consisted of a tank about the size of the pebble's own, along with a thruster engine at the rear. For most of its lifetime, the pebble would be held inside its "life jacket". This provided electrical power via a solar panel, included a star tracker to provide basic alignment information, and carried a laser communications transceiver. The shell itself was intended to provide protection from laser strikes and pellets from the known Soviet anti-satellite weapon, part of the Istrebitel Sputnikov program. Tests Only three all-up tests of the Pebbles concept were carried out before the program was cancelled. All three failed for various reasons. The first pebble test was carried out on 25 August 1990. This consisted of a basic airframe carrying an IR sensor, a star tracker and an attitude control system. It was to be launched to an altitude of over Wallops Island, Virginia, by a Black Brant sounding rocket. After launch, the pebble was to separate from the rocket, and then use its sensors to keep itself oriented with the still firing third stage of the Brant while also recording its orientation through the star tracker. The stage would be above the horizon and would take place at night, easing the problem of tracking. One of the explosive bolts that was supposed to separate the rocket fired 81 seconds into flight, much earlier than planned, causing the fairing to flip over to one side and pull the pebble partially out of the airframe. The only success on the mission was that another experiment, the ultraviolet plume instrument (UVPI) flying in orbit above the launch, was able to successfully track the rocket. As a result of the failure of the first test, the follow-up series was delayed by 10 months. The second test was carried out on 17 April 1991. In this case the interceptor was supposed to be looking down at the target against the daylight Earth, testing its capability to see targets in this orientation. Because of the failure of the first launch, it was decided to instead repeat the simpler nighttime test that was supposed to have occurred on the first flight. This test intended to have the interceptor separate from the launcher and then perform a programmed turn so it could see the launcher through several following test phases. The first phase was to simply acquire the target via its rocket plume and keep