long-period comet that was spectacularly visible to the naked eye, having a bright coma and a thick white dust tail. It was discovered independently in July 1995 by Alan Hale and Thomas Bopp, two American amateur astronomers, at the unusually far distance of 7 astronomical units (AU; about 1 billion km 【600 million miles】) from the Sun, well beyond Jupiter's orbit. The comet reached perihelion (closest distance to the Sun) at 0.914 AU on April 1, 1997, without ever coming very close to Earth (nearest distance 1.31 AU), because its orbit was almost perpendicular to that of Earth. In addition to its broad dust tail, the comet possessed a narrower bluish plasma tail slanting away from the dust tail. From the comet's rate of gas production, its nucleus was estimated to be at least 30 km (20 miles) in diameter.