At https://www.sciencealert.com/dark-comets-come-in-different-varieties-and-it-could-have-implications-for-life-on-earth …. once again, scientists think they understand things – until something comes along to make them look at it again. New and fascinating giblets of information. Space rocks might be confounding accepted theories. Not all of them have been categorised, it would seem. They had been safely tucked away in a box and locked up. Like in a museum cupboard or sliding drawer. Comets and asteroids are too such space rocks that were thought to be done and dusted – with consensus views defining them. It takes a lot to alter the mainstream view of comets, for example. They continue to be described as dirty snowballs – or is that just for the consumption of Joe Public. However, leaving that aside, astronomers have now discovered what they call ‘dark comets’ – comets that don’t throw out any light as they are largely defunct. Just the occasional whisper of outgassing. They are said to look like asteroids but behave like comets. The occasional outburst on their travels through the solar system.
In 2023, one such dark comet was followed by the discovery of six others. Now, in 2024, there are another seven. There appears to be two types, we are told, based on their size and orbits. They may have delivered material to Earth for the development of Life – volatiles and organics. Dark comets do not sport much in the way of tails trailing along behind them. They could not be seen in the older telescopes. Now they can be observed – although not entirely. Until now. Was it the outgassing that caught the attention of the new space telescopes – as intermittent as it might have been. A recent paper suggests there could be a lot of them travelling around in near space. Now, where have I heard that before. Clube and Napier, back in the 1990s, were writing about defunct comets, or bits and pieces of them, continuing to orbit out there, unobserved by astronomers. The idea does not seem to have been taken seriously at the time and has been forgotten, or a new generation of astronomers are unaware of Clube and Napier’s books and articles. Napier even played with the idea that 30,000 years ago, or so, a centaur object escaped the gravity of Jupiter and Saturn and managed to enter the inner solar system, causing periodic upheavals until the end of the Bronze Ages. Astromers are not entirely sure what centaurs are – having both the characteristics of asteroids and comets. Is this an explanation for the dark comets currently being searched for? We shall have to wait and see. In their initial prognosis it was a huge comet – and when centaurs became better known they seemed to fit the bill. They clearly thought their giant comet or centaur had split asunder into many constituent pieces that were still out there and periodically visited the inner solar system. As they had become defunct they were dark – or mostly invisible, sporting a whispy tail every now and then.
Remaining on the theme of threats from space, at https://phys.org/news/2024-12-moon-biggest-ancient-crater-circular.html … where we learn the Moon’s biggest crater is much more circular then previously conceived. NASAs data from the Lunar Reconnaissance Orbiter, and that from the Japanese Kaguya mission, provide a global view of the surface of the Moon. This shows clearly what is known as the South Pole Aitken basin. This is now considered to be the Moon’s oldest and largest visible impact crater. Not only is this evidence of the danger space rocks involve but it has been declared as the landing region for the upcoming Artemis mission – unless the Chinese get there first. As it has been found to be near circular this indicates the impact was direct, almost like dropping a stone in a rice pudding. Oval craters suggest a different trajectory. Debris from the impact seems to be distributed around it – a veritable treasure trove of geological information to be gleaned in future expeditions. In addition, India’s recent Chandrayaan3 rover detected minerals indicative of an impact, near the south pole of the Moon.
At https://www.sciencedaily.com/releases/2024/12/241209122556.htm … astronomers find the smallest asteroids so far detected in the asteroid belt. When Earth is hit by a large asteroid it is classified as a severe event – even causing extinction of some forms of life on earth. However, asteroids the size of a bus may strike the earth every few years. Not necessarily reaching the ground as many explode in the atmosphere – such as the 2013 Chelyabinsk meteor. And perhaps, the Tunguska object in 1908 – and very many other examples recorded in the relatively recent past. They still present a danger if they hit an area of heavy human habitation – like the bolide that exploded and destroyed Tall el-Hammam back in the Bronze Age.