Comet Hyakutake

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Comet Hyakutake ( Japanese: 百武彗星 Hyakutake suisei, IPA [çakɯtake sɯiseː]; formally designated C/1996 B2) is a comet that was discovered in January 1996, and passed very close to the Earth in March of that year. It was dubbed The Great Comet of 1996, and was one of the closest cometary approaches to the Earth in the previous 200 years, resulting in the comet appearing very bright in the night sky and being seen by a large number of people around the world. The comet temporarily upstaged the long-awaited Comet Hale-Bopp, which was approaching the inner solar system at the time, although Hyakutake was at its brightest for only a few days.

Scientific observations of the comet led to several notable discoveries. Most surprising to cometary scientists was the discovery of X-ray emission from the comet, the first time a comet had been found to be emitting X-rays. This emission is believed to be caused by ionised solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft also unexpectedly crossed the comet's tail at a distance of more than 500 million  km from the nucleus, showing that Hyakutake had the longest tail yet known for a comet.

Hyakutake is a long period comet. Before its most recent passage through the solar system, its orbital period was about 15,000  years, but the gravitational influence of the giant planets has now increased this to 72,000 years.

Discovery

Comet Hyakutake captured by the Hubble Space Telescope on April 4, 1996 with an infrared filter.

The comet was discovered on January 30, 1996 by Yuji Hyakutake, an amateur astronomer from southern Japan. He had been searching for comets for some years and had moved to Kagoshima partly for the dark skies in the nearby rural areas. He was using a powerful set of binoculars with six- inch objective lenses to scan the skies on the night of the discovery.

The comet he found was actually the second Comet Hyakutake, the first being comet C/1995 Y1, which Hyakutake had discovered just a few weeks earlier. While he was re-observing his first comet (which never became visible to the naked eye), Hyakutake happened to look at the patch of sky where he discovered it. To his great surprise there was another comet there, in almost exactly the same position his first had been. Although he could hardly believe he had discovered a second comet so soon after the first, Hyakutake reported his observation to the National Astronomical Observatory of Japan the following morning. Later that day, the discovery was confirmed by independent observations.

At the time of its discovery the comet was shining at magnitude 11.0 and had a coma about 2.5  arcminutes across. It was about 2  astronomical units (AU) from the Sun. Later, a pre-discovery image of the comet was found on a photograph taken on January 1, when the comet was about 2.4 AU from the Sun and had a magnitude of 13.3.

Orbit

When the first calculations of the comet's orbit were made, scientists realised that the comet was going to pass very close to the Earth on 25 March, just 0.1 AU away. Only three comets in the previous century had passed closer. As Comet Hale-Bopp was already being discussed as a possible "great comet", it took a while for the astronomical community to realise that Hyakutake too might become spectacular; its close approach to Earth meant it was very likely to become a great comet.

Also encouraging for the comet's chances of becoming bright was that its orbit showed it had last returned to the inner solar system about 17,000 years ago. This meant it was likely to have passed close to the Sun several times before, rather than being a fresh arrival from the Oort cloud, which contains comets having orbital periods of millions of years. Comets entering the inner solar system for the first time may brighten rapidly at first before fading as they near the sun due to a layer of highly volatile material being evaporated away: this was the case with Comet Kohoutek in 1973, which was initially touted as a comet of the century, but only appeared modestly bright. Older comets show a more consistent and predictable brightening pattern.

Besides approaching the Earth closely, the comet's path also meant that it would be visible throughout the night to northern hemisphere observers at its closest approach, passing very close to the pole star. Most comets are close to the Sun in the sky when they are at their brightest and thus may not be seen in a completely dark sky.

The comet passes the Earth

The comet on the evening of its closest approach to Earth on 25 March 1996.

Hyakutake became visible to the naked eye in early March. By mid-March, the comet was still fairly unremarkable, shining at 4th magnitude with a tail about 5  degrees long. However, as it neared its closest approach to Earth it rapidly became brighter, and its tail grew in length. By March 24, the comet was one of the brightest objects in the night sky; its tail stretched an impressive 35 degrees across the sky. The comet had a notably bluish-green colour.

The comet made its closest approach to Earth on 25 March, when it was moving across the night sky so rapidly that its movement could be detected against the stars in just a few minutes. It raced across the sky, moving about the diameter of a full moon (half a degree) every 30 minutes. Observers estimated its magnitude as around 0, and tail lengths of up to 80 degrees were reported. Its coma, now close to the zenith for observers at mid-northern latitudes, appeared some 1.5 to 2 degrees across, roughly four times the diameter of the full moon. Even to the naked eye, the comet's head appeared distinctly green, due to strong emission from diatomic carbon (C₂).

Because Hyakutake was at its brightest for only a few days, it did not have time to permeate the public imagination in the way that Comet Hale-Bopp did the following year. Many European observers in particular did not see the comet at its peak because of unfavourable weather conditions. However, many people who saw both Hyakutake and Hale-Bopp at their peaks contend that Hyakutake was more impressive.

Perihelion and afterwards

The SOHO satellite captured this image of Hyakutake as it passed perihelion, with a nascent coronal mass ejection also visible to the left of the Sun.

After its close approach to the Earth, the comet faded to about 2nd magnitude. It reached perihelion on May 1, 1996, brightening again and exhibiting a dust tail in addition to the gas tail well seen as it passed the Earth. By this time, though, it was very close to the Sun and was not seen as easily. It was observed passing perihelion by the SOHO Sun-observing satellite, which also recorded a large coronal mass ejection being formed at the same time. Its distance from the Sun at perihelion was 0.23 AU, well inside the orbit of Mercury.

After its perihelion passage, Hyakutake faded rapidly and was lost to naked-eye visibility by the end of May. Its orbital path carried it rapidly into the southern skies, and following perihelion it became much less monitored than before. The last known ground-based observation of the comet took place on October 24, 1996, when it was shining at magnitude 16.8 and no longer had an observable coma.

Hyakutake had previously passed through the inner solar system about 17,000 years ago; gravitational interactions with the gas giants during its 1996 passage stretched its orbit greatly, and it will not return to the inner Solar System again for approximately 72,000 years. However, there are conflicting orbital data suggesting a revolutionary period closer to 114,000 years.

Scientific results

Spacecraft passes through the tail

The Ulysses spacecraft made an unplanned and unexpected pass through the tail of the comet on May 1, 1996. The encounter was not noticed until 1998, when astronomers analysing old data found that Ulysses' instruments had detected a large drop in the number of protons passing, as well as a change in the direction and strength of the local magnetic field. They realised that this implied that the spacecraft had crossed the 'wake' of an object, most likely a comet, but did not identify the object responsible.

Two years later, two teams independently analyzed the same event. The magnetometer team realized that the changes in the direction of the magnetic field mentioned above agreed with the "draping" pattern expected in a comet's ion, or plasma tail. The magnetometer team looked for likely suspects. No known comets were located near the satellite, but looking further afield, they found that Hyakutake, some 500 million km away, had crossed Ulysses' orbital plane on April 23. The solar wind had a velocity at the time of about 750 km/s, at which speed it would have taken about eight days for the tail to be carried out to where the spacecraft was situated at 3.73 AU, approximately 45 degrees out of the ecliptic plane. The orientation of the ion tail inferred from the magnetic field measurements agreed with the source lying in Comet Hyakutake's orbital plane.

The other team, working on data from the spacecraft's ion composition spectrometer, discovered a sudden large spike in detected levels of ionised particles at the same time. The relative abundance of chemical elements detected indicated that the object responsible was definitely a comet.

Based on the Ulysses encounter, the comet's tail is known to have been at least 570 million km (360 million  miles; 3.8 AU) long. This is almost twice as long as the previous longest-known cometary tail, that of the Great Comet of 1843, which was 2.2 AU long.

Composition

Terrestrial observers found ethane and methane in the comet, the first time either of these gases had been detected in a comet. Chemical analysis showed that the abundances of ethane and methane were roughly equal, which is thought to imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules. Hyakutake's ices must have formed at temperatures of 20  K or less, indicating that it probably formed in a denser than average interstellar cloud.

The amount of deuterium in the comet's water ices was determined through spectroscopic observations. It was found that the ratio of deuterium to hydrogen (known as the D/H ratio) was about 3×10⁻⁴, which compares to a value in Earth's oceans of about 1.5×10⁻⁴. It has been proposed that cometary collisions with Earth might have supplied a large proportion of the water in the oceans, but the high D/H ratio measured in Hyakutake and other comets such as Hale-Bopp and Halley's Comet have caused problems for this theory.

X-ray emission

X-ray emission from Hyakutake, as seen by the ROSAT satellite.

One of the great surprises of Hyakutake's passage through the inner solar system was the discovery that it was emitting X-rays, with observations made using the ROSAT satellite revealing very strong X-ray emission. This was the first time a comet had been seen to do so, but astronomers soon found that almost every comet they looked at was emitting X-rays. The emission from Hyakutake was brightest in a crescent shape surrounding the nucleus with the ends of the crescent pointing away from the Sun.

The cause of the X-ray emission is thought to be a combination of several mechanisms. Reflection of solar X-rays is seen in other solar system objects such as the Moon, but is not likely to be able to explain the whole flux from Hyakutake, as the diffuse coma would be an inefficient X-ray reflector. Interactions between energetic solar wind particles and cometary material is also likely to contribute significantly to this effect. Observations of Comet LINEAR with the Chandra satellite in 2000 determined that X-rays observed from that comet were produced predominantly by collisions between nitrogen and oxygen ions in the solar wind and neutral hydrogen in the comet's coma.

Nucleus size and activity

The region around the nucleus of Comet Hyakutake, as seen by the Hubble Space Telescope. Some fragments can be seen breaking off.

Radar results from the Arecibo Observatory indicated that the nucleus of the comet was about 2 km across, and surrounded by a flurry of pebble-sized particles ejected from the comet at a few metres per second. This measurement of the nucleus size corresponded well with indirect estimates using infrared emission and radio observations.

The small size of the nucleus ( Halley's Comet is about 15 km across, while Comet Hale-Bopp was about 40 km 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.