Brilliant Pebbles was a ballistic missile defense (BMD) system proposed by Lowell Wood and Edward Teller of the Lawrence Livermore National Laboratory (LLNL) in 1987, near the end of the Cold War. The system would consist of thousands of small satellites, each with missiles similar to conventional heat seeking missiles, placed in low Earth orbit constellations so that hundreds would be above the Soviet Union at all times. If the Soviets launched its ICBM fleet, the pebbles would detect their rocket motors using infrared seekers and collide with them. Because the pebble strikes the ICBM before the latter could release its warheads, each pebble could destroy several warheads with one shot. The name is a play on the idea of Smart Rocks, a concept promoted by Daniel O. Graham as part of the Strategic Defense Initiative (SDI). This used large battle stations with powerful sensors, carrying dozens of small missiles, the rocks. To keep enough missiles above the Soviet Union at any given time, a minimum of 423 stations would be needed. The United States Air Force pointed out that this would require an enormous space lift capability, well beyond what was available. In meetings with Graham, Teller dismissed the concept as "outlandish" and vulnerable to attack by anti-satellite weapons. The SDI Office (SDIO) was similarly dismissive of the concept. Teller and Wood initially proposed their own BMD system, Project Excalibur. This used an X-ray laser driven by a nuclear warhead that could attack dozens of ICBMs at once. In 1986, Excalibur failed several critical tests. Soon after, the American Physical Society published a report stating that none of the directed-energy weapons being studied by SDI were remotely ready for use. Abandoning these approaches for the short term, SDIO then promoted a new concept that was essentially a renamed Smart Rocks. It was at this point that Wood introduced Pebbles, suggesting that advances in sensors and microprocessors meant there was no need for a central station—the missiles could host all the equipment they needed to act alone. To attack this system, anti-satellite weapons would have to be launched against every pebble, not every station. After considerable study, in 1990 Pebbles replaced Rocks as the baseline SDI design and in 1991 it was ordered into production and became the "crowning achievement of the Strategic Defense Initiative". By this time the Soviet Union was collapsing and the perceived threat changed to shorter-range