In that first part of a second, the computer searched the roster of station personnel and learned that no one present was even remotely affiliated with Project SETI—the Search for Extraterrestrial Intelligence. In that same part of a second, following protocol, the SETI bank fixed on Peterson, who was on duty and by profession an astrophysicist. His special interest was in anomalously large flares on dwarf stars, which was at best small preparation for recognizing the first evidence ever that starship engines were burning in the galactic night. Peterson was here searching for something else, which was typical of the history of exotic discoveries; but an astrophysicist would have to do, the computer decided.

And so, still within the first part of a second, Peterson’s pad went blank, blinked red, then displayed a star chart as the SETI bank called out, “We have an anomalous gamma ray source at the position displayed on your pad”—a red dot began to blink—“0.5 million electron volts. Intensity holding steady. Zero probability of Earth vessel in that sector matching these outputs. Your opinions and queries are urgently needed, Dr. Peterson.”

The intensity profile showed that hundreds of anomalous particles were coming in, all with the precise energy obtainable only from electronpositron annihilation. Even one such particle was proof enough that someone out there had manufactured antihydrogen and was using it to power an antimatter drive.

Peterson immediately rejected the notion that he was the butt of an elaborate joke; these days the penalties for even minor hacking offenses were too severe to be worth the risk. And he knew the computer’s fail-safe systems too well. He also knew the crew. Even though most of them were competent scientists, and young Steven Bilenkin seemed especially promising, outside their individual areas of expertise they were typical university brats: professor types utterly lacking in imagination. Within the first day of their arrival, they had all adapted to and become bored with Miranda’s landscape, and had stopped looking out the windows.

Peterson stared at the star chart and the blinking red dot, then glanced outside and found the same stars on the horizon. Where the dot should have been, just above the eastern edge of Miranda Canyon and Bardo’s Leap, nothing was visible to unaided eyes. But something was out there, undeniably, by everything he knew.

He said, “Give me distance, vector, and velocity.” The particles alone, emanating from a point source, would provide all the necessary information—requiring only a few calculations using a sphere whose radius extended from the gamma ray source to Miranda. The computer flashed numbers that would have, under different circumstances, made Peterson famous. They told him that the engine was five light-months away and approaching the solar system at 92 percent of light speed. The number of particles being emitted indicated that it was decelerating at a rate that would simulate a G-force of two against its occupants, assuming a vessel whose mass was that of a small space station.

He did some quick mental calculations. Five light-months away … five months since the gammas left their source … subtract five months deceleration at two gravities … assuming an approximately steady rate … and the vessel could be as little as four or five weeks away….