Two million seconds after first acceleration, the mirror-matter engine ran out of fuel. The Sniffer had reached the end of acceleration and the beginning of coast phase. Terminal velocity was measured as almost twenty thousand kilometers a second, matching the mission profile to better than one part in a million.

Nothing built by humans had ever traveled so fast. The Sniffer registered small reductions in its speed as the fading gravity field of the distant Sun slowed its progress. The deceleration was in the flight profile, and it called for no remedial action.

The Sniffer checked that the guide star of Alpha Centauri lay directly ahead. Then it banked down into power-conserving mode, with the internal clock speed slowed by a factor of four million. Almost dormant, the spidery structure glided through the void between the stars. The main functions of the mission still lay far in the future.

At four hundred and eighty million seconds, almost fifteen years after launch, the incident particle flux rose above a preset threshold. The Sniffer activated all sensors and began a fine profiling of the medium through which it was now moving. At once it found differences from the projected situation.

The supercooled central brain of Sniffer-A had no circuits that might be described as worriers, but it was built to register, record, and transmit anomalies. The great bow wave of charged particles generated by the Alpha Centauri supernova had been reached ahead of time. Also, the particle mixture was grossly different from that in the mission profile.

The Sniffer began its comparisons. The particle flux was more energetic than anticipated, but that was consistent with a greater overall velocity and early arrival. A more significant oddity lay in the unexpected abundance of nuclei heavier than the protons of bare hydrogen. Everything was too plentiful, from deuterium — too weakly bound to have survived the fires of the supernova — to uranium. Odder yet, the data suggested patterns within the particles, as though the ions were somehow maintaining their exact relative separations over large distances.