A comet 100 times brighter than Halley's comet is approaching Earth.At present, the Hale-Bopp comet cannot be seen in the sky over Japan because it is below the horizon, but it will be visible here in the last half of this month. But in April, the comet will make its closest approach to the Sun and will continue to be visible until early May. In July 1995, two U.S. astronomical observers, Alan Hale and Thomas Bopp, independently discovered the comet. At that time, the comet was still at a position inside Saturn's orbit. According to Junichi Watanabe, National Astronomical Observatory spokesman, the Hale-Bopp comet is a mass of ice from a region called the cloud of Oort, which surrounds the solar system. The last time the comet approached the Sun was about 4,000 years ago, and it will next approach the Sun about 2,500 years from now. The cycle varies because the comet is influenced by gravitational fields of planets.The Hale-Bopp comet is similar in size to Halley's comet, but its quantity of water becoming vapor, which determines its brightness, is 1,500 tons per second, 100 times that of Halley's comet. Halley's comet, whose cycle is 76 years, has been baked by the Sun many times and its ice has become sparse. Compared with Halley's comet, the Hale-Bopp comet is "full of vigor," according to Watanabe.Although its absolute brightness is in the top class among about 1,000 comets so far recorded, the Hale-Bopp comet will not be very bright when seen from the Earth because it is so far away. It is expected that the comet will be as bright as the Hyakutake comet, which came by in spring 1996.