nuclear diameter: 1-40 km (Halley:16 x 8 x 7 km) nuclear mass: 1014-1019g (Halley: 1017-1018g) nuclear mass loss per apparition: ~1 % coma radius: 104-105 km hydrogen cloud radius: 107 km dust tail length: 106-107 km dust tail particle size: 0.1-100 microns dust tail direction: antisolar, becoming curved as dust particles follow independent orbits ion tail length: 106-108 km ion tail direction: antisolar short-period comets: period < 200 years long-period comets
: period > 200 years comet discovery rate: ~12 per year number of comets discovered to date: ~4 000 average number of apparitions per year: ~17
pass close to Earth only 1% as often as NEAs do.
can be difficult to watch and understand in part because they take at least 200 years for each orbit around the sun, and sometimes can take thousands or millions of years.
In that case, 1992 QB1 may have been ejected from a far more distant -- and still hypothetical -- reservoir of long-period comets
called the Oort cloud.
Although the main Kuiper Belt ends around 48 a.u., there is another large reservoir of objects in the distant solar system from which the long-period comets
Weissman notes that Yanaka's orbit, while still somewhat unclear, indicates this object belongs to a group of long-period comets
that pass near the sun every 10,000 years or so.
Short-period comets come from the Kuiper belt (where temperatures are about -230[degrees]C, or -380[degrees]F) whereas most long-period comets
originate in the Oort cloud (about -260[degrees]C).
(orbital period P > 200 yr) typical discovery rate per year for new comets: 6 semi major axes: [10.sup.2]-[10.sup.5] AU
Dutch astronomer Jan Oort (1900-1992) inferred its existence in 1950, based on the observation that the orbits of long-period comets
(those with periods greater than 200 years) are large and randomly oriented in the sky.