The Week in Space and Physics: The Bones of Theia
On long buried fragments of Theia, Betelgeuse's past, Lucy's first asteroid and the next launch of Starship.
The Greeks knew Theia as one of twelve primordial Gods; mother of the Moon and a child of the Earth. Today we know it as a long lost planet; one that long ago crashed into the Earth and thus gave birth to the Moon.
When the Solar System was young, scientists now believe, the Earth formed from the slow accumulation of rocks and dust. Yet another planet – Theia - also formed in a similar orbit, and eventually reached a similar size to that of Mars today. At some point a collision took place: an event violent enough to shatter Theia and turn the outer layers of Earth into molten rock.
From the debris of this violent collision came the familiar shapes of the Moon and the Earth. This is not to say the Moon is Theia; rather the Moon consists of a mixture of both the primordial Earth and Theia. The outer layers of the Earth are formed from a similar mix – and so, geologists have found, the rocks of both the Earth and the Moon look similar.
This explains several things about our world and its satellite. The Moon is unusually large, at least compared to the other moons in our solar system, a curiosity that can be easily recreated in simulations of the long ago impact. It also explains the orbit of the Moon, and why evidence from moon rocks suggests the Moon was once largely molten.
Now researchers think the theory could also explain a mystery lying deep under the Earth’s surface. Thousands of miles under our feet sits the Earth’s core, a spinning shell of iron and nickel. Above to it, in the lower mantle, are two gigantic blobs; one lying deep under Africa and the other far under the Pacific.
The origin of these blobs has long puzzled scientists – but, according to a new study published in Nature, they may be long buried fragments of Theia. Based on clues found in moon rocks, scientists think that Theia contained more iron oxide than the Earth did, making it a slightly denser planet.
When Theia collided with the Earth, some large blobs of Theia’s mantle may have survived. Since they were more dense than the Earth's mantle, they should have started to sink. Over time, computer models suggest, those peices would have piled up at the base of the Earth’s mantle, forming the large blobs found there today.
Not all researchers are convinced by the idea. Other simulations have shown that the material from Theia and Earth should have throughly mixed after the collision. To settle the matter, researchers really need to get samples from the deep mantle blobs. Yet doing so is difficult, as drilling so deep into the Earth is impossible with current technology.
Still, if it is true the theory would settle a long standing mystery about the geology of our planet. It may also reveal scars left over from an ancient collision - an event that had a profound impact on both the Earth and its inhabitants.
Did Betelgeuse Eat a Star?
In recent years Betelgeuse has often captured the headlines. One day, astronomers believe, the star will explode; creating a celestial display unmatched in human history. Though this will probably not come to pass for another hundred thousand years or so, researchers still watch the star carefully, hoping to prise out the secrets of dying supergiant.
Their observations have revealed some strange details. Betelgeuse appears to be moving rapidly; speeding through space at thirty kilometres per second. It is also spinning more quickly that it should – rotating at a rate of five kilometres per second. That is unusually high, especially for a star as vast as Betelgeuse.
Back in 2020, a group of researchers at Louisiana State University suggested this rapid spin could point to a dramatic event in its history. Long ago, they think, Betelgeuse may have belonged to a binary system. Back then it would have been a smaller star; one still burning its original supply of hydrogen.
Yet as that fuel ran out, Betelgeuse would have cooled and expanded. If its binary partner had been close enough, Betelgeuse may have engulfed it; dragging the star down into its core. Simulations show this could have made the star spin faster, explaining why it rotates so rapidly today.
A newly released study from the same research group took at closer look at how all this would have played out. Had Betelgeuse merged with star roughly a quarter of its mass, they reckon, the outcome would look roughly like the star does today. The event could also explain some oddities in Betelgeuse’s atmosphere, especially the presence of nitrogen near its surface.
Still, the evidence is not yet conclusive that such a merger really took place. And even if it did, it is unlikely to bring the date of its supernova much closer, nor would it have much to do with the recent dimming and brightening of the star. Instead it is simply another twist in the tale of one of the most fascinating stars in the night sky.
Lucy Meets Dinkinesh
NASA’s Lucy spacecraft flew past an asteroid last week, the first of ten rocks the probe should visit over the next few years. The asteroid, Dinkinesh, is one of the smallest asteroids yet visited by humanity – measuring just a few hundred metres across.
The photographs sent back by Lucy also revealed a surprise: Dinkinesh is not alone. It appears, indeed, to be a binary asteroid – formed of one large rock with a much smaller one locked in orbit around it. Around one in seven asteroids close to Earth are thought to consist of such binary pairs, including one visited last year by the DART mission.
For NASA, however, the encounter was also an important validation of Lucy’s ability to track and image asteroids as it flies past them. Lucy should spend the next few years flying between a group of asteroids known as the Trojans. These share the orbit of Jupiter, but travel in two groups – one some way ahead of the giant planet, and the other some way behind.
In order to visit both of these groups, Lucy will follow a long, looping path across the Solar System. At present it is on the first outbound leg of that path, heading through the main asteroid belt towards the orbit of Jupiter. Dinkinesh, then, is a member of that asteroid belt, rather than being one of the Trojans.
Lucy’s next encounter with an asteroid will come in April 2025, when it passes within one thousand kilometres of another main belt asteroid. That will be followed, towards the end of 2027, by a series of encounters with Trojan asteroids. Lucy’s mission is expected to last until the early 2030s, though its looping path through the solar system is likely to persist for millennia more.
Starship: Time to Try Again?
Almost seven months after Starship lifted off for the first time, SpaceX have said they will make another launch attempt in the coming weeks. The move follows the completion of a safety review by the Federal Aviation Administration, though a separate environmental review is still to be completed.
That review could still take several months to finish, according to the administration. Despite this, SpaceX has started to push ahead with launch preparations, hinting that the Starship could next lift-off on November 13. As in April’s attempt, SpaceX will try to launch Starship onto a near orbital path, looping most of the way around the world before falling into the Pacific Ocean.
Whether it will get that far remains to be seen. The last time Starship flew, problems with the rocket’s engines left it spinning out of control long before it reached space. SpaceX will surely have addressed many of the problems uncovered by that flight, but engine reliability still seems to be a problem for SpaceX.