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🔗 Astronomy 🔗 Astronomy/Astronomical objects

2024 YR₄ is an asteroid between 40 and 90 metres (130 and 300 ft) in diameter, classified as an Apollo-type (Earth-crossing) near-Earth object. It was discovered by the Chilean station of the Asteroid Terrestrial-impact Last Alert System (ATLAS) on 27 December 2024. As of 5 February 2025, 2024 YR₄ was rated 3 on the Torino scale with a 1 in 53 (1.9%) chance of impacting Earth on 22 December 2032. NASA gives a Palermo Technical Impact Hazard Scale rating of −0.40 for 2024 YR₄, which corresponds to an impact hazard of 39.8% of the background hazard level. The discovery has triggered the first step in planetary defense responses, in which all available telescopes are asked to gather data about the object and United Nations-endorsed space agencies are prompted to begin planning for asteroid threat mitigation.

Preliminary analysis of spectra and photometric timeseries of this asteroid suggests it is a stony S-type or L-type asteroid with a rotation period near 19.5 minutes. The asteroid previously made a close approach of 828,800 kilometres (515,000 miles; 2.156 lunar distances) to Earth on 25 December 2024 (two days before its discovery), and is now moving away from Earth. It will make its next close approach around 17 December 2028. By early April 2025 and until June 2028, 2024 YR₄ will have moved too far away from Earth to be observed by ground-based telescopes. Space-based infrared telescopes such as the James Webb Space Telescope may be able to observe 2024 YR₄ when it is far from Earth.

🔗 Astronomy 🔗 History of Science 🔗 Astronomy/Astronomical objects 🔗 Astronomy/Solar System

Vulcan was a theorized planet that some pre-20th century astronomers thought existed in an orbit between Mercury and the Sun. Speculation about, and even purported observations of, intermercurial bodies or planets date back to the beginning of the 17th century. The case for their probable existence was bolstered by the support of the French mathematician Urbain Le Verrier, who had predicted the existence of Neptune using disturbances in the orbit of Uranus. By 1859 he had confirmed unexplained peculiarities in Mercury's orbit and predicted that they had to be the result of the gravitational influence of another unknown nearby planet or series of asteroids. A French amateur astronomer's report that he had observed an object passing in front of the Sun that same year led Le Verrier to announce that the long sought after planet, which he gave the name Vulcan, had been discovered at last.

Many searches were conducted for Vulcan over the following decades, but despite several claimed observations, its existence could not be confirmed. The need for the planet as an explanation for Mercury's orbital peculiarities was later rendered unnecessary when Einstein's 1915 theory of general relativity showed that Mercury's departure from an orbit predicted by Newtonian physics was explained by effects arising from the curvature of spacetime caused by the Sun's mass.

🔗 Physics 🔗 Astronomy 🔗 Physics/relativity 🔗 Astronomy/Astronomical objects

In general relativity, a white hole is a hypothetical region of spacetime and singularity that cannot be entered from the outside, although energy-matter, light and information can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter, light and information cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. This region does not exist for black holes that have formed through gravitational collapse, however, nor are there any observed physical processes through which a white hole could be formed.

Supermassive black holes (SBHs) are theoretically predicted to be at the center of every galaxy and that possibly, a galaxy cannot form without one. Stephen Hawking and others have proposed that these SBHs spawn a supermassive white hole/Big Bang.