Comet Hyakutake
Encyclopedia
Comet Hyakutake is a comet
, discovered on January 31, 1996, which 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 atom
s in the coma
of the comet. The Ulysses spacecraft
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 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.
, an amateur
astronomer
from southern Japan. He had been searching for comets for years and had moved to Kagoshima Prefecture
partly for the dark skies in nearby rural areas. He was using a powerful set of binoculars
with 150 mm (6 inch) objective lenses to scan the skies on the night of the discovery.
This comet was actually the second Comet Hyakutake; Hyakutake had discovered comet C/1995 Y1 several weeks earlier. While re-observing his first comet (which never became visible to the naked eye
) and the surrounding patch of sky, Hyakutake was surprised to find another comet in almost the same position as the first had been. Hardly believing a second discovery 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 approximately 2.5 arcminutes across. It was approximately 2 astronomical unit
s (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.
were made, scientists realized that it was going to pass just 0.1 AU from the Earth on 25 March. Only four comets in the previous century had passed closer. Comet Hale-Bopp
was already being discussed as a possible "great comet
"; the astronomical community eventually realised that Hyakutake might also become spectacular because of its close approach.
Moreover, the comet's orbit showed that it had last returned to the inner Solar System
approximately 17,000 years earlier. Because the comet had probably passed close to the Sun several times before, the approach in 1996 would not be a maiden arrival from the Oort cloud
, a place where comets with orbital periods of millions of years come from. Comets entering the inner Solar System for the first time may brighten rapidly before fading as they near the Sun, as a layer of highly volatile material evaporates. This was the case with Comet Kohoutek
in 1973; it was initially touted as potentially spectacular, but only appeared moderately bright. Older comets show a more consistent brightening pattern. Thus, all indications pointed that Comet Hyakutake would be bright.
Besides approaching close to the Earth, the comet would also be visible throughout the night to northern hemisphere
observers at its closest approach because of its path, passing very close to the pole star
. This would be an unusual occurrence, because most comets are close to the Sun in the sky when the comets are at their brightest, leading to the comets appearing in a sky not completely dark.
with a tail about 5 degrees
long. 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, and its tail stretched 35 degrees. The comet had a notably bluish-green colour.
The closest approach occurred on 25 March. Hyakutake was moving so rapidly across the night sky that its movement could be detected against the stars in just a few minutes; it covered 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 latitude
s, appeared approximately 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 emissions 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.
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, but following perihelion it became much less monitored. The last known observation of the comet took place on November 2.
Hyakutake had passed through the inner Solar System approximately 17,000 years ago; gravitational interactions with the gas giants
during its 1996 passage stretched its orbit greatly, and barycentric fits
to the comet's orbit predict it will not return to the inner Solar System
again for approximately 70,000 years.
s passing, as well as a change in the direction and strength of the local magnetic field
. This implied that the spacecraft had crossed the 'wake' of an object, most likely a comet; the object responsible was not immediately identified.
In 2000, 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, 500 million km away, had crossed Ulysses' orbital plane on April 23, 1996. The solar wind
had a velocity at the time of about 750 km/s, at which speed it would have taken 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 ion
ised 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.
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 may 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, which compares to a value in Earth's oceans of about 1.5. 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.
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 two mechanisms. Interactions between energetic solar wind particles and cometary material evaporating from the nucleus is likely to contribute significantly to this effect. Reflection of solar X-rays is seen in other Solar System objects such as the Moon
, but a simple calculation assuming even the highest x-ray reflectivity possible per molecule or dust grain is not able to explain the majority of the observed flux from Hyakutake, as the comet's atmosphere is very tenuous and diffuse. Observations of comet C/1999 S4 (LINEAR)
with the Chandra satellite
in 2000 determined that X-rays observed from that comet were produced predominantly by charge exchange collisions between highly charged carbon
oxygen
and nitrogen
minor ions in the solar wind, and neutral water, oxygen and hydrogen in the comet's coma.
indicated that the comet nucleus
was about 2 km 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 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 kg/s at the beginning of March, rising to 3 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.
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
, discovered on January 31, 1996, which passed very close to Earth in March of that year. It was dubbed The Great Comet
Great comet
A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great...
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
Comet Hale-Bopp
Comet Hale–Bopp was perhaps the most widely observed comet of the 20th century, and one of the brightest seen for many decades...
, which was approaching the inner Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
at the time.
Scientific observations of the comet led to several discoveries. Most surprising to cometary scientists was the first discovery of X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emission from a comet, believed to have been caused by ionised solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
particles interacting with neutral atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s in the coma
Coma (cometary)
frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...
of the comet. The Ulysses spacecraft
Ulysses probe
Ulysses is a decommissioned robotic space probe that was designed to study the Sun as a joint venture of NASA and the European Space Agency . The spacecraft was originally named Odysseus, because of its lengthy and indirect trajectory to near Solar distance...
unexpectedly crossed the comet's tail at a distance of more than 500 million km from the nucleus
Comet nucleus
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma...
, 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
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, its orbital period was about 17,000 years, but the gravitational perturbation
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
of the giant planets has increased this period to 70,000 years.
Discovery
The comet was discovered on January 31, 1996, by Yuji HyakutakeYuji Hyakutake
was a Japanese amateur astronomer who discovered Comet Hyakutake on January 30, 1996 while using 25×150 binoculars.His only other discovery was comet C/1995 Y1. The media has stated that Hyakutake became interested in astronomy after seeing Comet Ikeya-Seki .He died in 2002, at age 51, of an...
, an amateur
Amateur
An amateur is generally considered a person attached to a particular pursuit, study, or science, without pay and often without formal training....
astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
from southern Japan. He had been searching for comets for years and had moved to Kagoshima Prefecture
Kagoshima Prefecture
is a prefecture of Japan located on the island of Kyushu. The capital is the city of Kagoshima.- Geography :Kagoshima Prefecture is located at the southwest tip of Kyushu and includes a chain of islands stretching further to the southwest for a few hundred kilometers...
partly for the dark skies in nearby rural areas. He was using a powerful set of binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...
with 150 mm (6 inch) objective lenses to scan the skies on the night of the discovery.
This comet was actually the second Comet Hyakutake; Hyakutake had discovered comet C/1995 Y1 several weeks earlier. While re-observing his first comet (which never became visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
) and the surrounding patch of sky, Hyakutake was surprised to find another comet in almost the same position as the first had been. Hardly believing a second discovery so soon after the first, Hyakutake reported his observation to the National Astronomical Observatory of Japan
National Astronomical Observatory of Japan
The is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii. It was established in 1988 as an amalgamation of three existing research organizations - the Tokyo Astronomical Observatory of the University of Tokyo, International Latitude...
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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
11.0 and had a coma approximately 2.5 arcminutes across. It was approximately 2 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (AU) from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. 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 orbitOrbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
were made, scientists realized that it was going to pass just 0.1 AU from the Earth on 25 March. Only four comets in the previous century had passed closer. Comet Hale-Bopp
Comet Hale-Bopp
Comet Hale–Bopp was perhaps the most widely observed comet of the 20th century, and one of the brightest seen for many decades...
was already being discussed as a possible "great comet
Great comet
A Great Comet is a comet that becomes exceptionally bright. There is no official definition; often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great...
"; the astronomical community eventually realised that Hyakutake might also become spectacular because of its close approach.
Moreover, the comet's orbit showed that it had last returned to the inner Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
approximately 17,000 years earlier. Because the comet had probably passed close to the Sun several times before, the approach in 1996 would not be a maiden arrival from the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
, a place where comets with orbital periods of millions of years come from. Comets entering the inner Solar System for the first time may brighten rapidly before fading as they near the Sun, as a layer of highly volatile material evaporates. This was the case with Comet Kohoutek
Comet Kohoutek
Comet Kohoutek, formally designated C/1973 E1, 1973 XII, and 1973f, was first sighted on 7 March 1973 by Czech astronomer Luboš Kohoutek. It attained perihelion on 28 December that same year....
in 1973; it was initially touted as potentially spectacular, but only appeared moderately bright. Older comets show a more consistent brightening pattern. Thus, all indications pointed that Comet Hyakutake would be bright.
Besides approaching close to the Earth, the comet would also be visible throughout the night to northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
observers at its closest approach because of its path, passing very close to the pole star
Pole star
The term "Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star.In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent...
. This would be an unusual occurrence, because most comets are close to the Sun in the sky when the comets are at their brightest, leading to the comets appearing in a sky not completely dark.
The comet passes the Earth
Hyakutake became visible to the naked eye in early March 1996. By mid-March, the comet was still fairly unremarkable, shining at 4th magnitudeApparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
with a tail about 5 degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
long. 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, and its tail stretched 35 degrees. The comet had a notably bluish-green colour.
The closest approach occurred on 25 March. Hyakutake was moving so rapidly across the night sky that its movement could be detected against the stars in just a few minutes; it covered the diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
of a full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
(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
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
for observers at mid-northern latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s, appeared approximately 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 emissions from diatomic carbon
Diatomic carbon
Diatomic carbon is a diatomic molecule of carbon , which occurs in electric arcs; in comets, stellar atmospheres and the interstellar medium; and in blue hydrocarbon flames.-Chemistry:...
(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
Comet Hale-Bopp
Comet Hale–Bopp was perhaps the most widely observed comet of the 20th century, and one of the brightest seen for many decades...
did the following year. Many European observers in particular did not see the comet at its peak because of unfavourable weather conditions.
Perihelion and afterwards
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 seen as it passed the Earth. By this time, however, it was close to the Sun and was not seen as easily. It was observed passing perihelion by the SOHOSolar and Heliospheric Observatory
The Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
Sun-observing satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, which also recorded a large coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
being formed at the same time. Its distance from the Sun at perihelion was 0.23 AU, well inside the orbit of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
.
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, but following perihelion it became much less monitored. The last known observation of the comet took place on November 2.
Hyakutake had passed through the inner Solar System approximately 17,000 years ago; gravitational interactions with the gas giants
Gas Giants
Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...
during its 1996 passage stretched its orbit greatly, and barycentric fits
Barycentric coordinates (astronomy)
In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies.The barycenter is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other...
to the comet's orbit predict it will not return to the inner Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
again for approximately 70,000 years.
Spacecraft passes through the tail
The Ulysses spacecraft made an unexpected pass through the tail of the comet on May 1, 1996. Evidence of the encounter was not noticed until 1998. Astronomers analysing old data found that Ulysses' instruments had detected a large drop in the number of protonProton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s passing, as well as a change in the direction and strength of the local magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. This implied that the spacecraft had crossed the 'wake' of an object, most likely a comet; the object responsible was not immediately identified.
In 2000, 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, 500 million km away, had crossed Ulysses' orbital plane on April 23, 1996. The solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
had a velocity at the time of about 750 km/s, at which speed it would have taken 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
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
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 ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ised 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
Great Comet of 1843
The Great Comet of 1843 formally designated C/1843 D1 and 1843 I, was a long-period comet which became very bright in March 1843 . It was discovered on February 5, 1843 and rapidly brightened to become a great comet...
, which was 2.2 AU long.
Composition
Terrestrial observers found ethaneEthane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
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 may 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
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
or less, indicating that it probably formed in a denser than average interstellar cloud.
The amount of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
in the comet's water ices was determined through spectroscopic
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
observations. It was found that the ratio of deuterium to hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(known as the D/H ratio) was about 3, which compares to a value in Earth's oceans of about 1.5. 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
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 ROSATROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...
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 two mechanisms. Interactions between energetic solar wind particles and cometary material evaporating from the nucleus is likely to contribute significantly to this effect. Reflection of solar X-rays is seen in other Solar System objects such as the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, but a simple calculation assuming even the highest x-ray reflectivity possible per molecule or dust grain is not able to explain the majority of the observed flux from Hyakutake, as the comet's atmosphere is very tenuous and diffuse. Observations of comet C/1999 S4 (LINEAR)
C/1999 S4
C/1999 S4 is a long-period comet discovered on September 27, 1999, by LINEAR.The comet made its closest approach to the Earth on July 22, 2000, at a distance of . It came to perihelion on July 26, 2000, at a distance of 0.765 AU from the Sun....
with the Chandra satellite
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
in 2000 determined that X-rays observed from that comet were produced predominantly by charge exchange collisions between highly charged carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
minor ions in the solar wind, and neutral water, oxygen and hydrogen in the comet's coma.
Nucleus size and activity
Radar results from the Arecibo ObservatoryArecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
indicated that the comet nucleus
Comet nucleus
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma...
was about 2 km 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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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 kg/s at the beginning of March, rising to 3 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.