© Walter Pacholka, Astropics/SPLHale-Bopp, seen here from Joshua Tree National Park, California, was one of the brightest comets of the 20th century.
Few cosmic apparitions have inspired such awe and fear as comets. The particularly eye-catching Halley's comet, which last appeared in the inner solar system in 1986, pops up in the
Talmud as "a star which appears once in seventy years that makes the captains of the ships err". In 1066, the comet's appearance was seen as a portent of doom before the Battle of Hastings; in 1456, Pope Callixtus III is said to have excommunicated it.
Modern science takes a more measured view. Comets such as Halley's are agglomerations of
dust and ice that orbit the sun on highly elliptical paths, acquiring their spectacular tails in the headwind of charged particles streaming from the sun. We even know their source: they are Kuiper belt objects (KBOs) tugged from their regular orbits by Neptune and Uranus.
But there's a problem.
Certain comets, such as Hale-Bopp, which flashed past Earth in 1997, appear simply too infrequently in our skies. Their orbits must be very long, far too long to have an origin in the Kuiper belt. The conclusion of many astronomers is that the known solar system is surrounded in all directions by a tenuous halo of icy outcasts, thrown from the sun's immediate vicinity billions of years ago by the gravity of the giant planets.