The Week in Space and Physics: Ancient Black Holes
On ancient black holes, how to deflect an asteroid, the risk from Apophis and the third flight of Starship
Twenty-six thousand light years from Earth lies a supermassive black hole. It is, paradoxically, both vast and tiny. The equivalent of four million suns are crushed within it, held in a sphere less than forty million miles across. It is, in all probability, fantastically old. The black hole has lurked at the heart of our galaxy for billions of years, steadily growing along with the Milky Way.
Exactly how old it is, however, and how it gained its supermassive status, are still open questions. We do know that such black holes are remarkably common. Supermassive black holes have been found in almost every galaxy studied, often sitting close to their centres. Some are far larger than ours - that in Andromeda is at least thirty times bigger; another in the Messier 87 galaxy weighs in at over six billion solar masses.
Astronomers know such black holes can grow by consuming stars and gas clouds, of course, and from time to time they collide and merge. But there are limits on how fast this can realistically happen, and many black holes seem to be far larger than those limits allow.
Answering the puzzle of these fast growing black holes is one of the main aims of the James Webb Space Telescope. It has already spotted red dots of light in the ancient universe that seem to be giant black holes. And - as with so many other areas of cosmology - its observations are challenging what astronomers had long thought to be true.
When the James Webb first found these dots of light, astronomers had two questions. Are they really black holes, as they seemed to be? And just how common are they? Initial results had suggested there were far more of them than counts of nearby black holes and quasars suggested, and they seemed to appear much earlier in the cosmic timeline than previously thought.
Still, some astronomers thought it might be a coincidence. The James Webb had only examined a fraction of the sky, and so it may simply have stumbled across an unusually crowded area, or been misled by the distorting effects of nearby stars and galaxies. That could have made these dots look far more common than they really are.
Data released since then, however, has only confirmed that these red dots seem to be everywhere in the early universe. Other studies have concluded they really are black holes: clouds of hydrogen gas have been seen hurtling around them at such speeds that only the intense gravity of a black hole could be accelerating them
Most of them, however, are small compared to today’s supermassive black holes. They seem to exist in dusty galaxies, which reddens their colour, and appear capable of growing quickly. Some of them may be early forms of quasars, a powerful and energetic form of massive black hole seen by other telescopes. But the questions of how these early black holes appeared, why there are so many, and just how they got so big so quickly, remain to be answered.
What DART Did to Dimorphos
Eighteen months ago, NASA’s DART probe smashed into Dimorphos, a small moon of the asteroid Didymos. The aim was to shift the orbit of Dimorphos and thus prove the ability to move dangerous asteroids onto a safer path.
In this the mission was a success. DART pushed Dimorphos into a faster orbit around Didymos, cutting thirty minutes off its twelve hour long circuit. Part of that extra speed came from the momentum of DART, but part also came thanks to a cloud of debris that exploded out from the impact.
We know that both of these factors were important in shifting the orbit of Dimorphos. But we don’t yet know exactly how much each contributed. The details depend on the structure of Dimorphos, and particularly on its mass and density. That in turn depends on how the asteroid is made up, and whether it is more like a loose pile of rubble or a single fragment of rock.
Later this year the European Space Agency will launch a probe named Hera to find out. After it arrives at the Didymos system in late 2026, Hera will spend several months examining Dimorphos. That will allow researchers to figure out how dense it is, and so how it is structured internally.
Until then, however, researchers have been using computer models to investigate. In one recent study they recreated the impact of DART using a variety of possible asteroid structures. The one that best corresponds to the actual impact, they found, is an asteroid made up of small boulders loosely held together by gravity.
DART’s impact, according to the simulation, would have disrupted that pile, changing its shape in the process. Hera, it predicts, is unlikely to find a crater from DART. Instead it is likely to spot signs that the collision reshaped and resurfaced Dimorphos, changing not just its orbit but also its appearance.
This kind of information could prove invaluable if we ever need to deflect an asteroid for real. DART proved that it can be done. Hera will now help fill in the details, and so give us more certainty that such a deflection would actually work in a more urgent and terrible situation.
How Risky is Apophis?
On Friday, 13 April 2029, the asteroid Apophis will pass just twenty thousand miles over the Atlantic Ocean. That will put the asteroid roughly ten times closer than the Moon, and just below the ring of geostationary satellites orbiting our planet.
When Apophis was first discovered, and its orbital parameters less well known, this close approach was thought to present some danger. If our initial measurements of its position and velocity happened to be slightly off, the asteroid might have turned out to be on a direct collision course. If so, it presented a catastrophic risk: the asteroid is big enough to destroy a city, or, if it were to land in water, to send terrifying waves rushing across an ocean.
Careful measurements of its movements, however, have ruled this risk out. Astronomers have accounted for everything from the way it spins to the way it radiates heat, and thus calculated rather precisely - to within two miles - its position on that Friday in 2029.
What they had not accounted for, at least until now, was the prospect of an earlier collision between Apophis and another asteroid. In theory such an encounter could subtly shift the orbit of Apophis, placing it on track to smash into the Earth. In practice this is rather unlikely - although there are a lot of asteroids, one would have to approach Apophis in exactly the right way to send it careening towards us.
Nevertheless, astronomers have now done the necessary calculations. After screening the orbits of thousands of asteroids, they found just one likely to pass close to Apophis before 2029. The risks of a collision, they find, are miniscule. Earth, it seems, is safe for another few years.
Starship Flies Again
The largest rocket ever built took flight for the third time this week. Starship lifted off from SpaceX’s Boca Chica spaceport on Thursday morning, flew high over the Atlantic Ocean and southern Africa before experiencing a fiery re-entry above the Indian Ocean.
Although the flight got further than either of the two previous attempts, it was not fully successful. SpaceX was hoping to test the ability of Starship to re-enter the atmosphere at high speed, and then to see it splash down into the ocean. Unfortunately problems seem to have occurred with the spacecraft’s control systems, causing SpaceX to cancel a planned manoeuvre just before re-entry. Afterwards controllers lost contact with the spacecraft, probably because it exploded high in the atmosphere.
Despite that, the mission did chalk up a number of successes. SpaceX were able to open Starship’s cargo bay doors in space for the first time, and apparently carried out a demonstration of fuel transfer inside the rocket. Both of these technologies will be crucial for future uses of Starship.