New simulations suggest Venus' extremely slow backward rotation may have been triggered by a high-angle collision with a fast-moving object roughly one-tenth its mass. The impact could have dramatically altered Venus' spin and melted nearly its entire mantle. Universe Today reports: Venus' bizarre and extraordinarily slow retrograde rotation on its axis has long puzzled planetary scientists. But in a new paper presented at the recent European Geosciences Union General Assembly in Vienna, the authors argue that their models indicate that a high angle moon-sized, high-velocity impactor likely triggered Venus's strange 248-day rotation. And it probably happened within the first 50 million years of Venus' formation. [...] The team found that an impactor that is about a tenth of Venus' mass hitting the planet at a high angle could drastically slow the early young planet's rotation. Depending on the actual impact parameters, we can slow down a rapidly rotating early Venus to rotation rates that are that are compatible with long-term evolution towards a slow rotating planet, says [Cedric Gillmann, the paper's lead author and a planetary scientist at ETH Zurich]. Or even in some cases with large energetic impact that happen with a tangential impact that would even put planets early on in already a retrograde but faster rotation, he says. In the simulations, giant impacts expectedly produce surface magma oceans, the paper's authors note. Their relative depths vary depending on impact properties: from a shallow melt layer in the order of 100km thick to a fully molten mantle, they note. If the surface can radiate heat to space efficiently, the magma ocean cools down quickly, they write. If Gillmann and colleagues are correct, Venus' likely impactor also melted some 99 percent of Venus' mantle. That is, the interior structure that extends between its core and crust. You will get rid of that impact heat pretty efficiently, and after a few hundred million years, you end up seeing an evolution that is very difficult to distinguish from a case where you don't have an impact, says Gillmann. What role the impact may have played in Venus' lack of plate tectonics, however, remains open for debate. But it's known that Venus' lack of a large-scale carbon recycling mechanism likely led to its current runaway greenhouse.

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A Chinese Zhuque-2E rocket's upper stage broke apart shortly after last week's June 9 launch, likely creating 100 to 150 pieces of debris in a busy region of low-Earth orbit crossed by the ISS and lower-altitude Starlink satellites. Most fragments should reenter within months because of atmospheric drag, but experts say the incident adds to a worsening trend as China leaves more large rocket bodies in orbit while expanding its launch rate. Ars Technica reports: The US Space Force confirmed the breakup event in a post on space-track.org, a website used by the military to distribute orbit data to the public. "The tracked pieces are being incorporated into routine conjunction assessment to support spaceflight safety," the Space Force wrote in an advisory. "There are currently no threats to human spaceflight. Analysis is ongoing." So far, the Space Force has not added any of the debris fragments to the official catalog of human-made space objects. [...] The bad news is that the Zhuque-2E's breakup is the latest chapter in China's growing contribution to the space junk problem. After decades of leaving spent rocket bodies in orbit, launch operators in most countries now reserve enough fuel to steer their upper stages back to Earth for controlled reentries. Rocket bodies attributed to Russia and the former Soviet Union account for the bulk of the launch-related debris in long-lived orbits, followed by China and the United States. But the Russian and American numbers are declining or holding steady, while the mass of Chinese rocket bodies in these long-lived orbits has grown by more than 150 percent in the past five years, according to a new analysis by Space Domain Awareness expert Jim Shell. The increase comes as China ramps up launches of its own megaconstellations designed to compete with SpaceX's Starlink. Rocket bodies are the most concerning sources of space debris because they are typically fairly large in size and mass, often with residual propellant and high-pressure gases that can trigger an explosion. There is no way to maneuver or dispose of them if left abandoned in orbit after releasing their payloads. McKnight characterized the recent breakup of the Zhuque-2E rocket as a "slight space safety issue," but the trend is not good. China's Long March 6A rocket has an especially bad track record, including two explosions that littered a higher-altitude low-Earth orbit with more than 1,000 debris fragments, where they will remain for decades or centuries. "Three of the top four breakup events in LEO are of Chinese origin, with two of these events being from Chinese (rocket body) explosions in the last four years," McKnight said.

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