“What do the letters mean?”
“The first shows the half month when the asteroid was discovered. The second is the order of discovery in that half month. So 1986AA is the first asteroid to be discovered in the first half of January, 1986.”
Malenfant eyed the numbers with dismay. “Shit. There must be dozens, just for 1986.”
“More in later years; asteroid watches have gotten better.”
“So which one is ours?”
Cornelius smiled and pointed to the second column. “As soon as enough observations have been accumulated to determine the asteroid’s orbit, it is given an official designation, a permanent number, and sometimes a name.”
The official numbers, Malenfant saw with growing excitement, were in the range 3700-3800. Cornelius scrolled down until he came to a highlighted line.
1986TO 3753 0.484 1.512 0.089…
The key numbers jumped out at Malenfant: 1986 3753.
“Holy shit,” he said. “It’s there. It’s real”
“Not only that,” Cornelius said. “This little baby, 1986TO, is like no other asteroid in the solar system.”
“How so?”
Cornelius smiled. “It’s Earth’s second moon. And nobody knows how it got there.”
George Hench stomped out to “go bend some tin,” glaring at Cornelius as he did so.
Cornelius, unperturbed, called up more softscreen data and told Malenfant what little was known about asteroid number 3753.
“It is not in the main belt. In fact, it’s a near-Earth object, like Reinmuth. What the astronomers call an Aten.”
Malenfant nodded. “So its orbit mostly lies inside Earth’s.”
“It was discovered in Australia. Part of a routine sky watch run out of the Siding Springs observatory. Nobody’s done any careful spectral studies or radar studies. But we think it’s a C-type: a carbonaceous chondrite, not nickel-iron, like Rein-muth. 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 farther than Mars. “And,” Cornelius said, “it has an inclined orbit.” Cornelius’ 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 toward 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.
“Overall,” Cornelius said, “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, counterclockwise.
“Now, what’s interesting is what happens when the kidney bean approaches Earth again.”
The traced-out bean worked its way slowly toward the blue dot. The bean seemed to touch the Earth. Malenfant expected it to continue its spiraling 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 spiraled all the way back again 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 time scale further, until the kidney-bean ellipses arced back and forth around the sun.
“It’s quite stable,” Cornelius said. “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 gotten 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.”