The discovery of Comet C/1996 B2 (Hyakutake) has generated a great number
of inquiries from the news media and the general public. This information
sheet addresses the most commonly asked questions. (Please see the Appendix
at the end for definitions of astronomical terms in this discussion.)
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1) What's this I hear about a comet that was recently discovered and
was to become bright in March and April?
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A comet was discovered in late January that became the brightest comet
visible in a dark sky from the earth in 20 years, in the second half of
March 1996. The comet is now visible only to southern-hemisphere observers,
having passed perihelion on May 1. The comet has been followed by observers
in the southern hemisphere, as it faded from fourth magnitude in May to
sixth magnitude in late June (a binocular object), and the faint comet
is now only visible in large telescopes. This comet did not appear spectacular
to the general public, though astronomers have been pretty excited because
few comets become as bright as the brightest stars, and many useful observations
have already been made from visible wavelengths to ultraviolet to infrared
to radio (as the comet emits radiation at all of these wavelengths). The
comet was visible from all parts of the world until about March 23 or 24
(when southern-hemisphere observers will have more difficulty viewing it
until May), but faded as it moved away from the earth and toward the sun's
glare in April. It then re-emerged from the solar glare in mid-May after
having moved far south in the sky.
The comet passed rather close to the earth (0.102 AU, or about 15 million
kilometers or 9.3 million miles) on March 25 (at about 7 hours Greenwich
Mean Time, or about 2 a.m. USA Eastern Standard Time). At that time, C/1996
B2 was moving some 10-20 degrees per day across the northern sky, and the
comet will in fact be visible all night long from the U.S., Canada, Europe,
and Japan (weather and light pollution conditions permitting) through the
last week in March. Then it was visible in the northwest each evening in
late twilight during the last week of April, though it was a difficult
object due to low altitude and twilight. Comet C/1996 B2 (Hyakutake) temporarily
"upstaged" the anticipated good showing of comet
C/1995 O1 (Hale-Bopp) in early 1997, though C/1995 O1 may be substantially
brighter than was C/1996 B2.
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1a) What is a comet?
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It is a small body in our solar system that orbits the sun much as do the
earth and other planets. It has a "nucleus", or solid body, that is usually
around 1-10 km across and is supposedly a "dirty snowball" consisting of
ices and dust and rock. When far from the sun in the outer reaches of the
solar system, there is very little activity coming off such a nucleus.
However, when the comet nucleus gets closer to the sun, the sun's radiation
warms the nucleus, causing the ices to sublimate (or "steam") outwards
from the nucleus from various vents, carrying along various atoms and molecules
that constitute different ices and dust and rock in the original nucleus.
This venting outwards creates both the coma (or atmosphere) surrounding
the nucleus --- out to thousands, hundreds of thousands, and sometimes
millions of kilometers from the nucleus itself --- and also the tail of
material that generally streams in the anti-sunward direction from the
nucleus. Once this venting activity "turns on", the true nucleus is almost
invariably invisible from Earth, as intense material in the inner coma
then tends to mask the tiny nucleus.
Note that comets are NOT the same as meteors (or so-called "shooting
stars"; see definition in Appendix below); while
meteors typically streak through our atmosphere in a second or two and
are sometimes much brighter than even a "bright" comet (when they are called
"fireballs"), comets are much further away than the moon and move slowly
with respect to the background stars from night to night, rising and setting
each day just as do the sun, the moon, the planets, and the stars.
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2) How was this comet discovered?
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On January 30 Universal Time (Greenwich Mean Time), a Japanese amateur
named Yuji Hyakutake from Hayato-machi, Aira-gun, Kagoshima-ken, Japan,
made a visual discovery of a previously-unknown comet (his second) using
25x150 binoculars (these large binoculars have lenses that are 6 inches
across). This comet was very close in the sky to the discovery location
of another comet, C/1995 Y1, found by the same observer one month previously!
The comet was rapidly confirmed by other experienced observers in Japan,
and the results were reported to the worldwide clearinghouse for comet
discoveries in Cambridge, Massachusetts, the Central
Bureau for Astronomical Telegrams (CBAT). The Central Bureau, which
is operated by the Smithsonian Astrophysical Observatory for the International
Astronomical Union (IAU) then issued an IAU
Circular announcing the discovery (IAUC 6299, 1996 January
31), as is the practice for new comet, nova, and supernova discoveries.
The designation C/1996 B2 (Hyakutake) was assigned by the Central Bureau
upon computation of preliminary orbital elements (IAUC 6303). [See
question 11, below, for further information on IAU
Circulars.]
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3) What is the proper name of this comet, and how did it get that name?
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The proper designation as used by the International Astronomical Union
on its IAU Circulars is "Comet C/1996 B2 (Hyakutake)". Comets are
normally named for their discoverers, and this is done by consultation
between the CBAT and a special committee of nine astronomers within the
IAU. The designation "C/1996 B2" means that this was the second comet found
in the second halfmonth of January (letter B plus number 2) in the year
1996; halfmonths are given as letters, with "A" covering Jan. 1-15, "B"
covering Jan. 16-31, "C" covering Feb. 1-15, etc. ("I" being omitted and
"Z" not needed); the "C/" indicates that this is a long-period comet (that
is, one with a solar-orbiting period of more than 200 years). Similar to
the system used for designating asteroids, this system was brought into
use for comets on 1995 Jan. 1.
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4) How far away is the comet now, and how close will it come to Earth?
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The comet was 2.0 Astronomical Units (the equivalent of 300 million kilometers,
or 186 million miles) from the sun at discovery, and 1.83 AU (or 275 million
km, or 170 million miles) from the earth. The comet will come no closer
to us than about 0.10 AU (15 million km, or 9.3 million miles), which is
some 40 times the earth-moon distance, and that happened around 7 hours
Greenwich Mean Time (about 2 a.m. USA Eastern Standard Time) on 1996 March
25; the comet's velocity when closest to the earth in late March was roughly
150,000 kilometers per hour (for comparison, the earth orbits the sun at
about 108,000 km/hr). The comet will reach perihelion (closest approach
to the sun) on 1996 May 1 at a distance of 0.23 AU (34 million km, or 21
million miles) from the sun; the comet's velocity at that time will be
roughly 316,000 km/hr.
The approach of C/1996 B2 to the earth on March 25 (0.102 AU) was the
closest for any comet since 1983 (when there were two comets coming to
0.06 AU and 0.03 AU within a month of each other), and it is the fifth
closest approach of any comet during the past century. What is unique about
this comet is that no other comet is known then to have gone on to pass
anything like as close to the sun as this one does (0.23 AU on May 1).
One of the 1983 comets had about twice this comet's perihelion distance,
but the approach to the earth was well after perihelion. There was possibly
a comet with a perihelion distance comparable to this one that came closer
to the earth after perihelion in the year 400, but that is very uncertain.
The time interval between passage near the earth and subsequent passage
near the sun is longer for C/1996 B2 (37 days) than for any closer earth
approach since that of the famous Lexell comet in 1770 (43 days), that
comet holding the record confirmed approach to the earth (0.015 AU). C/1996
B2 is intrinsically the brightest earth-approacher since the early eighteenth
century, and the 55 days between discovery and earth approach is a record
for a preperihelic earth approach. A list of the closest
cometary approaches to the earth is available.
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4a) How do we know these distances and where the comet will be at any
given time?
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The distances of the comet now and in the future are known because of the
existence of many accurate positional measurements of the comet, made mostly
by amateur astronomers around the world using CCD cameras on numerous nights
as the comet moves slowly with respect to the "fixed" background stars
(whose positions are known from catalogues). As many positional (or astrometric)
observations pour in to our offices from observers around the world, we
are able to compute the comet's path, or orbit, about the sun. Two pre-discovery
photographs containing faint images of C/1996 B2 were found by a Japanese
amateur who had taken them on January 1, and these have permitted the comet's
orbit about the sun to be more definitively determined. The Central Bureau
and its associated Minor Planet Center maintain such positions and orbits
of comets and minor planets. Comet C/1996 B2 (Hyakutake) apparently had
an orbital period (i.e., the time it takes to make one complete revolution
about the sun) of about 8000 years, prior to its current voyage to the
inner solar system; the perturbations by the major planets will alter this
figure, such that its period may increase to roughly 14,000 years (according
to figures computed by Brian G. Marsden on 1996 March 28).
An orbital
diagram for this comet is available. This plot shows a view from the
north pole of the ecliptic, which is the plane of the earth's orbit about
the sun (on our sky, the ecliptic is the apparent annual path of the sun,
which is inclined with respect to the celestial equator by about 23 degrees);
the dashed line for the comet shows its path when it is below this plane,
and the solid line shows when it is north of the ecliptic (and thus best
placed for northern-hemisphere observers). The small inset shows a side
view in the plane of the earth's orbit, showing how the comet's orbit slices
through the ecliptic at a high angle.
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5) Can members of the general public see the comet now?
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(see also question 5, above) Only if you are in the southern hemisphere,
and then only with the aid of a large telescope. The best way for inexperienced
observers to see any comet usually is to contact a local astronomy club,
planetarium, or college observatory to find out about upcoming star parties
or public observatory nights in which the comet will be shown to interested
members of the public. For example, the Center for Astrophysics in Cambridge,
MA, holds monthly
observatory nights for the public on the third Thursday of the month
throughout the year; call 617-495-7461 for additional information; the
CfA's Whipple Observatory near Tucson, Arizona, holds quarterly star parties
(call 520-670-5707 for information there).
When comet C/1996 B2 (Hyakutake) was readily visible to observers in
the northern hemisphere several months ago, it passed several degrees away
from the bright star Arcturus (magnitude 0) during March 22-23, then a
similar distance away from the stars in the handle of the Big Dipper on
March 24-25. On March 26, comet C/1996 B2 passed by the Little Dipper,
and on March 27 passed a couple of degrees away from Polaris, the North
Star (at which time it was at a rather constant altitude above the horizon,
looking due north, for northern-hemisphere observers all night long). The
comet then moved toward the evening sky, from whence it was visible to
the northwest in late evening twilight and during the evening hours during
early- to mid-April. After mid-April, the comet neared the sun (and thus
the sun's glare), becoming more of a twilight object that set around the
time it got dark.
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6) So how bright was this comet expected to become?
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Comets are very difficult to predict in terms of brightness, especially
when first discovered. The comet is now well past its expected peak brightness.
It should now slowly fade for southern-hemisphere objects as it gradually
recedes to more distant parts of the solar system. While C/1996 B2 (Hyakutake)
is certainly the brightest comet visible in a dark sky since 1976 (and
one of the most fascinating this century, due to its close approach to
the earth), it will probably not be considered as the first "spectacular"
naked-eye comet since comet C/1975 V1 (West), which in the morning skies
of March 1976 was as bright as Sirius (the brightest star) or even the
planet Jupiter. Such an accolade may be assigned to C/1995 O1 (Hale-Bopp)
next year, and such accolades must partly be answered with respect to "the
eye of the beholder" (see question 10 below); it would appear at this time
that comet C/1995 O1 (Hale-Bopp) will become brighter in early 1997 than
C/1996 B2 (Hyakutake) was in late March.
Comet C/1996 B2 (Hyakutake) peaked in brightness around magnitude 0
during the last week in March. It faded to magnitude 2 during April. This
brightness refers to the brightness of the comet's coma (or head or atmosphere);
note that the comet's coma is still now a half degree or more across (or
equal to or larger than the apparent size of the moon), and it was as much
as 2 or more degrees across around March 25-26. This means that C/1996
B2 is not likely to be as obvious as a star or planet of the same brightness,
because the latter objects have the same amount of light concentrated in
(essentially) a point. Thus, while experienced observers were measuring
the "integrated" brightness of the entire coma of the comet to be near
magnitude 0 or even brighter around March 25, in effect the comet appeared
at a glance to be about as obvious as a first-magnitude star, because that
0-magnitude brightness was spread out over the better part of a degree
on the sky (as opposed to such light appearing as a point for stars).
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7) Did this comet have a nice tail?
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Tail lengths when the comet was close to the earth in late March were reported
to be well over 60 degrees, and the naked-eye lengths approached 80 to
100 degrees in April --- very rare for a comet. Not all comets have tails.
There are two types of tails --- gas (or ion) tails and dust tails. Gas
tails tend to be more common in comets, but they are also usually fainter
than dust tails to the naked eye; this is because gas tails emit light
by fluorescence, in which gas atoms emitted from the comet's nucleus interact
with solar-wind radiation, and they re-transmit energy received from solar
radiation at different wavelengths. This fluoresced light in comet tails
is very blue, which is difficult for the human eye to perceive. Dust tails
tend to become prominent in comets that travel inside the earth's orbit
(i.e., less than 1 AU from the sun), in regions where the warming solar
radiation more strongly interacts with ice in the comet's nucleus, causing
much overall coma and tail activity. Most of the so-called 'bright' comets
of this past century displayed prominent naked-eye dust tails.
While experienced observers in dark skies reported tail lengths of more
than 60 degrees in late March (and up to 100 degrees in mid-April), this
refers to a very faint, tenuous gas tail that is all but invisible from
more urban areas that suffer from heavy light
pollution; such urban viewers should expect only to see a fuzzy
star, or a large bright, fuzzy ball in binoculars that is progressively
brighter toward the comet's center. However, the brighter part of the tail
(especially any dust tail, which is more easily seen by the human eye)
was expected to grow a bit during April, as the comet draws rapdily nearer
to the sun.
The comet in March exhibited mainly a bluish gas tail, which makes it
difficult to see with the human eye. There are some signs that more of
a yellowish dust tail is forming now, but the coma is sure to be dominant
in brightness over the tail during most of April.
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8) Do we know how large this comet is?
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Some recent radar bouncing has suggested that the nucleus may be on the
order of 1-3 km in size, which is typical for such long-period comets (which
normally are much more active than are short-period comets). The coma size
is on the order of hundreds of thousands, or even millions, of kilometers
(miles) across. The source of this activity is actually a much tinier nucleus,
or solid, dirty snowball. Most comets have nucleus sizes around 1-10 km;
comet 1P/Halley had an oblong nucleus of size 8x15 km. Because of the dense
shroud of coma material around the nucleus, we cannot tell the size of
the nucleus itself while the coma is active (without a close rendezvous
by an artificial spacecraft, as with 1P/Halley).
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9) How frequently are comets discovered?
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During 1990-1994, an average of about 12 comets per year were discovered
(plus about one rediscovery per year of a "long-lost" short-period comet),
with roughly four discovered by amateur astronomers. However, at the end
of 1994, two major professional search programs for comets ceased at Palomar
Mountain in southern California, and these programs had discovered four
or more comets per year over the past 10-12 years. In 1995 there were five
discoveries of previously-unknown comets, plus one discovery of a comet
that had been lost for 150 years (122P/de Vico). Comet C/1996 B2 was the
third new comet discovery of this year.
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10) How frequently do 'spectacular' comets become visible?
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It depends on your definition of "spectacular", but the range is roughly
every 20 years or so (or a couple of times in a lifetime), especially if
one defines "spectacular" as being as bright as the brightest planets or
brighter. By that definition, comet C/1996 B2 is not likely to become "spectacular".
The increase in light
pollution made comet C/1996 B2 (Hyakutake) harder to see for many
people, regardless of its brightness. This, combined with a high standard
for "spectacular" activities, could detract from public perception of this
comet.
Be wary, then, that many members of the general public --- who
are used to fireworks being spectacular (where fireworks are typically
between the moon and sun in brightness) --- may not find anything fainter
than a crescent moon (mag -8 or so) to be spectacular! Realize that there
is a broad spectrum of listeners and readers out there! Light pollution
is much bigger today than 20 or 30 years ago, and those stuck in a large
city are perhaps unlikely to be impressed. We may be treated to two bright
comets in the span of a single year, with C/1995 O1 (Hale-Bopp) expected
to become a naked-eye object for early 1997.
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11) What are the IAU Circulars?
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The Circulars are a publication of the Central Bureau for Astronomical
Telegrams of the International Astronomical Union. The IAUCs
are available both in paper form (by postal mail) and in electronic form
via the CBAT
Computer Service and via e-mail. IAUCs are the original source
for discovery information regarding all new comets, novae, and supernovae.
Newspapers and magazines, as well as libraries and professional and amateurs
astronomers, subscribe to these useful astronomical news circulars. For
subscription information either check out the
on-line subscription information, send e-mail to iausubs@cfa.harvard.edu
or send postal mail to:
Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory
60 Garden St.
Cambridge, MA 02138; U.S.A.
And check out the Central Bureau's World Wide Web page with useful information
at the following URL:
http://cfa-www.harvard.edu/cfa/ps/cbat.html
Contact: Daniel W. E. Green (Associate Director, Central Bureau for
Astronomical Telegrams). E-mail dgreen@cfa.harvard.edu.
Telephone 617-495-7440.
Written by D. W. E. Green, with input from B. G. Marsden, G. V. Williams,
J. Hoskins, J. Corliss, and J. Cornell. [Updated 1996 November 20.]