Part Two

When Paolo woke and joined ver in the scape, Yatima said, "I'm trying to decide what we should tell them. When they ask why we came after them."

Paolo laughed grimly. "Tell them about Lacerta."

"They'll know about Lacerta."

"As a blip on a map. They won't know what it did. They won't know what it meant."

"No." Yatima gazed at Weyl, at the center of the blue shift. Ve didn't want to antagonize Paolo with questions about Atlanta, but ve didn't want to shut him out either. "You know Karpal, don't you?"

"Yes." Paolo accepted the present tense with a faint smile.

"And wasn't he on the moon, running TERAGO "

Paolo said coldly, "He did everything he could. It wasn't his fault the whole planet was sleepwalking."

"I agree. I don't blame him for anything." Yatima spread vis arms, conciliatory. "I just wondered if he'd ever talked about it. If he ever told you his side of things."

Paolo nodded grudgingly. "He talked about it. Once."

4

LIZARD HEART

Bullialdus observatory, Moon

24 046 104 526 757 CST

2 April 2996, 16:42:03.911 UT

Karpal lay on his back on the regolith for a full lunar month, staring up into the crystalline stillness of the universe and daring it to show him something new. He'd done this five times before, but nothing had ever changed within reach of his unaided vision. The planets moved along their predictable orbits, and sometimes a bright asteroid or comet was visible, but they were like spacecraft wandering by: obstacles in the foreground, not part of the view. Once you'd seen Jupiter close-up, firsthand, you began to think of it more as a source of light pollution and electromagnetic noise than as an object of serious astronomical interest. Karpal wanted a supernova to blossom out of the darkness unforeseen, a distant apocalypse to set the neutrino detectors screaming—not some placid conjunction of the solar system's clockwork, as noteworthy and exciting as a supply shuttle arriving on time.

When the Earth was new again, a dim reddish disk beside the blazing sun, Karpal rose to his feet and swung his arms cautiously, checking that none of his actuators had been weakened by thermal stress. If they had, it wouldn't take long for his nanoware to smooth away the microfractures, but each joint still needed to be tested by use in order to notice the problem and call for repairs.

He was fine. He walked slowly hack to the instrumentation shack at the edge of Bullialdus crater; the structure was open to the vacuum, but it sheltered the equipment to some degree from temperature extremes, hard radiation and micrometeorites. Looming behind it was the crater wall, seventy kilometers wide; Karpal could just make out the laser station on top of the wall, directly above the shack. The beams themselves were invisible from any vantage, since there was nothing to scatter the light, but Karpal couldn't picture Bullialdus from above without mentally inscribing a blue L, a right-angle linking three points on the rim.

Bullialdus was a gravitational wave detector, part of a solar-system-wide observatory known as TERAGO. A single laser beam was split, sent along perpendicular journeys, then recombined; as the space around the crater was stretched and squeezed by as little as one part in ten-to-the-twenty-fourth, the crests and troughs of the two streams of light were shifted in and out of alignment, causing fluctuations in their combined intensity which tracked the subtle changes of geometry. One detector, alone, could no more pinpoint the source of the distortions it measured than a thermometer lying on the regolith could gauge the exact position of the sun, but by combining the timing of events at Bullialdus with data from the nineteen other TERAGO sites, it was possible to reconstruct each wavefront's passage through the solar system, revealing its direction with enough precision, usually, to match it to a known object in the sky, or at least make an educated guess.

Karpal entered the shack, his home for the last nine years. Nothing had changed in his absence, and little had changed since his arrival; the racks of optical computers and signal processors lining the walls looked as gleamingly pristine as ever, and his emergency spares kit and macro repair tools had barely been moved from where he'd first placed them. He wasn't quite alone on the moon—there were a dozen gleisners doing paleoselenology up at the north pole—but he was yet to receive a visitor.

Almost every other gleisner was in the asteroid belt, either working on the interstellar fleet, providing some kind of support service, or generally playing camp follower. He could have been there himself, in the thick of it—the TERAGO data was accessible anywhere, and being physically present at one site offered few advantages when overseeing repairs for all twenty—but he'd been tempted by the solitude here, and the chance to work without distractions, devoting himself to a single problem for a week, or a month, or a year. Lying on the regolith gazing up at the sky for a month at a time hadn't been in his original plans, but he'd always expected to go slightly crazy, and this seemed like a mild enough eccentricity. At first, he'd been afraid of missing an important event: a supernova, or a distant galactic core's black hole swallowing a globular cluster or two. Every speck of data was logged, of course, but even when the gravitational waves had taken millennia to arrive there was a certain thrill of immediacy about monitoring them in real time; to Karpal, now was a transect of space-time ten billion years deep, converging on his instruments and senses at the speed of light.

Later, the risk of being away from his post became part of the attraction. Part of the dare.

Karpal checked the main display screen, and laughed softly in pulse-coded infrared; the faint heat echoed back at him from the walls of the shack. He'd missed nothing. On the list of known sources, Lac G-1 was highlighted as showing an anomaly but it was always showing anomalies; this no longer qualified as news.

As well as recording any sudden catastrophes, TERAGO was constantly monitoring a few hundred periodic sources. It took an event of rare violence to produce a burst of gravitational radiation sufficiently intense to be picked up halfway across the universe, but even routine orbital motion created a weak but dependable stream of gravitational waves. If the objects involved were as massive as stars, orbited each other rapidly, and weren't too remote, TERAGO could tune into their motion like a hydrophone eavesdropping on a churning propeller.

Lacerta G-1 was a pair of neutron stars, a mere hundred light years away. Though neutron stars were far too small to be observed directly—about twenty kilometers wide, at most—they packed the magnetic and gravitational fields of a full-sized star into that tiny volume, and the effects on any surrounding matter could be spectacular. Most were discovered as pulsars, their spinning magnetic fields creating a rotating beam of radio waves by dragging charged particles around in circles at close to lightspeed, or as X-ray sources, siphoning material from a gas cloud or a normal companion star and heating it millions of degrees by compression and shock waves on its way down their tight, steep gravity well. Lac G-1 was billions of years old, though; any local reservoir of gas or dust which might have been used to make X-rays was long gone, and any radio emissions had either grown too weak to detect, or were being beamed in unfavorable directions. So the system was quiet across the electromagnetic spectrum, and it was only the gravitational radiation from the dead stars' slowly decaying orbit that betrayed their existence.