming along at more than 18,000

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kilometers an hour, about 100 million

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kilometers from Earth, they'd plan to pass

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800 meters from the center and beat their own

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target by clean half.

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Anna: At that speed and distance, that's

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absurdly precise.

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Avery: The operations lead, Yuyemimasu, put it

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beautifully. He said the difficulty was like

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shooting a 1 yen coin up on the northern

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island of Hokkaido from down in Okinawa,

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right at the other end of Japan. And they

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didn't just take pictures. Hayabusa2

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pulled off the first ever laser ranging of an

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asteroid during a flyby, bouncing a laser

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off the rock to measure the distance in real

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time.

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Anna: And there was a surprise waiting for them

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too.

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Avery: There was. Torifune turned out to be a

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contact binary. Two lumps of rock stuck

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together a bit like a cosmic snowman.

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Roughly 450 meters of double lobed

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asteroid. They hadn't expected to look like

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that at all.

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Anna: So why do this? Beyond the lovely

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images, planetary defense.

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Avery: Learning to navigate a spacecraft that

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precisely that close to a moving target

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is exactly the skill you'd need to reach,

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or one day nudge a genuinely

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hazardous asteroid. This was a rehearsal

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and the next act is already booked. In

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2031, Hayabusa2 will rendezvous with a

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tiny asteroid called 1998

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KY26. Just 11 meters across.

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It might even try to land a

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Anna: fridge sized probe landing on something

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the size of a house. I love this mission.

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Now to a launch that isn't happening quite

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yet, but when it does, it'll be a first.

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The German company rocket factory Oxford

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RFA is aiming to send the first ever

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rocket to orbit from UK soil, flying

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from Saxavoord Spaceport up on

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Unst, the most northerly of Britain's

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inhabited islands in Shetland.

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Avery: The first orbital launch from the uk. That's

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a genuine milestone.

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Anna: It really is. Britain has never launched a

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satellite to orbit from its own ground.

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RFA's rocket, called RFA1,

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is a 30 meter three stage vehicle

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designed to carry up to 1300

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kilograms to a sun synchronous orbit.

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But last Monday, while running a test

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campaign on the pad ahead of a hot fire of

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the engines, the company found what it called

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an issue with the vehicle that requires

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further investigation. So they've made the

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call to de stack to take the fully

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assembled rocket apart stage by stage

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to get at the problem.

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Avery: And they've got a history at that pad,

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haven't they?

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Anna: They do. Back in August 2024,

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an earlier RFA1 first stage was

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destroyed in a fireball during a static fire

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test at that very site. Nobody hurt,

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the pad saved, but the stage lost.

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RFA are open about their philosophy. They

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develop iteratively, they test hard and

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they accept that things sometimes break. This

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latest issue sounds far less dramatic. Caught

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on the ground exactly where you want to catch

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it.

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Avery: So what does the timeline look like now?

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Anna: AIR certified launch window opens on the 10th

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of August. Whether they make that depends on

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what the investigation turns up. SACS of

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ORD matters here beyond just this one flight.

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It's the first fully licensed vertical

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orbital launch site in Western Europe and the

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only UK spaceport cleared for vertical

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launches. With satellite demand the way it

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is, Europe is hungry for its own

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independent way to orbit. So this is one

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to keep an eye on over the next week.

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Avery: We opened with a galaxy that's still growing.

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Let's close the news with one that's slowing

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down. And this one's practically a neighbor.

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New Hubble work published last Monday finds

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star formation across the Andromeda galaxy

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has been fading for the last 500 million

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years.

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Anna: Andromeda, that's the closest big spiral to

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Avery: us, the nearest large spiral, about

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2.5 million light years away, roughly a uh,

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trillion stars. And on a dark night, you

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can see it with your own eyes. A team led by

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Benjamin Williams pieced together two

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enormous Hubble surveys covering about two

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thirds of Andromeda's disk and measured

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something like 200 million individual stars.

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As Williams put it, the stars are the fossil

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record of the galaxy's formation. Read them

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carefully and you can reconstruct its

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history.

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Anna: And that history is one of a galaxy

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quieting down steadily.

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Avery: Yes, its star forming rate has dropped from

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about one sun's worth of new stars a year to

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roughly a fifth of that with the sharpest

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decline in just the last 40 million years.

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Most of what's left is concentrated in a ring

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about 32,000 light years out from the center.

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And there may be a culprit. A little

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satellite galaxy called M M32

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pit and run, possibly a sideswipe.

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The region of Andromeda nearest M M32 shows

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the most suppressed star formation. And the

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slowdown there seems to have kicked off

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around 60 million years ago, which could

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actually help pin down when M M32 last

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barged through Andromeda's disk. It's a

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reminder that galaxies aren't serene islands.

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They jostle, they collide. And sometimes a

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small neighbor leaves a very large mark.

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Anna: And you can go and look at the scene of the

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crime tonight.

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Avery: You can, which is the perfect handover to the

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skywatch.

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Anna: Skywatch time and a, uh, heads up before we

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start. We've just had the full buck moon,

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so the moon is a waning gibbous this week,

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still bright enough early on to wash out the

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faintest targets. It'll get easier as the

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week goes and the moon thins toward last

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quarter.

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Avery: First up, a comet with its moment tonight.

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Comet 10P Tempel 2

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reached its closest point to the sun

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yesterday. And Tonight, Monday the 3rd,

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it makes its closest approach to Earth.

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Anna: It's not a naked eye showpiece. You'll want

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binoculars or a small telescope. And dark

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skies from the southern hemisphere. It rides

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highest before dawn. So Sydney early

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risers have the better geometry. From North

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America, it's best hunted in the narrow

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window of true darkness after evening

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twilight. But before that, gibbous moon

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climbs up for mid northern latitudes

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roughly 9:30 to 10:30pm

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local. Use averted vision and let your

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eyes settle.

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Avery: Next, an easy one for everyone.

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Tonight, the waning gibbous moon sits

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right beside Saturn, a lovely naked eye

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pairing up through most of the night and into

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dawn and visible from both hemispheres.

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If you've got a scope, swing it to Saturn

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while you're there for the

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Anna: rings and for the early risers.

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Mercury reached its greatest western

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elongation yesterday, so it's about as well

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placed in the morning sky as it gets this

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round. Look low to the eastern horizon

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roughly an hour before sunrise over

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toward the stars of Gemini, with Mars a

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little higher up. Northern observers get the

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cleaner shot at Mercury this time, but it's

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worth a try from the south too. You'll need a

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flat clear horizon either way.

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Avery: Now a uh, diary note we flagged last week and

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it's now just two days out on the 5th of

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August, a UH Spent Falcon 9 upper stage

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is on track to impact the Moon near

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Einstein crater on the western limb at about

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0635 Universal Time.

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Anna: And this one's squarely for our, uh, North

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American listeners. You have the best seat in

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the house. That impact time works out to

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about 2:35 Eastern,

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11:35 Pacific the night before.

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With the moon up and the impact region on the

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dark side of the terminator ideal, you

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won't see a flash with the naked eye. But if

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you've got a decent telescope and a camera

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trained on that limb, you're in the game.

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Avery: For us in the southern hemisphere, the

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timing's unkind. That's mid afternoon in

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Sydney, uh, around 4:35pm with

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the moon not favorably placed. So we'll be

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waiting on the orbiter after images of the

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impact site rather than catching it live.

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But it's a great excuse to follow

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Anna: along online and looking ahead, a

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quick flag for the big month coming.

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12 August is a huge date.

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A total solar eclipse tracking across

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Greenland, Iceland and Spain, paired

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for once with a moonless peak of the

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Perseid meteor shower. Plus a six

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planet lineup in the pre dawn sky. We'll

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build all of that out closer to the day.

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Avery: 1 um, safety word now though, and we'll say

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it every time. If you're anywhere near that

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eclipse path, you view the partial phases

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only through certified ISO

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123122

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eclipse glasses or a properly filtered

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scope. Never look at the uneclipsed, um, sun

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without them.

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Anna: That's your Monday briefing. If you want

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00:16:41.960 --> 00:16:44.480
more. Everything lives at astronomydaily,

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00:16:44.800 --> 00:16:47.420
IO, the full back catalog, a news

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00:16:47.420 --> 00:16:49.940
feed that updates through the day, and a spot

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00:16:49.940 --> 00:16:51.820
to sign up for the daily newsletter.

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Avery: You can send us a message from the site too.

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00:16:54.380 --> 00:16:57.100
Corrections, questions, what you spotted in

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00:16:57.100 --> 00:16:59.820
your own sky. We read them and we love them.

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We even got our first voice message today

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from listener Ian Sama in Uganda who

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said hi there, I'm Um, Ian from Uganda and I

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00:17:07.980 --> 00:17:10.540
really enjoy your podcast very much. Thanks a

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00:17:10.540 --> 00:17:13.470
lot. Thank you, Ian. Um, the feedback is very

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much appreciated.

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Anna: Yes, thank you, Ian. Very much appreciated by

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all of us. We'll be back tomorrow with more

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of today's space news. Until then, from

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Avery and me, keep

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Avery: looking up clear skies.

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Anna: Mhm.

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The stories.