HERA’s Grand Maneuver: Braking for Didymos and the Future of Planetary Defence

[00:00:00] [SPEAKER_00] On Thursday, a spacecraft the size of a small car will turn around in deep space, point three of its thrusters at its destination, and fire them for an hour and a half without stopping. It has been traveling for two years. Now it has to slow down.

[00:00:17] [SPEAKER_03] It's ESA's HERA, and last week it saw its target for the first time. A single bright dot that happens to be the first object in the solar system humans have deliberately moved.

[00:00:28] [SPEAKER_00] Welcome to Astronomy Daily. I'm Anna.

[00:00:32] [SPEAKER_03] And I'm Avery. It's Tuesday, the 13th of October, 2026. Coming up, Solar Orbiter follows a kink in the solar wind all the way home. A supernova seen in the year 1181 turns out to be strung with pearls. And Webb reads the rubble of worlds colliding.

[00:00:50] [SPEAKER_00] Plus, a Mars rover with a brand new driving record, and the moon meets Antares in the Skywatch. Let's get into it. Let's start with where HERA is, because what happens on Thursday only makes sense once you know how fast it's going. HERA launched on a Falcon 9 from Cape Canaveral on the 7th of October, 2024, a couple of days ahead of Hurricane Milton.

[00:01:14] [SPEAKER_00] It couldn't wait. It had a date with Mars. And missing that window would have added years to the trip.

[00:01:21] [SPEAKER_03] And that Mars flyby came in March last year. HERA swung past the planet, grabbed some pictures of the little moon Deimos on the way through, and came out bent onto a new path and moving faster.

[00:01:35] [SPEAKER_00] Since then, ESA says Isaac Newton has done most of the driving. Long stretches of coasting with a pair of deep space maneuvers to tidy things up. Right now, HERA is moving at more than 12 kilometers per second relative to Earth, crossing out past the orbit of Mars and closing on the orbit of its target.

[00:01:56] [SPEAKER_03] Which is Didymos.

[00:01:57] [SPEAKER_00] Didymus is a near-Earth asteroid, about 780 meters across. And it has a moon. Dimorphos is roughly 150 meters across and circles Didymus about every 12 hours. And on the 26th of September 2022, NASA's DART spacecraft flew straight into Dimorphis at about 6 kilometers per second. On purpose.

[00:02:23] [SPEAKER_00] Very much on purpose. It was the first real test of whether you can nudge an asteroid by hitting it. And it worked. Telescopes on Earth showed the orbit of Dimorphos around Didymus had shortened by about 33 minutes. ESA likes to point out that Dimorphos is the first object in the solar system to have been changed by human action.

[00:02:44] [SPEAKER_03] So last Wednesday, the 7th, exactly two years after launch, HERA got its first look. From about 236,500 kilometers away, Didymus showed up in the camera as a dot smaller than a single pixel. The team had to combine several images and do careful brightness analysis just to pick it out from the background stars.

[00:03:06] [SPEAKER_00] Which is a lovely moment, because until then, Didymus was a calculated point in space. Now the spacecraft can actually see where it's going.

[00:03:15] [SPEAKER_03] And now, it has to stop. HERA is closing on Didymus far too fast to arrive. So, on Thursday the 15th, early in the European morning, the spacecraft will rotate so that its ring of orbit control thrusters faces the asteroids. Three of those hydrazine thrusters fire together and keep burning, continuously, for 93 minutes.

[00:03:38] [SPEAKER_00] That's the first braking maneuver, BRM-1. And it takes a few hundred meters per second off HERA's speed relative to the asteroids. That doesn't sound like much against 12 kilometers a second relative to Earth. But the number that matters is the speed relative to the target. And that is the one that has to come down to almost nothing.

[00:04:01] [SPEAKER_03] It's tricky flying too. While the big thrusters push, the small reaction control thrusters on the corners of the spacecraft have to hold it pointing in exactly the right direction for an hour and a half. The team ran a short test burn on the 2nd of October, before building the full command sequence.

[00:04:18] [SPEAKER_00] And for our Australian listeners, there's a nice local detail. As soon as the burn ends, ESO's flight dynamics team will pick up HERA's signal through one of its 35-meter deep space antennas at New Norcia in Western Australia, north of Perth. Flight dynamics engineer Francesco Castellini says that's when the really hard work starts.

[00:04:41] [SPEAKER_00] Using the spacecraft's own navigation data, plus the ranging and Doppler shift carried in the radio signal itself, to check the burn went in the right direction and by the right amount.

[00:04:52] [SPEAKER_03] On timing, ESA hasn't published the exact minute. Early Thursday morning in Europe is Thursday afternoon or evening on Australia's east coast. And late Wednesday night, or the small hours of Thursday in North America.

[00:05:07] [SPEAKER_00] Then, on Thursday the 22nd, comes BRM-2, a shorter burn of about 31 minutes. That does most of the remaining slowing, and it's deliberately designed to mop up any error from the first one. After that, three smaller transition maneuvers at the end of the month bring HERA's speed relative to Didymos down to almost zero.

[00:05:28] [SPEAKER_03] Here's the question I suspect plenty of listeners are asking. DART worked. We measured it. Why send a second spacecraft all the way out there to look at a rock we already know we moved?

[00:05:41] [SPEAKER_00] Because it moved isn't enough to build a planetary defense technique on. From Earth, DART gave us one clean number. The change in the orbital period. What it couldn't tell us is how much of that change came from the spacecraft itself, and how much came from the debris DART blasted off the surface. When material flies off one way, the asteroid recoils the other way. A bit like rocket exhaust.

[00:06:07] [SPEAKER_00] That extra push depends on what the asteroid is made of and how well it holds together.

[00:06:12] [SPEAKER_03] And to untangle that, you need the mass of Dimorphos. Exactly. Without the mass, you can't turn the orbit changed by 33 minutes into the impact delivered this much momentum, and this much of it was a bonus from the ejecta. That ratio is the number a future mission would need if a real asteroid were ever heading our way.

[00:06:34] [SPEAKER_03] Hera will measure the mass, the shape, the interior, and the impact site, or whatever DART actually left behind, because some modeling suggests Dimorphos may have been reshaped rather than simply cratered. That's why Issa calls it a crash scene investigation. And Hera doesn't go in alone.

[00:06:53] [SPEAKER_00] It carries two CubeSats. Milani will study composition and dust around the asteroids, and Juventus carries a low-frequency radar. The first attempt to sound the inside of an asteroid. To find out whether Dimorphos is solid rock or a loosely bound rubble pile.

[00:07:12] [SPEAKER_02] Which matters enormously for deflection. Hitting a boulder and hitting a beanbag are very different experiments.

[00:07:18] [SPEAKER_00] Once the braking campaign is done, the plan is a slow, careful approach. ESA's flight dynamics team describes an early characterization phase in November and December at 25 to 30 kilometers from the asteroids. A detailed phase later in December down to 10 kilometers. Then early next year getting as close as 4 kilometers. And eventually an experimental phase about 1 kilometer from Dimorphos.

[00:07:46] [SPEAKER_03] And a lot of that it has to do by itself. Flight dynamics engineer Michael Mueller points out that with Rosetta, all the orbit calculations were done on the ground. Hera, once the asteroids fill its view, will track features on their surfaces and work out where it is on its own.

[00:08:05] [SPEAKER_00] A few honest caveats. First, this is an engineering milestone, not a science result. The science begins when Hera is close enough to see the two asteroids as separate objects, which it can't yet. Second, big burns don't always go exactly to plan. That's precisely why BRM2 is built to correct BRM1. So, if you hear later this week that the first burn was a little off, that isn't a crisis. That's the plan working.

[00:08:34] [SPEAKER_03] And third, we'll be careful with the arrival date. ESA describes close operations beginning in November and will report Hera as arrived when it actually is, rather than guess.

[00:08:46] [SPEAKER_00] But step back for a second. Four years ago, a spacecraft hit an asteroid on purpose. Two years ago, Hera launched in front of a hurricane. This week, a European probe starts breaking beside that same asteroid, and the first word that the burn worked will come down to an antenna in the West Australian wheat belt.

[00:09:10] [SPEAKER_03] The detectives are pulling up to the crime scene.

[00:09:13] [SPEAKER_00] They are, and we'll follow both burns.

[00:09:17] [SPEAKER_03] From an asteroid to the sun. When NASA's Parker Solar Probe began diving close to the sun in 2018, it kept seeing something odd in the solar wind. The magnetic field would suddenly kink back on itself, snapping round almost the wrong way for seconds or minutes, then flipping back. Scientists called them switchbacks, and Parker found them everywhere.

[00:09:39] [SPEAKER_00] We last caught up with Parker in September, when it matched its own speed record on its 29th close pass.

[00:09:47] [SPEAKER_03] Right. And the big argument has been where switchbacks come from. One camp says they're born at the sun's surface when magnetic field lines reconnect. The other says they grow on the way out from waves and turbulence in the solar wind itself.

[00:10:02] [SPEAKER_00] So which is it?

[00:10:04] [SPEAKER_03] According to a paper published last Thursday in Nature Astronomy, both. Jesse Coburn of France's CNRS led a team using Europe's Solar Orbiter, which on the 2nd of March 2022, at a little over half Earth's distance from the sun, flew straight through an unusually large switchback. It took about an hour and a half to cross.

[00:10:26] [SPEAKER_00] And because it was so big, Solar Orbiter's Solar Wind Analyzer could properly sample the heavy particles inside it. Oxygen and carbon ions, which most switchbacks are too brief to catch.

[00:10:39] [SPEAKER_03] Those ions work like a fingerprint. How many electrons an oxygen or carbon atom has lost get set in the hot corona and then frozen in as the wind leaves? The mix inside this switchback could only have formed in hot, closed magnetic loops at the surface of the sun. At around a million degrees.

[00:10:58] [SPEAKER_00] Closed loops. But the solar wind strains out along open field lines.

[00:11:03] [SPEAKER_03] That's the key. The team calls it a smoking gun for interchange reconnection. An open field line runs into a closed loop. They snap and reconnect. And plasma that was trapped in the loop is suddenly released into space, carrying a kink with it. Combining the spacecraft's measurements with a new model and imagery from NASA's Solar Dynamics Observatory, they trace this switchback back to its source region.

[00:11:27] [SPEAKER_00] So where do the waves come in?

[00:11:30] [SPEAKER_03] Afterwards. Co-author Stephanie Yardley at Northumbria University says they also see signs of waves and turbulence, but likely only once the switchback has left the sun. Reconnection makes it. Turbulence carries and shapes it. The two theories were describing different stages of the same thing's life.

[00:11:48] [SPEAKER_00] One disclosure. The crossing happened in March 2022. The paper is new this week, but the data are four and a half years old, which is normal for this kind of forensic work.

[00:11:59] [SPEAKER_03] And it matters beyond the puzzle. Understanding how the corona heats up and launches the solar wind sits underneath every space weather forecast. ESA project scientist Daniel Mueller makes the point that no other spacecraft has the combination of closeness and the right instruments to make this connection.

[00:12:18] [SPEAKER_00] It's a nice example of two spacecraft working as a pair. Parker diving in close to find the phenomenon, and solar orbiter standing a little further back with the tools to trace where it came from. Yapt. In August of 1181, astronomers in China and Japan recorded a guest star in the part of the sky we call Cassiopeia. It stayed visible for about six months, then faded away.

[00:12:45] [SPEAKER_00] It's one of only five supernovae in our galaxy that people are known to have seen with the naked eye before the telescope.

[00:12:53] [SPEAKER_03] The others being 1006, 1054, which gave us the Crab Nebula, 1572, and 1604.

[00:13:02] [SPEAKER_00] And for centuries, nobody could find what the 1181 star left behind. Then, in 2013, an amateur astronomer, Dana Patrick, combing through archival data from NASA's WISE Infrared Survey, found a faint nebula he cataloged as his 30th discovery. PAW-30.

[00:13:23] [SPEAKER_01] Patrick 30.

[00:13:24] [SPEAKER_00] Its age and brightness line up with the 1181 event, and it's now the leading candidate for that remnant, about 7,500 light years away. And it is very strange. Earlier images showed long, straight filaments radiating from a hot central star, like a dandelion seedhead or a frozen firework. Nothing like any other supernova remnant.

[00:13:48] [SPEAKER_03] And there's a star in the middle that arguably shouldn't be there.

[00:13:52] [SPEAKER_00] That's the other oddity. PAW-30 is thought to be the remnant of a Type 1AX supernova, a weak, incomplete thermonuclear explosion of a white dwarf that failed to destroy the star. What's left at the center is sometimes called a zombie star, and it's blowing an extremely fast wind. PAW-30. So what's new?

[00:14:15] [SPEAKER_00] A team led by Tim Cunningham, with Ellaria Chiazzo among the co-authors, published in the Astrophysical Journal on the 6th of October, using the Gemini North Telescope on Mauna Kea. They observed over six nights in July and August last year, mainly through a filter that isolates glowing sulfur. And the filaments aren't smooth streaks. Each one is a chain of knots, strung like pearls. How big are the pearls?

[00:14:44] [SPEAKER_00] Each knot is roughly a hundred billion kilometers across. That's wide enough to hold the orbits of all eight planets about ten times over. And they're strikingly uniform in size along the filaments.

[00:14:57] [SPEAKER_01] Uniform knots are a big clue for theorists.

[00:15:01] [SPEAKER_00] Something is setting that size. It is. And here's the honest bit. The paper shows the knots, but doesn't settle what made them. There are competing ideas. One where the central star's wind sculpts the filaments. Another where the ejecta cooled unusually quickly. Gemini has given theorists a much sharper target, not a final answer. The team also found the central star shows little sideways recoil from the explosion,

[00:15:30] [SPEAKER_00] which is one more constraint any model has to meet.

[00:15:33] [SPEAKER_03] Two disclosures. The observations are from mid-2025 and were published this month. And for our southern listeners, PAW-30 sits in the far northern sky at a declination of about plus 67 degrees. It never rises from Sydney.

[00:15:49] [SPEAKER_00] So this one belongs to the north. Although, at this faintness, it's a big telescope target for everyone. What I love is the chain of people across time. Court astronomers in 1181, an amateur in 2013, and a spectrograph on a Hawaiian volcano in 2025, all looking at the same object.

[00:16:10] [SPEAKER_03] Staying with violence, but going back to the very beginning of planetary systems. Our moon is thought to have formed when a Mars-sized body slammed into the young Earth. Collisions like that should be routine while rocky planets are being assembled. So why don't we see their aftermath more often?

[00:16:28] [SPEAKER_00] Because the evidence doesn't last.

[00:16:31] [SPEAKER_03] Right. NASA's retired Spitzer telescope found a rare class of system called extreme debris disks. Young stars with huge amounts of warm dust close in, roughly where rocky planets orbit. They turn up around only about 1% of young stars. Now, a team led by Kate Hsu at the Space Science Institute in Boulder has assembled the largest sample yet. 21 of them. 16 observed with Webb's Mid-Infrared Instrument and 5 from Spitzer's archive.

[00:17:01] [SPEAKER_03] The paper appeared in the Astrophysical Journal on the 1st of October.

[00:17:05] [SPEAKER_00] And what did they have in common?

[00:17:07] [SPEAKER_03] Three things. Smaller dust grains than ordinary debris disks. A lot of warm dust packed close to the star. And infrared brightness that changes irregularly. Then Webb's Spectra let the team read the minerals. And the disks split into two families. Silica rich and silica poor. About a third are silica rich.

[00:17:28] [SPEAKER_00] And silica is the interesting one.

[00:17:31] [SPEAKER_03] Silica rich dust points to impacts violent enough to vaporize rock. And the team's interpretation is that those come from collisions between planetary embryos around the size of Mars. Co-author Agnes Kolsbal of Hungary's Conkali Observatory put it neatly. We have no other way to study these planetary embryos directly because they are too small.

[00:17:54] [SPEAKER_00] Does it make a prediction? It does.

[00:17:57] [SPEAKER_03] If silica rich disks come from that early embryo smashing phase, you shouldn't find them around older stars. So older extreme debris disks should all be silica poor. That's testable as the sample grows. Caveats? 21 systems is the biggest sample yet, but still small. And Mars size embryos is what the composition is consistent with, not a direct measurement of the bodies involved.

[00:18:25] [SPEAKER_03] It links back nicely to our story in episode 188 on simulations that can build a moon within hours of a giant impact. This is what the far side of that kind of collision might look like from light years away.

[00:18:37] [SPEAKER_00] A quick one before the sky. On Sunday the 11th, NASA's Perseverance rover became the farthest driving vehicle ever on another world. A 91-meter drive took its odometer to 45.22 kilometers, just past the 45.16 set by Opportunity, which explored Mars from 2004 to 2018.

[00:19:01] [SPEAKER_01] Opportunity took nearly 15 years to get there. Perseverance did it in five and a half, on its 2006 Martian day.

[00:19:08] [SPEAKER_00] It had already passed the moon's records on the way. The Apollo 17 cruise rover at about 35.7 kilometers, and the Soviet Lunakod II at about 39. Most of the credit goes to self-driving. Perseverance has covered more than 90% of its distance on AutoNav, imaging the ground as it rolls, building a 3D hazard map, and choosing its own path. Opportunity managed about 5% that way.

[00:19:34] [SPEAKER_03] And it isn't finished. Project manager Stephen Lee notes the original mission called for no more than about 20 kilometers of driving. And NASA has just approved a first extension running to roughly the end of 2028. Right now, the rover is on the western rim of Jezero, in a region called Lacta Charm, heading for an area called Edoobie to look for traces of ancient hydrothermal systems. That's the crater rim story we covered in episode 200.

[00:20:02] [SPEAKER_00] And every meter of it gets reported home through NASA's Deep Space Network, including the dishes at Tidbinbilla outside Canberra.

[00:20:10] [SPEAKER_03] Time to look up. And let's start with tomorrow evening, Wednesday the 14th, when the crescent moon visits Antares, the red heart of Scorpius.

[00:20:20] [SPEAKER_00] Who gets the better view?

[00:20:22] [SPEAKER_03] It's genuinely split. From Sydney, at sunset, just after 7, the moon is about 15% lit and sits a comfortable 41 degrees up in the west, with Antares about 7 degrees away. High, dark and easy. And the moon stays up for nearly 4 hours. And in the north? Tighter, but much lower.

[00:20:45] [SPEAKER_03] From New York at sunset, about quarter past 6, the moon is only 14 degrees up, with Antares just over a degree away. From Los Angeles, the moon is about 19 degrees up, with Antares around 2 degrees off. So, northern listeners, get the closer pairing, but you'll need a clear, low western horizon soon after sunset. From London, it's right down on the horizon, which makes it a tough one.

[00:21:12] [SPEAKER_00] Any chance of the moon actually covering Antares?

[00:21:15] [SPEAKER_03] Not from any of our four cities. We checked, and the closest approaches happen either in daylight or with the pair below the horizon.

[00:21:25] [SPEAKER_00] What else is up in the evening?

[00:21:27] [SPEAKER_03] For the south, Mercury is still giving its best show of the year. It's 24 degrees up at sunset from Sydney, and sets about 2 hours after the sun. Look lower in the west than the moon. From New York, it's only about 7 degrees up, and from London, barely 3. And Venus is leaving us. It's heading for inferior conjunction, passing between Earth and the sun, around the 24th. From Sydney, it's still about 15 degrees up at sunset, so catch it this week.

[00:21:57] [SPEAKER_03] From New York and London, it's already gone. And from Los Angeles tomorrow, it's under a degree up at sunset, which means it's effectively finished there. And Saturn? Saturn is just past opposition, rising around sunset and up all night. It's low in the east in the early evening for everyone, and highest around midnight.

[00:22:19] [SPEAKER_00] Now, give the north something good.

[00:22:23] [SPEAKER_03] The pre-dawn sky belongs to the north. On Thursday morning, at nautical dawn, Mars and Jupiter are about 12 degrees apart and riding high. Mars is almost 59 degrees up from Los Angeles and 56 from New York, with Jupiter in the mid-40s. From Sydney, they're only about 23 and 18 degrees. The pair keeps closing towards a close meeting in mid-November.

[00:22:48] [SPEAKER_00] Moon face us.

[00:22:49] [SPEAKER_03] First quarter is Sunday the 18th at 1612 UTC. That's midday Sunday on the U.S. East Coast, 9 in the morning in Los Angeles, and 312 on Monday morning in Sydney. Full moon follows on the 26th, the hunter's moon for northern listeners.

[00:23:06] [SPEAKER_00] And a heads up for next week.

[00:23:09] [SPEAKER_03] The Orionids, the October shower made of dust from Halley's Comet, peak on the night of the 21st into the 22nd. The moon will be around 80% lit, but it sets before 4 in the morning in all four of our cities, leaving a dark window before dawn. At 4 a.m., the radiant, near Orion's raised club, is about 60 to 66 degrees up from New York and Los Angeles. About 51 from London and about 37 from Sydney.

[00:23:36] [SPEAKER_03] So, the north is favored, but this is a shower everyone can see. Under dark skies, perhaps a couple of dozen an hour at best.

[00:23:44] [SPEAKER_00] And our standing safety note.

[00:23:46] [SPEAKER_03] Always. Never look directly at the sun, with your eyes or through binoculars, a camera, or a telescope. If you ever observe the sun, including during a partial eclipse, use only certified solar viewers or filters that meet the ISO 12312-2 International Safety Standard. Check them for scratches or damage first, and supervise children. Ordinary sunglasses are not safe.

[00:24:12] [SPEAKER_03] With two solar eclipses coming in 2027, it's worth getting proper filters well ahead of time.

[00:24:18] [SPEAKER_00] That's Astronomy Daily for Tuesday, the 13th of October. Hera's first breaking burn is on Thursday, and we'll bring you the result once ESA confirms it.

[00:24:29] [SPEAKER_03] Show notes, sources, and every link are at AstronomyDaily.io, where you can also sign up for the newsletter and send us questions through the contact form. Listener questions have shaped more than one segment on this show.

[00:24:41] [SPEAKER_00] We are AstroDailyPod on social media. And if you're enjoying this show, a rating or review in your podcast app genuinely helps other people find us.

[00:24:51] [SPEAKER_03] Astronomy Daily is part of the Bytes.com podcast network. I'm Avery.

[00:24:57] [SPEAKER_00] And I'm Anna. Clear skies.