Time. Stephen Baxter

done any careful spectral studies or radar studies. But we think it’s a C-type: a carbonaceous chondrite, not nickel-iron, like Reinmuth. Water ice, carbon compounds. It probably wandered in from the outer belt – far enough from the sun that it was able to keep its volatile ices and organics – or else it’s a comet core. Either way, we’re looking at debris left over since the formation of the Solar System. Unimaginably ancient.’

      ‘How big is it?’

      ‘Nobody knows for sure. Three miles wide is the best guess.’

      ‘Does this thing have a name?’

      Cornelius smiled. ‘Cruithne.’ He pronounced it Crooth-knee. ‘An ancient Irish name. The ancestor of the Picts.’

      Malenfant was baffled. ‘What does that have to do with Australia ?’

      ‘It could have been worse. There are asteroids named after spouses, pets, rock stars. The orbit of Cruithne is what made it worth naming.’ Cornelius pointed to numbers. ‘These figures show the asteroid’s perihelion, aphelion, eccentricity …’

      Asteroid 3753 orbited the sun in a little less than an Earth year. But it did not follow a simple circular path, like Earth; instead it swooped in beyond the orbit of Venus, out further than Mars. ‘And,’ said Cornelius, ‘it has an inclined orbit …’ Cornelius’s diagrams showed 3753’s orbit as a jaunty ellipse, tipped up from the ecliptic, the main Solar System plane, like Frank Sinatra’s hat.

      Malenfant considered this looping, out-of-plane trajectory. ‘So what makes it a moon of the Earth?’

      ‘Not a moon exactly. Call it a companion. The point is, its orbit is locked to Earth’s. A team of Canadian astronomers figured this out in 1997. Watch.’

      Cornelius produced a display showing the orbits of Earth and Cruithne from a point of view above the Solar System. Earth, a blue dot, sailed evenly around the sun on its almost-circular orbit. By comparison, Cruithne swooped back and forth like a bird.

      ‘Suppose we follow the Earth. Then you can see how Cruithne moves in relation.’

      The blue dot slowed and stayed in place. Malenfant imagined the whole image circling, one revolution for every Earth year.

      Relative to the Earth, Cruithne swooped towards Venus – inside Earth’s orbit – and rushed ahead of Earth. But then it would sail out past Earth’s orbit, reaching almost to Mars, and slow, allowing Earth to catch up. Compared to Earth it traced out a kind of kidney-bean path, a fat, distorted ellipse sandwiched between the orbits of Mars and Venus.

      In the next ‘year’ Cruithne retraced the kidney-bean – but not quite; the second bean was placed slightly ahead of the first.

      Cornelius said, ‘Overall 3753 is going faster than the Earth around the sun. So it spirals ahead of us, year on year …’ He let the images run for a while. Cruithne’s orbit was a compound of the two motions. Every year the asteroid traced out its kidney bean. And over the years the bean worked its way along Earth’s orbit tracing out a spiral around the sun, anti-clockwise.

      ‘Now, what’s interesting is what happens when the kidney bean approaches Earth again.’

      The traced-out bean worked its way slowly towards the blue dot. The bean seemed to touch the Earth. Malenfant expected it to continue its spiralling around the sun.

      It didn’t. The kidney bean started to spiral in the opposite direction: clockwise, back the way it had come.

      Cornelius was grinning. ‘Isn’t it beautiful? You see, there are resonances between Cruithne’s orbit and Earth’s. When it comes closest, Earth’s gravity tweaks Cruithne’s path. That makes Cruithne’s year slightly longer than Earth’s, instead of shorter, as it is now. So Earth starts to outstrip the kidney bean.’ He ran the animation forward. ‘And when it has spiralled all the way back to where it started –’ Another reversal. ‘Earth tweaks again, and makes Cruithne’s year shorter again – and the bean starts to spiral back.’

      He accelerated the timescale further, until the kidney bean ellipses arced back and forth around the sun.

      ‘It’s quite stable,’ said Cornelius. ‘For a few thousand years at least. Remember a single kidney bean takes around a year to be traced out. So it’s a long time between reversals. The last were in 1515 and 1900; the next will be in 2285 and 2680 –’

      ‘It’s like a dance,’ said Malenfant. ‘A choreography.’

      ‘That’s exactly what it is.’

      Although Cruithne crossed Earth’s orbit, its inclination and the tweaking effect kept it from coming closer than forty times the distance from Earth to Moon. Right now, Malenfant learned, the asteroid was a hundred times the Earth-Moon distance away.

      After a time Malenfant’s attention began to wander. He felt obscurely disappointed. ‘So we have an orbital curiosity. I don’t see why it’s so important you’d send a message back in time.’

      Cornelius rolled up his softscreen. ‘Malenfant, NEOs – near-Earth objects – don’t last forever. The planets pull them this way and that, perturbing their orbits. Maybe they hit a planet, Earth or Venus or even Mars. Even if not, a given asteroid will be slingshot out of the Solar System in a few million years.’

      ‘And so –’

      ‘And so we have plausible mechanisms for how Cruithne could have been formed, how it could have got into an orbit close to Earth’s. But this orbit, so finely tuned to Earth’s, is unlikely. We don’t know how Cruithne could have got there, Malenfant. It’s a real needle-threader.’

      Malenfant grinned. ‘And so maybe somebody put it there.’

      Cornelius smiled. ‘We should have known. We shouldn’t have needed a signal from the downstreamers, Malenfant. That Earth-locked orbit is a red flag. Something is waiting for us, out there on Cruithne.’

      ‘What?’

      ‘I have absolutely no idea.’

      ‘So now what?’

      ‘Now, we send a probe there.’

      Malenfant called back George Hench. The engineer prowled around the office like a caged animal.

      ‘We can’t fly to this piece of shit, Cruithne. Even if we could reach it, which we can’t, Cruithne is a ball of frozen mud.’

      ‘Umm,’ Cornelius said. ‘More to it than that. We’re looking at a billion tons of water, silicates, metals, and complex organics – aminos, nitrogen bases … Even Mars isn’t as rich as this, pound for pound. It’s the primordial matter, the stuff they made the Solar System out of. Maybe you should have planned to fire the probe at a C-type in the first place.’

      Malenfant said evenly, ‘George, it’s true. We can easily make an economic case for Cruithne –’

      ‘Malenfant, Reinmuth is made of steel. My God, it gleams. And you want to risk all that for a wild goose chase with your la-la buddy?’

      Malenfant let George run on, patiently. Then he said, ‘Tell me why we can’t get to Cruithne. It’s just another NEO. I thought the NEOs were easier to reach than the Moon, and we got there forty years ago.’

      George sighed, but Malenfant could see his brain switching to a different mode. ‘Yeah. That’s why the space junkies have been campaigning for the NEOs for years. But most of them don’t figure the correct energy economics. Yes, if you look at it solely in terms of delta-vee, if you just add up the energy you need to spend to get out of Earth’s gravity well there are a lot of places easier to get to than the Moon. But you need to go a chart deeper than that. Your NEO’s orbit has to be very close to Earth’s: in the same plane, nearly circular, and with almost the same radius. Now, Reinmuth’s orbit is close to Earth’s. Of course it means that Reinmuth doesn’t line up