page_title: Comet Hyakutake
section_title: Nucleus size and activity
hierarchical_section_title: Comet Hyakutake / Scientific results / Nucleus size and activity
caption_attribution_description: English: Hyakutake comet nucleus. Español: Núcleo del cometa Hyakutake.
context_page_description: Comet Hyakutake is a comet, discovered on 31 January 1996, that passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night sky and was widely seen around the world. The comet temporarily upstaged the much anticipated Comet Hale–Bopp, which was approaching the inner Solar System at the time.
  Scientific observations of the comet led to several discoveries. Most surprising to cometary scientists was the first discovery of X-ray emission from a comet, believed to have been caused by ionised solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft unexpectedly crossed the comet's tail at a distance of more than 500 million kilometres from the nucleus, showing that Hyakutake had the longest tail known for a comet.
  Hyakutake is a long-period comet. Before its most recent passage through the Solar System, its orbital period was about 17,000 years, but the gravitational perturbation of the giant planets has increased this period to 70,000 years.
context_section_description: Radar results from the Arecibo Observatory indicated that the comet nucleus was about 4.8 km (3.0 mi) across, and surrounded by a flurry of pebble-sized particles ejected at a few metres per second. This size measurement corresponded well with indirect estimates using infrared emission and radio observations.
  The small size of the nucleus (Halley's Comet is about 15 km (9.3 mi) across, while Comet Hale–Bopp was about 60 km (37 mi) across) implies that Hyakutake must have been very active to become as bright as it did. Most comets undergo outgassing from a small proportion of their surface, but most or all of Hyakutake's surface seemed to have been active. The dust production rate was estimated to be about 2×10³ kg/s at the beginning of March, rising to 3×10⁴ kg/s as the comet approached perihelion. During the same period, dust ejection velocities increased from 50 m/s to 500 m/s.
  Observations of material being ejected from the nucleus allowed astronomers to establish its rotation period. As the comet passed the Earth, a large puff or blob of material was observed being ejected in the sunward direction every 6.23 hours. A second smaller ejection with the same period confirmed this as the rotation period of the nucleus.
caption: The region around the nucleus of Comet Hyakutake, as seen by the Hubble Space Telescope. Some fragments can be seen breaking off.
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