Research › Daylight
This page features research centered around observations or models of comets and asteroids too close to the Sun to be fully studied at night, requiring specialized space- and/or ground-based instruments adapted to safely look near the Sun.
An Asteroid That's Secretly a Comet
Minor planet (139359) 2001 ME1 has always looked like an inert rock whenever it was observed at night, while beyond Earth's orbit. Its orbit, however, takes it as close to the Sun as Mercury at perihelion, where solar heating is 10× stronger. Zhang et al. (2026; PSJ, 7, 21) found that a bright comet reported in coronagraphic images from SOHO's LASCO cameras in 2018 was actually this same object, now clearly actively outgassing near perihelion. Moreover, its position almost directly between the Sun and Earth/SOHO enabled efficient forward scattering of sunlight by its dust grains, making it appear ~7 magnitudes brighter than when seen from the side. This object is likely a very old comet whose near-surface ices have been nearly exhausted.
Rapid Brightening of 3I/ATLAS
Interstellar comet 3I/ATLAS was opposite the Sun from Earth when it made its closest approach to the Sun. Solar coronagraphs and heliospheric imagers continued monitoring the comet while it was hidden from ground-based optical observers in the glare of the Sun, revealing a rapid surge in activity as the comet heated up. Zhang & Battams (2026; PASP, 138, 014403) presented observations from cameras onboard the STEREO-A, SOHO, and GOES-19 spacecraft that tracked this brightening.
Sodium Vapor from Asteroid (3200) Phaethon
Asteroid (3200) Phaethon is best known for the dense stream of dust it presumably laid along its orbit, producing Geminids meteors each December, yet has always looked like an inert rock through nighttime telescopes. Phaethon, however, also approaches the Sun to 0.14 au—less than half the distance Mercury does—where it does briefly turn active with a visible tail under the intense solar heating. Zhang et al. (2023; PSJ, 4, 70) used coronagraphic and heliospheric imagery by the SOHO and STEREO spacecraft, respectively, to show that this visible activity captures Phaethon's ongoing loss of sodium vapor, which fluoresces brightly in sunlight, and could serve as a tracer of more substantial mass loss in the past.