A star found by staring may finally measure how fast our black hole spins

The star that only years of looking could find

Sagittarius A* is the black hole at the centre of our galaxy. It holds about 4.3 million times the mass of the Sun, and the paper puts it 8,277 parsecs away - roughly 27,000 light years [1]. Dozens of stars loop around it. Tracking them is how we know it is there.

On Wednesday a team reported a new star in Nature [1]. They found it with GRAVITY, an instrument that combines the light of the European Southern Observatory’s Very Large Telescope in Chile into one sharper view [1]. They call it S301.

S301 goes round once every 8.7 years. That is the shortest orbit ever measured at the galactic centre; the previous record was 12 years [1]. Its path is a long thin ellipse. At its closest it skims about 136 or 142 times the radius of the black hole’s point of no return [1]. The paper gives two numbers because the orbit can face either of two ways, and nothing yet tells them apart [1]. Either way that is roughly ten times closer than S2, the star that has carried most of this work until now [1]. At that instant it travels 25,000 to 25,600 kilometres a second, between 8.3% and 8.5% of the speed of light [1].

The closeness is the whole point. Einstein’s theory says a spinning black hole drags space around with it, and that effect fades very fast as you move away. No star found so far passes near enough to feel it. S301 does. The team estimates a fair chance of measuring that spin within about a decade [1]. Their forecast assumes roughly ten measurements a year out to 2035, plus ten more around the closest pass [1].

So why did nobody see it before? Because it is faint - magnitude 19.3 in infrared light, on a scale where bigger numbers mean dimmer [1]. The team has watched this patch monthly since 2017, in week-long runs from March to September, spending 80 to 100 hours a year on it [1]. The light arrives in six-minute chunks that are stacked together [1]. S301 surfaced in spring 2023. Once they had a rough orbit they went back through the archive and found it again in 2021, and weakly in 2017 [1]. It had been sitting in data they already owned. Those early positions are the least precise, and the paper says plainly why: fewer exposures were taken at those pointings [1].

One wrinkle matters more than it looks. Sgr A* itself flickers and is bright. Adding up more frames does not make it go away. The team had to model its light in each exposure and subtract it, then combine what was left and search that [1]. Even so, a separate instrument that splits light into its colours could not pick S301 out at all [1]. That is why its speed towards or away from us is still unknown [1].

The same week, the same currency

Time on a telescope was the price of almost every result this week.

A comet called 240P/NEAT split in two. David Jewitt’s group at UCLA worked out why by watching it from October 2025 to April 2026 [4]. That is two months before its closest pass by the Sun, and four months after [4]. They used a 2.56-metre telescope, small by modern standards [4]. Six months of patience produced dust-loss rates, sizes for both fragments and a separation speed [4]. It also ruled out tides, impacts and internal pressure [4]. What is left is the comet spinning itself apart [4]. The brighter piece is 400 to 600 metres across; the fainter one is harder to measure precisely because it is fainter [4]. (Sky & Telescope’s picture captions name the Astrophysical Journal, but its own citation and the paper’s identifier point to the Astronomical Journal.)

NASA has put online the transcript of a two-day symposium on Hubble held in Washington on 4 and 5 June [2]. Read as a ledger, it is a record of hours bought. One programme, ULLYSES, spent a thousand Hubble orbits on massive stars [2]. Another, Rocky Worlds, is spending 250 orbits watching small stars flare [2]. Giada Arney is project scientist for the proposed Habitable Worlds Observatory, and put the scale plainly. The faintest galaxies in the Hubble Ultra Deep Field are about as bright as the habitable planets it would hunt [2].

That is why the plan for it is so slow. To gather enough light, it would have to hold on a single planet for days to weeks [3]. That is why studying 25 worlds is expected to take a decade [3].

Elsewhere: astronomers reported GJ 523b, a rocky world 2.55 times Earth’s width but 23.5 times its mass, circling its star every 17.75 days. TESS flagged it; the 3.5-metre WIYN telescope at Kitt Peak measured the tug it puts on its star [5]. The system is about 170 million years old, which makes the density awkward - a core that heavy should have pulled in gas and become a small Jupiter. “This isn’t what we expected at all,” said lead author Max Kroft [5]. And Europe’s FLEX satellite was fuelled in French Guiana for a 15 September launch [6]. Its job is to measure the faint glow plants give off while photosynthesising, a signal invisible to the eye [6].

The sky that will not stay dark

Every one of those results depends on a background that stays quiet. This week two pieces of research argued that it will not.

Astronomers led by Aaron Boley of the University of British Columbia modelled the night sky under proposed orbital data centres. They posted the study on 3 August to arXiv, where physicists share work before formal review [7]. Companies have filed with US regulators for fleets of anywhere between 20,000 and one million satellites [7]. The favoured orbit runs over the poles along the day-night line, where sunlight never stops [7]. “People have lost the sense of the night sky because of the light-polluted cities, and now we’re rewriting that story with artificial satellites,” Boley said [7].

Roughly 15,000 satellites are up now, and there are plans on paper for nearly two million [8]. Hugh Lewis, a long-standing space-debris researcher, posted a separate study on runaway collisions [9]. His version accounts for satellites that dodge each other, and that get replaced when they die [9]. On disposal, Ars Technica ran the arithmetic on SpaceX’s proposed million-satellite fleet [10]. At a five-year life for the computer chips aboard, about 200,000 would be retired every year [10].

On the pad

China is expected to launch Chang’e-7 late on Sunday US Eastern time, which is Monday 24 August in China [11][12]. A Long March 5 was rolled out at Wenchang on 19 August [11]. The stack carries an orbiter, a lander, a rover and a hopping robot built to jump from sunlight into permanently shadowed craters near the south pole, hunting water ice [11][13]. Ice has been spotted from orbit; nobody has ever measured it on the ground [11]. Landing is set for later this year - Scientific American reports November [12]. Brown University’s James Head called it “the most ambitious, comprehensive, and complex robotic lunar mission ever attempted by any nation or entity” [13].

LandSpace landed the first stage of its ZhuQue-3 rocket in Gansu on 19 August local time, China’s first booster recovery on land using legs [14][15]. Video showed a fire in the tail afterwards [14]. It was the vehicle’s second flight; the first crashed on landing [14].

SpaceX’s 100th mission of the year was a Starlink flight from California [16]. It took the fleet past 11,000 satellites in orbit [18]. Days earlier it set a record with two Falcon 9s 38 minutes apart [17]. A Cape Canaveral flight carrying the year’s 2,000th Starlink was then stalled by a last-minute abort [19]. President Trump signed a memo on Thursday aimed at 1,000 US launches and re-entries a year by 2030, against 178 last year [20]. Muon Space raised $250 million, valuing it at $1.5 billion according to a person familiar with the round [21]. A new San Jose factory is meant to build 500 satellites a year by 2027 [21].

On the station, Anil Menon and Sophie Adenot spent six hours 23 minutes outside and removed a failed antenna, but ran out of time to fit the spare [22]. Adenot became the first French woman to walk in space [22]. Starship’s Ship 40, which splashed down intact in the Indian Ocean on 24 July, reached Christmas Island after 24 days under tow [23]. On Mars, Curiosity found a field of honeycomb cracks 4 to 8 centimetres across, a shape it had never met before [24]. Perseverance watched what appears to be the moon Phobos cross the Sun on 13 August, a day after the eclipse on Earth [25].