The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.
âWell, hereâs another nice mess youâve gotten me into,â you think to yourself as you explore your new surroundings. ââIntoâ â thatâs funny, it literally is âintoâ.â
But quoting the ancient Laurel and Hardy movie you watched during the long trip here isnât going to help anything. Itâs completely black outside. You turn your spacesuitâs light on using the chin controls, and the view is exactly the same as the last time you tried: a jumbled mess of reflections and shadows, the light from the top of your helmet hitting rocks around you and scattering illuminations back toward you. You turn the light off again to save power. The suitâs emergency locator signal is still being broadcast, so theyâll find you eventually.
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âLetâs stop at the little moon first,â you had said. âThe big rock can wait. Iâve never seen a little 200-metre asteroid up close!â
Yeah, that was a brilliant idea. The approach was without incident: the ship parked a short way off the surface. The gravity of the tiny rock was so small the ship hardly had to do anything to remain in station-keeping. Out the airlock youâd gone, confident that youâd be the first human to set foot on the asteroid. And you were, for a fraction of a second, at least. You dropped down to the surface at a measly 18 inches per second, slower than walking speed. You braced your knees, ready to absorb the impact and stand up tall.
Except the impact had been less of a landing and more of a burrowing. You plunged right through the surface, hardly slowing. You fell right inside the asteroid. The scratching and scraping noise from the rocks and pebbles outside your suit was horrible, and you had worried your comm antenna had broken off. But no; the suit reports itâs fine.
You might not get the deposit on it back, though.
Visit to a small (rubble-pile) planet
In countless sci-fi movies, Our Heroes wind up â through accident or by plan or because theyâre being chased by The Villain â on (or in) an asteroid. They usually land their ship on it, and at some point get out and walk around. They walk normally and see a landscape of solid rock punctuated by the odd crater or two, and .
But is this what it would really be like?
Allow me to put on my âtut-tuttingâ astronomer persona and say: no. Even at the time most of these movies were made we knew enough about asteroids to preclude such activities. But recent discoveries made both from Earth and by spacecraft that visited these space rocks have shown us that these scenes are even wronger.
And that, as usual, reality is far weirder and way cooler than we had imagined.
Asteroids are, essentially, material left over from the formation of the planets when the solar system was very young. A substantial amount of matter orbiting the infant sun at the time formed planets or was blown away by the sun when it switched on and became a proper star. But a lot of it had already condensed to form objects as small as pebbles up to mini-worlds hundreds of kilometres across.
Most of this leftover detritus orbited the sun between Mars and Jupiter, and the gravity of that mighty latter planet yanked and tugged on them, preventing them from coalescing into a planet. There they remain, what we now call asteroids in what we call the Main Asteroid Belt.
Itâs common in media to portray them as roughly spherical grey lumps with some craters scattered about the surface. However, most are irregular in shape, usually elongated enough to look more like space potatoes than what you see in movies. They lack the size and mass needed to crush themselves into balls like the much larger planets do. This also means their surface gravity is very low, so walking on them would be impossible. Youâd be more likely to launch yourself into space than to perambulate across them.
Asteroids do have craters, and they do tend to be grey, but after that reality and entertainment tend to part company.
As early as the 1990s scientists using ground-based telescopic observations were becoming suspicious that smaller asteroids, ones roughly a mile across or less, werenât solid. Spacecraft sent to visit these tinier rocks clinched it: they werenât monolithic, but instead were rubble piles â literally a collection of individual rocks barely held together by their own feeble gravity.
Collisions with other small asteroids over the aeons have broken them up, creating deep fissures and fractures inside them. Sometimes a collision is severe enough to totally break apart the asteroid, but not quite enough to fling all its pieces away into space. They move outward a bit, then settle back down together due to their mutual gravity.
Two of these rubble-pile asteroids have had spacecraft rendezvous with them quite recently. Ryugu is about a kilometre across and was the target of the Japanese Hayabusa2 mission, and Bennu, about 500 metres wide, was visited by the NASA spacecraft OSIRIS-REx. The images the crafts took showed that the surfaces of both asteroids are just junk piles, rocks strewn everywhere. Some are huge, boulders as big as 10-storey buildings, but most are much smaller.
Another surprise scientists got was their shape. Neither asteroid is round, in the spherical sense. The look more like squat cones glued base to base. While the reason behind this isnât totally understood, itâs certainly due to their weak gravity, rotation and rubbly makeup.
An asteroidâs spin gives a rock on the surface a weak push outwards due to centrifugal force. This force is zero at the pole and a maximum at the equator. If the surface is disturbed â say from a small impact by another asteroid â the ground will shake, knocking rocks loose and theyâll naturally roll toward the equator because of the outward force. Friction between the moving rocks and the ones underneath causes the falling rocks to form a conical pile, just like sand in an hourglass, and over the aeons the diamond shape forms.
Amazingly, they get weirder.
The rocks in these rubble piles only rest loosely on top of one another. This is very likely true all the way down to their centres; they fit together so poorly that most of the asteroid is made of voids between the rocks â empty space. This makes their density very low, but it also presents a serious problem to any would-be tourist.
A major part of the OSIRIS-REx mission was to and return them to Earth. It very slowly approached the surface and extended a long boom with a collector at the end, designed to grab rocks once contact with the surface was made. However, the boom actually pierced right through the surface, crushing and brushing aside rocks as it did so, burying itself a half-metre into the asteroid before low-power gas jets fired and backed the spacecraft out. Had those jets not worked, itâs not clear if the entire spacecraft wouldâve buried itself in Bennu.
This means landing on the surface of a small asteroid is a tricky proposition. Even a very slow approach could mean entombing yourself inside it. Coupled with a gravitational pull that would give you roughly the same weight as a mini-marshmallow, your best bet is to park your spaceship near the asteroid and fly yourself around in your jetpack/spacesuit without making physical contact. Otherwise youâre asking for trouble.
Donât worry; youâll probably get rescued. But you might not live down the embarrassment.
If we continue to explore space, then asteroids will be a beguiling target. Many contain water ice and other materials that could make living in space easier, and the science behind them is rich and fascinating. Asteroids are time capsules to the birth of the solar system and we have much to learn from them. I have no doubt humans will be visiting them, perhaps even in the next decade or two.
But when they do, I hope theyâre very, very careful.
Phil Plaitâs new book, , is out now.
