We’re all familiar with the Moon, Earth’s cosmic companion. So familiar with it in fact, that we probably all think we know the theory behind how it got to be where it is – the result of a “giant impact” far back in Earth’s early history. However, a new study, published on October 31st in Nature, suggests what actually led to the creation of the Moon was possibly a lot more elegant than previously realised.
The Moon is actually quite unique among the solar system’s satellites. It’s relatively large when compared to its parent planet, and it is a made of pretty much the same stuff, minus some more volatile compounds that evaporated long ago. Other moons tend to be a lot more chemically diverse when compared to one another and their parent worlds.
The accepted theory of lunar formation has it that not long after primordial Earth formed, a Mars-sized object grazed it, throwing off a mass of material from which the Moon subsequently condensed. This impact set the angular momentum for the Earth-moon system, and gave the early Earth a five-hour day. Then, over the aeons, the Moon slowly receded from the Earth (as it continues to do so to this day), and Earth’s rotation has slowed to our current 24-hour day.
It’s a theory all worked out be a combination on mathematics based on the moon’s current orbit, the angular momentum of the Earth-Moon system, the influence of various tidal forces, a little bit of guesswork, etc. However, it does have a couple of holes in it.
The first is that if the Moon was formed as a result of material set free during a slight collision between Earth and another body, then that material should have been a mix of debris from both Earth and the other body, giving rise to a lunar composition that should be at least somewhat different to that of Earth. The second is that if the Moon condensed from a disk of material rotating around Earth’s equator, it should be in orbit over the equator – but instead, its orbit is tilted 5 degrees off the equator.
Both of these issues have previously been explained in terms of “intervening steps” between what we see today and the original “giant impact”. However, a team of scientists led by Sarah Stewart, professor of earth and planetary sciences at the University of California, have posited an alternative explanation, which requires no “intervening steps”, but always natural mechanics to explain everything.
In their model, the “giant impact” still occurs – but it completely destroys the nascent Earth and whatever hit it, leaving a mass of vaporised and molten material orbiting the Sun, which eventually condenses to form a “new” Earth and the Moon – thus giving them similar chemical compositions. Initially, the Earth would have likely been tipped so its axis was pointing towards the Sun while spinning in a two-hour day.
Then, as angular momentum was dissipated through tidal forces, the Moon started receding from Earth, eventually reaching a point called the “LaPlace plane transition”. At this point the forces from the Earth on the Moon became less important than gravitational forces from the sun, resulting in some of the angular momentum of the Earth-Moon system transferring to the Earth-Sun system, causing the Earth to tip “upright”, while leaving the Moon in a very highly inclined orbit relative to Earth’s equator. However, as the Moon continued to slowly and naturally recede from the Earth, it eventually reached the Cassini transition, gradually reducing the Moon’s angle of inclination relative to the Earth’s equator, bringing it to the five-degree offset we see today.
Thus, with this model, no exotic intermediary steps are required to account for the Moon’s composition or why it is where it is today; everything can be explained through the application of mathematics and planetary mechanics, offering a compelling alternative to the accepted theory of lunar evolution.
China Launches the Long March 5 Heavy Lifter
China’s newest and biggest heavy-lift rocket, the Long March 5 (Chang Zheng-5) lifted-off from the Wenchang launch centre on Hainan Island, off China’s southern coast, at 12:43:14 UT or 20:43 Beijing time on Thursday, November 3rd, carrying an experimental satellite designed to test electric-propulsion technology.
With a 25 tonne low Earth orbit payload capacity, the Long March 5 stands on a par with the current crop of heavy lift launch vehicles in operation around the world. The product of two decades of research and development, it is destined to become a centrepiece of China’s growing space ambitions.
Among its may missions, the Long March 5 will play a leading role in the construction of China’s upcoming space station, starting with the launch of the core Tianhe (“Harmony of the Heavens”) module in 2018. When completed in 2022, the 60-tonne station will comprise the core module supported by the Wentian (“Quest for the Heavens”) and Mengtian (“Dreaming of the Heavens”) pressurised experiments modules, all of which will be linked by a multi-port adaptor / EVA airlock.
In addition, the station will be accompanied by a “free flying” space telescope platform called Xuntian (“Heavenly Cruiser”). This will have a slightly smaller primary mirror than the Hubble Space Telescope (2 metres as opposed to 2.4 metres), but will reportedly have a field of view 300 times greater than Hubble.
The station will be supported by Tiangong derived Tianzhou automated resupply vehicle, with crews delivered to the station initially via Shenzhou capsules, and eventually by its in-development and Apollo-like replacement due to enter service some time in the 2020s.
The Long March-5 is a two-stage rocket, the first stage comprising a central core supported by four strap-on boosters. These give the vehicle 10 rocket motors at launch, fuelled by a mix of liquid hydrogen / liquid oxygen or liquid kerosene/liquid oxygen, marking a significant move away from more environmentally hazardous fuels for the Chinese pace programme.
Following launch, at seven minutes into the flight, the boosters and first stage separated. allowing the second stage motors to fire for 6 minutes to place the vehicle in a preliminary low-altitude parking orbit. After coasting on orbit for about 14 minutes, the second stage motors re-ignited, pushing the vehicle into a higher orbit where the Shijian 17 communications satellite mounted atop a Yuanzheng (“Expedition”) “space tug” separated, allowing the latter to lift the payload up to its assigned final orbit.
The launch marked the second flight from China’s newest space facility at Wenchang, the first being the inaugural flight of the Long March 7 earlier in 2016.
The Hexagonal Storm of Saturn
Ever since the Voyager 2 made its historic flyby of Saturn in august 1981, astronomers have been fascinated by a persistent hexagonal storm sitting over the giant planet’s north pole. Created by six-sided jetstream, it measures 13,800 km (8,600 mi) across, making it bigger than Earth is wide, and is a source of immense power.
It is also a storm which is now in a state of flux, offering new insights into the complex nature of Saturn’s atmosphere. Images of Saturn’s northern regions taken by NASA’s Cassini, space vehicle in November 2012 and September 2016 reveal the storm has undergone a substantial change in colour from a bluish haze to a golden-brown hue.
The precise cause of the change remains unknown, but the theory is that Saturn’s deep atmosphere is undergoing a seasonal change as the planet approaches its summer solstice (which will occur in mid-2017). Like the Earth, Saturn is tilted relative to its orbital plane (26.73°), giving rise to distinct seasons in its hemispheres.
However, as it takes 30 Earth years to orbit the Sun, the seasons on Saturn last for around seven terrestrial years apiece. Past observations of the gas giant reveal that its atmosphere can undergo dramatic changes that coincide with the changing of the seasons, with the great hexagonal storm in particularly going through changes.
Between November 1995 and August 2009, for example, a time which coincided with Saturn going from its autumnal to its spring equinox, the north polar atmosphere became clear of aerosols produced by photochemical reactions, most likely as a result of the northern polar region receiving less sunlight. But since 2009, the polar atmosphere has been exposed to continuous sunlight, and it is thought this has been driving an upturn in the production of photochemical hazes in the upper atmosphere.
The significance here is that the great hexagonal storm has appeared to act as a physical barrier preventing the particles produced by these photochemical reactions in the rest of the atmosphere, which give Saturn its golden hue, from entering the north polar region, which remained deep blue. Now, with the summer solstice approaching, it appears the chemical composition of the polar region is now changing and becoming more like the rest of the planet. Thus, scientists are being offered a new means to observe and attempt to understand the complex processes at work within the thick atmosphere of the second largest planet in the solar system.
SpaceX Now Looks To December to Resume Falcon 9 Launches
SpaceX boss Elon Musk has indicated that the company now expects to resume Falcon 9 flights around mid-December 2016, just a month later than the company had originally hoped.
Flights were suspended after a Falcon 9 and its satellite payload were destroyed during a “dry run” pre-launch test on September 1st. Such tests are not uncommon when space vehicles are on the launchpad; however, SpaceX runs very unique tests in which not only are the vehicle’s fuel tanks loaded with propellants, but the main engines are fired for a brief period in a static fire test.
The cause of the September 1st explosion has now been traced to a specific set of circumstances surrounding how propellants and inert helium (commonly used to purge rocket motor systems) are loaded into the vehicle. In other words, the explosion was not the result of any structural defect in the rocket, but rather a failure of process which created an unexpected – and unheard of to that point – reaction.
“It’s never happened before in history, so that’s why it took us a while to sort it out,” Musk said when discussing the status of the investigation and the company’s hopes on Friday, November 4th. “This was the toughest puzzle to solve that we’ve ever had to solve.”
However, while the issue may not be down a structural failure within the rocket, the September 1st loss has once again given rise to concern within NASA that SpaceX’s approach of fuelling rockets immediately before launch could pose a serious threat to crewed missions.
The long-standing approach to launches by NASA and the United States Air force – and adopted by most other US-based launch companies – is that a rocket’s cargo or crew is added only after the vehicle has been fully fuelled. However, SpaceX uses rocket-grade kerosene propellant rather than the more usual liquid hydrogen. This allows more fuel to be pumped into a much small space than would otherwise be required, reducing the Falcon 9’s overall mass.
However, it comes at a price. Liquid kerosene can rapidly sublimate into gas, expanding as it does so. This in turn increases the pressure within the fuel tank, exposing it to possible rupture. Thus, fuelling of a SpaceX vehicle is held until everything else is in place, so the vehicle can launch as soon as possible after fuelling has completed.
To combat any risk to a crew during the fuelling process, SpaceX has put considerable effort into developing an abort system designed to blast a crew clear of the rocket even before it has launched. This system was successfully tested in May 2015, and SpaceX are convinced it is capable of saving a crew should a launchpad malfunction occur.
Nevertheless, on October 31st, 2016, NASA’s Space Station Advisory Committee, met to discuss the September 1st Falcon 9 accident, and again indicated they have serious concerns over SpaceX’s approach to fuelling their vehicles. Should they deem that last-minute fuelling poses too great a risk for crewed missions, SpaceX may yet have to reconsider how it manages Dragon 2 launches to the ISS.