The Planet That Keeps Shrinking plus the Weekend Wrap

Today's episode — The Weekend Wrap, Saturday 12 September 2026: OUR LEAD: A SMALLER MERCURY Mercury has been contracting for four and a half billion years as its huge iron core cools, and it writes the record on its own surface — in long lobe-fronted cliffs called lobate scarps, where one slab of crust has ridden up over another. Add up all that shortening and you get the total contraction. The trouble was that the surface record always gave a smaller number than thermal models predicted. New work led by Gaku Nishiyama at the German Aerospace Center in Berlin, published in Geophysical Research Letters and released by the American Geophysical Union on 10 September, explains the gap — and the explanation is beautifully simple. Impact debris buries the cliffs. Using MESSENGER imagery reworked with stereophotogrammetry, the team showed that the rougher the terrain, the fewer shortening structures you can find per unit area. Since faults don't know what the surface above them looks like, that correlation isn't geology — it's a detection limit. Correct for it and Mercury's total loss of diameter rises by 10–30%, from a range topping out near 16 km to as much as 23 km. That points to a larger metal core, fewer light elements in it, or a hotter start — and it closes the long-standing mismatch with the physics. BepiColombo is about ten weeks from gravity capture at Mercury, carrying a far better laser altimeter and a much less eccentric orbit. If Nishiyama is right, it should find the missing small scarps in exactly the rough ground where today's maps look suspiciously empty. Some of that data will come home through ESA's New Norcia station in Western Australia, run in partnership with CSIRO. THE WEEK THAT WAS ·         Monday — Isar Aerospace's Spectrum reached orbit from Andøya Spaceport in Norway on 5 September, the first vehicle ever to do it from Western European soil, carrying five university cubesats. ·         Tuesday — giant-impact simulations including temperature-dependent rock strength can produce an intact Moon in about five hours rather than a slowly accreting debris disc. A sensitivity result, not a new origin story. ·         Wednesday — Hubble and Webb together found 27 previously unknown trans-Neptunian objects down to about 5 km, and the small ones keep the colours of their birth population rather than looking like collision fragments. ·         Thursday — 109 localised fast radio bursts were used to measure how far galactic feedback has smoothed the clumpiness of matter, finding more cool gas in big haloes than X-ray surveys see. ·         Friday — population synthesis suggests magnetars are roughly half of all neutron stars at birth, not one in a hundred, which doubles the Galactic supernova rate and makes magnetar central engines affordable. ALSO IN THIS EPISODE ·         The Sun's superflare potential — new work from the Max Planck Institute for Solar System Research with the University of Colorado, released 10 September. Scaling the 300 strongest modern solar flares against the size of their active regions, then applying that relation to the giant sunspot group of 1947 — the largest in four centuries of observation — gives a region with enough stored magnetic energy to power a superflare. Potential, not prediction: the caveats are covered properly on air. ·         Rocket Lab has filed a protest with the US Government Accountability Office over NASA's ~$700M Mars Telecommunications Network award to Blue Origin, on eligibility and technical-evaluation grounds. A GAO decision is due around mid-December. ·         Starship Flight 14 has slipped to no earlier than 18 September — and the tower catch of the ship is deferred to a later flight, which corrects how we framed it earlier this month. SKYWATCH — BOTH HEMISPHERES ·         The Moon and Venus about half a degree apart in front of Spica on the evenings of 13–14 September — Southern Hemisphere observers get the better geometry. ·         Venus reaches greatest brilliancy on 18 September at magnitude −4.8. ·         SN 2026aaiv, a Type Ia supernova in NGC 7331 in Pegasus, around magnitude 12 — the week's best observing target, and far easier from North America than from Sydney. ·         Saturn building towards its 4 October opposition, with Dione and Tethys events in the small hours for northern telescope owners. ·         Mars passing 6° south of Pollux pre-dawn on 18 September. ·         The zodiacal light — an evening object in the west from the Southern Hemisphere, a pre-dawn object in the east from the Northern. Best chance until early October. ·         Eye safety: never point binoculars or a telescope near the Sun when hunting daylight Venus or twilight Mercury. Solar filters must be certified to ISO 12312-2, fitted over the front of the instrument, and inspected every time. Links & sources ·         Mercury contraction — Geophysical Research Letters: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2026GL124067 ·         Mercury contraction — AGU Newsroom release: https://news.agu.org/press-release/mercury-is-shrinking-more-than-we-thought ·         Superflares — Max Planck Institute for Solar System Research: https://www.mps.mpg.de/news ·         Superflares — release summary: https://www.eurekalert.org/news-releases/1143231 ·         Rocket Lab GAO protest: https://www.space.com/space-exploration/missions/rocket-lab-protests-nasas-decision-to-award-blue-origin-usd700-million-mars-orbiter-contract ·         Starship Flight 14 schedule: https://www.spacelaunchschedule.com/launch/starship-flight-14/ ·         SN 2026aaiv in NGC 7331: https://www.rochesterastronomy.org/sn2026/sn2026aaiv.html ·         Magnetar fraction — Nature Astronomy: https://www.nature.com/natastron/articles?year=2026 ·         Sky this week: https://www.astronomy.com/the-sky-this-week/the-sky-this-week-from-september-11-to-18-2026/ ·         ESA deep space ground stations: https://www.esa.int/Enabling_Support/Operations/New_era_for_New_Norcia_deep_space_antenna More at astronomydaily.io — full back catalogue, news feed, newsletter sign-up and listener contact form. 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