The Week in Space and Physics: The Odds of Impact
On asteroids and impacts, the search for alien technology, the minerals of Bennu, and a nearby super-Earth.
On the morning of June 30, 1908, something big exploded over Siberia. An area larger than London was destroyed, and eighty million trees were flattened by the blast. Shockwaves echoed across the planet: instruments in Russia picked up signs of an earthquake, those in Europe saw waves of pressure moving through the atmosphere.
For days afterwards skies across the world glowed strangely. People speculated about a volcanic eruption, yet for years nobody actually went to look. But then rumours of a meteor strike crept out, and in 1921 a scientific expedition led by Leonid Kulik set out to investigate.
He concluded the rumours were right. Later studies confirmed the idea: on that day a small asteroid, no more than sixty metres wide, had smashed into the Earth at tremendous speed. No crater exists, so most likely it exploded in the air before striking the ground. Nevertheless it had done substantial damage: the meteor had detonated with the force of a fifteen megaton atomic bomb.
On December 27, 2024, a telescope in Chile spotted a similar sized asteroid. Such things happen all the time, and the report was filed with the name 2024 YR4. But then follow-up observations revealed something alarming: this asteroid was on course for a very close approach to Earth.
On December 22, 2032, the asteroid has a small — about one in forty — chance of colliding with the Earth. If it does, the impact could occur anywhere along a broad path stretching from the Pacific coast of Central America to the shores of the Indian Ocean in Bangladesh. And as in 1908, the impact would be devastating on a local scale.
That prospect was worrying enough to trigger asteroid defense mechanisms for the first time. For now that consists of getting more observations of its movements, a step that should allow us to get a better fix on its orbit. Since it was only discovered recently, and is small and hard to see, both its future path and subsequent risk of impact are still somewhat uncertain.
Unfortunately, time is short. The asteroid is small and moving away from Earth. Though it is still visible to our largest observatories, it will be lost to their sight within weeks. The James Webb will scan it in March and May, but afterwards it will fade even from that telescope’s powerful gaze.
In the summer of 2028 the asteroid is expected to reappear. That will give another chance to study it and to confirm or rule out an impact. But if it is on a collision course then it may be too late to do anything about it. In theory we have options — everything from impactors to nuclear blasts could deflect it — but first we need to be sure of its path and find the time to prepare a spacecraft to visit.
Still, the good news is the risks are low. And though headlines are likely to make much of rising probabilities in the coming days, that does not mean impact is certain. Indeed, probabilities can drift up as we get a gradually better fix on its movements, only to suddenly drop when the range of possibilities finally excludes Earth.
The Search for Alien Technology
How might we find an alien civilization? Some ways are obvious: they might, for example, choose to send us a message, or turn up with a fleet of ships. Others are more subtle. Telescopes could pick out the signs of artificial city lights, spot industrial pollutants in an atmosphere, or find evidence of engineering projects being conducted on a vast scale.
These latter things are called technosignatures, and they refer to a way a civilization might betray its presence through its technology and industry. In theory, an alien observer of the Earth might trace our own presence in such a way. We are noisy, emitting a lot of radio waves that could be picked up at great distances. Our industry has changed the atmosphere, adding chemicals that alien telescopes could see. And our city lights are visible across billions of miles.
Yet many of these things, a recent paper from the SETI Institute found, are unlikely to aid alien astronomers. We have changed the Earth, true, but those changes are relatively small. To distant observatories our world would look like a rocky planet dominated by oxygen and nitrogen. They might conclude we have oceans and the potential for life, but they are unlikely to spot the subtle signs of our presence.
Finding a civilization of our technological level, then, seems out of the question for now. But what about a radically more advanced one? Such a civilization might, the physicist Freeman Dyson once speculated, surround their stars with arrays of solar panels. That would give them access to huge amounts of power — and leave a signature we could detect.
Maybe. In truth we have little idea of what such advanced technology might look like, just as stone age people could not have imagined the Internet or the hydrogen bomb. Any civilization capable of building a Dyson array certainly has a far deeper understanding of nature than we do, and surely has access to power sources unimaginable today.
Nevertheless, some searches for Dyson arrays have been conducted. They have, so far, found nothing.
Studying Bennu
In 2018, the OSIRIS-REx spacecraft arrived at the asteroid Bennu. For two years the probe studied the surface of the asteroid, looking for the right place to touch down and grab a sample. In 2020 it succeeded, scooped up a chunk of rock and rubble, and then headed back to Earth.
At the end of 2023, researchers finally got their hands on that sample. Yet a problem with opening the canister — it had been slightly damaged during re-entry — delayed the start of scientific work until the middle of last year. A preliminary analysis came late last year, but now, nine years after OSIRIS first launched, researchers have published their detailed results.
They make for some interesting reading. The asteroid Bennu is thought to represent material from the early solar system. It was probably once part of a larger asteroid or small planet, but one that shattered several billion years ago. Ever since Bennu has remained untouched, a relic of a long gone time when the solar system was home to many small worlds.
Some of the minerals found in OSIRIS’s sample can only have formed as salty water evaporated. That strongly suggests Bennu was once part of a watery world, perhaps one similar to some of the moons of Jupiter and Saturn. Intriguingly, analysis also found many of the essential ingredients of life.
In particular, it found amino acids within the sample. These chemicals combine to form proteins and are key to making both DNA and RNA. That they were found on Bennu tells us they must be widespread — and, therefore, that the chemical ingredients of life are likely scattered across the solar system and on the moons of Jupiter and Saturn.
A Rocky Super-Earth?
Astronomers reported the discovery of a potentially habitable world orbiting a nearby star. Dubbed HD 20794d, the planet lies about twenty light years away and seems to be a few times larger than the Earth.
It is positioned, the researchers say, in the right place for it to have oceans of liquid water. Yet its orbit is far more eccentric than the Earth’s, and so the planet must experience wild seasons. During one part of the year it ventures close to its star, bringing an intense summer. After it swings away, marking the dawn of a long and cold winter.
Still, the world is both close enough and interesting enough for further study. Some of that will come soon: telescopes will try to spy its atmosphere and divine more about its possible oceans. But the really exciting stuff will come later — astronomers are already making lists of planets for a future survey of habitable worlds, and this will be a top candidate.
FWIW: If intelligent life requires at least a half-dozen conditions with 1:10⁴ odds (right star, right planet, outer gas giants, et cetera), then the odds for intelligent life are 1:10²⁴. Compared to about 10¹¹ stars in our galaxy or about 10²² stars in the visible universe (assuming 10¹¹ galaxies). Intelligent life might be exceedingly rare.