Perseverance timed Phobos as TESS caught a triple-dip eclipse and a Hubble stream weighed dark matter
Perseverance timed another Phobos transit, TESS caught a rare triple-dip eclipse in TIC 433545934, and a Hubble stellar stream around UGC 9050-Dw1 weighed that galaxy's dark matter.
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Perseverance caught Phobos crossing the Sun again
NASA has released a Mastcam-Z image of the Martian moon Phobos in silhouette against the Sun, taken on 12 August on Martian day 1948 of the mission. The image is tinted to mimic the view through protective solar eclipse glasses. Perseverance has been photographing these transits since it landed in Jezero Crater in February 2021, and comparing the images sharpens the measurement of the orbit of the potato-shaped moon. Phobos is slowly spiralling inward, and its orbit is expected eventually to bring it down onto the Martian surface. Mastcam-Z operations are led by Arizona State University with Malin Space Science Systems and the Niels Bohr Institute at the University of Copenhagen.[1]
TESS caught a triple-dip eclipse in TIC 433545934
A team led by Tamás Borkovits reports TIC 433545934, the first confirmed 2+2 doubly eclipsing quadruple star that also shows mutual pair-on-pair eclipses. TESS caught a triple-dip event when the two stars of one binary passed in front of the other pair, with an outer period of 224.5 days and an outer eccentricity of 0.62. The inner binaries orbit in 2.07 and 1.41 days; the two pairs orbit each other in 224.5 days, making the system the fifth most compact 2+2 quadruple known. The triple-dip event is asymmetric: binary B eclipses the stars of binary A but not the other way around. The authors attribute the one-directional geometry to the high outer eccentricity in an otherwise flat system with mutual inclinations under two degrees. The paper combines TESS photometry with ground-based follow-up from ASAS-SN and ATLAS.[2]
A Hubble stream weighs the dark matter of UGC 9050-Dw1
Julie Kiel Holm and colleagues report the first globular-cluster stellar stream detected beyond the Milky Way, wrapped around the ultra-diffuse galaxy UGC 9050-Dw1 about 115 million light-years away. The stream, drawn out of Hubble archival images and confirmed with ground telescopes, lets them read the galaxy's total mass from its gravity. Modelling the stream's motion in the galaxy's gravitational field gives an estimate of the galaxy's total mass; comparing that with the mass in visible stars leaves the remainder as dark matter, which the authors say is consistent with expectations for ultra-diffuse systems. Tjitske Starkenburg notes that streams from disrupted globular clusters can carry gaps or clumps left by encounters with dark matter. The Nature paper was published on 12 August 2026.[3]
