Produced at the Harvard-Smithsonian Center for Astrophysics (CfA),
Cambridge, Massachusetts, U.S.A.
The discovery of Comet C/1995 O1 (Hale-Bopp) generated a great
number of inquiries from the news media and the general public. This
information sheet (which has been available at this website since shortly
after the comet's discovery, and which has been updated numerous times)
addresses the most commonly asked questions.
(Please see the Appendix at the end for definitions
of astronomical terms in this discussion.)
- 1) What's this I hear about a bright comet that was visible
from late 1996 and into early 1997?
- A comet was discovered in July 1995 that became bright enough in 1997
to be easily
seen with the naked eye, even from urban sites, and remained easily visible to
northern-hemisphere observers through April/May 1997. Comet C/1995 O1
(Hale-Bopp) orbits the sun, with closest approach (perihelion) having occurred
on 1997 April 1. Ever since discovery, the comet has shown high activity and
has been intrinsically very bright at relatively large distances from the sun;
in fact, it is evidently the brightest comet *intrinsically* (NOT *apparently*!)
whose orbit passes inside the earth's orbit in over 400 years (since the great
comet of 1577). Its high rate of activity, and the fact that it was a naked-eye
object for many months, is why much excitement has been present in the
C/1995 O1 (Hale-Bopp) became brighter than was
C/1996 B2 (Hyakutake) in late March 1996. Comet C/1995 O1 was probably
the best-observed comet in the history of mankind (not because its
*apparent* brightness was so great, but because it was a relatively easy
naked-eye object in the evening sky for northern-hemisphere observers for
a solid two months, with plenty of media attention to drive people outdoors
in the early evening to have a look).
- 2) What is a comet?
- 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
Note that comets are NOT the same as meteors (or so-called "shooting
stars"; see definition in Appendix below); while
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.
- 3) How was this comet discovered?
- On 1995 July 23, two observers first each spotted the comet while
looking at a cluster of stars known as Messier 70 (M70) in the
constellation Sagittarius. Within minutes of each other, Alan Hale in
New Mexico and Thomas Bopp in Arizona independently recognized a fuzzy
object near M70 that was fainter than the star cluster itself, and both
reported it 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 6187, 1995
July 23), as is the practice for new comet, nova, and supernova
discoveries. [See question 11, below, for further
information on IAU Circulars.]
- 3a) Who are the discoverers?
- Alan Hale has a Ph.D. in astronomy from New Mexico State
University and resides in Cloudcroft, NM. He is one of the world's
most active visual observers of comets and has seen nearly 200
different comets over many years. Thomas Bopp, who lives in a suburb
of Phoenix, AZ, is an amateur astronomer who was observing at a "star
party" with other amateur astronomers in the desert about 90
miles south of his home.
- 3b) What is the proper name of this comet, and how did it get that
- The proper designation as used by the International Astronomical
Union on its IAU Circulars, and the formal usage of professional
scientists in the refereed astronomical literature, is "Comet C/1995 O1
(Hale-Bopp)", though many people have referred to it popularly as "Comet
Hale-Bopp". 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/1995 O1" means that this was
the first comet found in the second halfmonth of July (letter O plus number 1)
in the year 1995; 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.
Tip: Whenever one comes across a reference to comet C/1995 O1 (Hale-Bopp) as "the Hale-Bopp comet", one
can be pretty sure that the information attached to that phrase is compiled by somebody unfamiliar with
comets and thus prone to error and misinformation. (People familiar with comets will either use their proper
designations or refer to them as "comet Hale-Bopp", "Halley's Comet", or simply "Hale-Bopp", but never "the
Halley comet" or "the Hale-Bopp comet"). [This might be analagous to one saying "game ballfoot" iustead of
"football game", or "office post" instead of "post office", or "Church Catholic Roman" instead of "Roman Catholic
Church" --- not strictly incorrect, perhaps, but obviously odd to those familiar with those subjects.]
- 4) How far away is the comet now, and how close will it come to
- Comet C/1995 O1 (Hale-Bopp) was 7.16 Astronomical Units (the equivalent of
1.07 billion kilometers, or 666 million miles) from the sun at discovery, and
6.20 AU (or 930 million km, or 577 million miles) from the earth. In late
April 1996, the comet was about 4.5 AU from both the earth and the sun, and in
September 1996 the comet was about 3 AU from both the earth and the sun. On
1997 January 7, the comet was 2.47 AU from the earth and 1.68 AU from the sun;
on February 1, the comet was 2.00 AU from the earth and 1.37 AU from the sun.
The comet came no closer to us than about 1.315 AU (197 million km, or 122
million miles), and that happened around 1997 March 22. This means that the
comet came no closer to the earth than 1.3 times the sun-earth distance.
[Contrast this with the minimum earth-comet distance of 0.10 AU, or 9.3
million miles, for C/1996 B2 (Hyakutake) on 1996 March 25. But because C/1995
O1 is intrinsically much brighter than C/1996 B2, it was brighter at 1.3 AU
from the earth than was C/1996 B2 at 0.10 AU.] Comet C/1995 O1 (Hale-Bopp)
reached perihelion (closest approach to the sun) on 1997 April 1 at around
3h30m Greenwich Mean Time (which corresponds to the following times on March
31 in the United States: 10:30 p.m. EST; 9:30 p.m. CST; 7:30 p.m. PST) at a
distance of 0.91 AU (136 million km, or 85 million miles) from the sun.
(For comparison, the comet's
aphelion distance, at which point it is furthest from the sun in its orbit,
is around 372 AU.)
The comet's velocity with respect to the sun in September 1996 (when it
was about 3 AU from the sun) was about 2.09 million kilometers per day (1.3
million miles per day), or about 24 km per second (15 miles/sec). When the
comet was closest to the sun (0.91 AU from the sun) in early April 1997, it
was travelling at about 44 km/sec (27 miles/sec).
- 4a) How do we know these distances and where the comet will be at any
- Orbit computation became possible with the publication in 1687 of
Sir Isaac Newton's Principia, in which he produced the classical laws
of physical motion for objects in the solar system. Over the centuries,
numerical techniques (and now powerful electronic computers) have greatly
improved the manner in which orbits of solar-system objects are calculated.
There are six so-called orbital elements that are unique (like a
fingerprint) for each solar-system
object orbiting the sun, defining such quantities as the object's time
of perihelion passage (closest point to the sun), the minimum (perihelion)
distance from the sun, and the orientation of the orbit in space. The
orbital elements, which are refined by additional observations,
can then be used to predict an object's future course (and astronomers
produce so-called ephemerides that list the object's position on the sky,
and predicted brightness and distances from the earth and sun, by date).
Our web pages contain both orbital elements and ephemerides for all
currently observable comets.
Early observations suggested that comet C/1995 O1 was quite far away from us
because of very small parallax seen in near-simultaneous observations made
by observers in Australia and Japan. As many positional (or astrometric)
observations poured in to our offices from observers around the world, we
were able to compute the comet's path, or orbit, about the sun. The orbit
of C/1995 O1 (Hale-Bopp) is almost perpendicular to the earth's own orbital
plane and takes the comet out quite far from the sun.
A single, apparent
image of the comet was found by astronomer Robert H. McNaught of the
Anglo-Australian Observatory from a wide-field photographic plate taken in
late April 1993 (when the comet was about 13 AU from both the sun and
earth). This observation strengthened the early orbital calculations by
greatly extending the arc of observation. The
comet has an orbital period of a few thousand years and extends out to
some ten times the distance of Neptune at its furthest point. Recent
orbital calculations indicate that comet C/1995 O1 (Hale-Bopp) last passed
through the inner solar system about 4210 years ago (or around 2214 BC), and
that it will return again in about 2380 years from now (or around 4377 AD) --
its orbital period being greatly shortened at its current "apparition" due to
gravitational perturbations by the major planets. There is an uncertainty of
a few years in these orbital periods.
We do not know where comet C/1995 O1 (Hale-Bopp) originated, though
the most widely accepted current theories have comets forming in the region
of the outermost major planets (Uranus and Neptune) some billions of years
ago and being scattered into both much larger and much larger orbits over
time by gravitational interactions with the major planets (and perhaps
occasional stars passing by).
- 5) Where in the sky is the comet now?
- The comet moved southward and is observable now chiefly from
southerly latitudes. In 2000, the comet has still be faintly visible
in large amateur telescopes, as it continues to recede from the sun and
earth and fades.
The comet was observable in the morning sky into late March or early
April 1997 (becoming lower in the northeast sky as each day passes), and became
better visible in the evening sky during April (and still from the northern
hemisphere). Beginning around the middle of March 1997, as comet C/1995 O1
neared peak brightness (a plateau lasting several
weeks into the end of April), the comet became better placed for evening
viewing. This comet was not a southern-hemisphere object in 1997 until about
June (after which time it will now be chiefly observable from the earth's
southern hemisphere and not the northern hemisphere).
At discovery, and for a few months afterwards, the comet was
moving slowly with respect to the background stars from night to night
in the evening sky in Sagittarius. It was visible in small amateur
telescopes (with mirrors or lenses 4-6 inches across) in the second
half of 1995 at visual magnitude 10-11, before becoming lost in the
sun's glare during December 1995 and January 1996; comet C/1995 O1
(Hale-Bopp) emerged from its proximity to the sun in February, and was
visible all night long during mid-1996 as an object of total visual magnitude
about 6, and the first naked-eye detection was reported
by experienced observer S. J. O'Meara at Volcano, Hawaii, on May 18.
The comet then became a relatively easy binocular object, as it continued to
move slowly northward through Sagittarius during April, May, and June 1996,
and into Scutum during July (when it passed opposition, the point opposite
the sun in the sky, meaning that it rose around the time it gets dark in
the evening was up all night). It gradually faded from binocular visibility
during 1998, and is now only visible with relatively large telescopes.
ephemerides (giving the object's position on the sky as a function of date)
have been published in the annual
the Minor Planet Circulars
and the IAU Circulars; an ephemeris
is also available on the World Wide Web at
- 5a) Can members of the general public see the comet now?
- Not easily, as explained above, and chiefly from the southern hemisphere.
One would probably have 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 -- noting, however, that nothing has faint as C/1995 O1 now
is should be considered visible from urban and suburban sites due to light
pollution. But other fairly bright (meaning binocular-visibility) comets
are present in our night sky usually for some weeks each year, and such
objects can frequently be observed at public-open nights at observatories.
[For example, the Center for Astrophysics in Cambridge, MA,
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).]
- 6) So how bright did this comet become?
- Comet C/1995 O1 (Hale-Bopp) reached peak brightness in March and
April 1997 around total visual magnitude -1, which
is rare for comets. This refers to the brightness of the comet's coma (or
head or atmosphere). C/1995 O1 is not the brightest comet of the 20th century,
though only a handful of comets have been brighter.
Comet C/1995 O1 appears to be rather large as comets go, and astronomers
have reported that its production rate has been running many times greater
than that of Halley's comet at the same distance from the sun.
This allowed the comet to give us the best naked-eye comet performance in 20
years [when comet C/1975 V1 (West), in the morning skies of March 1976,
was as bright as Sirius (the brightest star)].
[Some observers who saw comet C/1996 B2 in a dark sky will argue that that
comet put on a better naked-eye performance. Comet C/1996 B2
(Hyakutake) in March and April 1996 was a very nice comet to observers in a
very dark sky, with a naked-eye tail extending a third or more of the way
across the sky, but most urban viewers would not call that a "spectacular"
comet because the tail had such low surface brightness that it appeared merely
as a fuzzball.]
One concern of astronomers regarding earlier brightness predictions
for this comet had involved the fact that no long-period comet with a
perihelion distance near or inside the earth's orbit had been observed so far
before perihelion passage; this means that we did not have previous such
examples on which to base predictions. Also, comet C/1995 O1 exhibited
much dust, carbon-monoxide (CO), and other gaseous emission during the first
year of observation following discovery --- considerably more emission than is
usually seen in comets at such large distances from the sun; most bright comets
are thought to be fueled mostly by water-ice emissions, and theory suggests
that in the vacuum of space, significant water-ice sublimation did not begin
until the comet is within 3 AU of the sun (which did not occur until late
September or October 1996). Emission from such molecules as the hydroxl radical
(OH), diatomic carbon, and cyanogen (CN) were detected with the comet well
over 4 AU from the sun. The unusual early strong detection of these emissions
gave strong support to a good visual performance of this comet in early 1997.
If this is a comet that is fueled more by CO sublimation (or by
some other parent ices) than water sublimation, the peak brightness in 1997
might have been brighter or fainter than the projections, but this connection
will be sorted out later. Just as important as the brightness figure for the
general public's ability to see (or not to see) this comet as an impressive
naked-eye comet in early 1997 was the development of a dust tail [see
question 7, below]. Comet C/1995 O1
developed a very interesting pair of tails several degrees long, but
they did not develop the high surface brightness that was hoped for.
Numerous comets have been observed completely to fall apart
on approach to perihelion, as many comets are thought to have nuclei that
are very loosely held together. This comet never appeared likely to fall
apart. There were unsubstantiated reports of comet C/1995 O1 (Hale-Bopp)
fragmenting (splitting); some observers have
likely seen clumps of dust and/or gas coming off of the comet's nucleus, but
there has been no confirmed breaking/fragmenting/splitting of the comet's
- 6a) Was comet C/1995 O1 (Hale-Bopp) "the comet of the century"?
- Probably not, either from a scientific standpoint or from a popular
standpoint. The "comet of the century" in scientific terms was Halley's
comet, the only comet to have been visited at close range by artificial
imaging spacecraft. The "comet of the century" in popular terms would be
up for debate, for many reasons, not the least of which is the fact that
the world's communications/media industry/infrastructure is so much more
advanced (and thus set up for rapid dissemination of results and images
of any observed comets) in the 1990s than ever before; for this reason,
comets such as C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) have
received more media attention than other comets (and the public
possibly heard more about these comets than they did about Halley's
comet in 1909-1910 or 1985-1986 simply because of the modern-day size
and extent of the electronic and print media industry). What criteria
make for a "comet of the
century" must be first outlined, and depending on the specific criteria,
one will probably get different answers. Numerous other comets in the
20th century certainly have been brighter than C/1995 O1 became; numerous
other comets have had longer tail lengths than the maximum tail length of
C/1995 O1; and while we've had a couple of years to talk about comet
C/1995 O1 (Hale-Bopp), we've talked all century
about Halley's comet and its last/next return(s). But few comets are
visible from the northern hemisphere and remain as bright as (or
brighter than) magnitude 0 or +1 for 2-3 months, as occurred for C/1995 O1
during March-May 1997. Because of this expected duration of significant
brightness and because of widespread media attention, it is possible that a
larger number of people on the planet saw comet C/1995 O1 (Hale-Bopp)
than any other single comet in history.
So the answer to this question is necessarily uncertain.
- 6b) What about that 'infamous' comet Kohoutek in 1973-1974?
- Comet C/1973 E1 (Kohoutek) earned a bad reputation by "not living up
to" the media hype, including stories about it being possibly the 'Comet
of the Century'. [Some people said the same thing about 1P/Halley in
1985-86 and C/1996 B2 (Hyakutake) in March 1996.]
It turns out that C/1973 E1 (Kohoutek) had a very
stable light curve; what we did not know then was that light curves for
comets that are thought to be entering the inner solar system for the
first time (as apparently did C/1973 E1) very often have much slower
rises in brightness than do comets that have made many journeys through
the inner solar system. By comparison, C/1995 O1 (Hale-Bopp) has evidently
made numerous trips through the inner solar system, every few thousand
years, and there was reason to think that it would rise more rapidly
in brightness than did Kohoutek. But other comets have
very discontinuous light curves that make them totally unpredictable,
such as comet C/1989 X1 (Austin), which first rose steeply in brightness
and then leveled off to finish several magnitudes fainter than expected;
and numerous comets such as the recent C/1999 S4 (LINEAR) break apart
and fade from view (as C/1999 S4 did in July and August 2000).
- 7) Did this comet have a nice tail?
- 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. Comet C/1996 B2 (Hyakutake) in March 1996
was an exception; its long, faintish gas tail was readily visible because
that comet passed only 0.10 AU from the earth on March 25. One potential
problem with C/1995 O1 (Hale-Bopp) was that its perihelion
distance is much further out than most of the other "spectacular" comets
of the 20th century (perihelion distance = 0.91 AU). The Great Comet
of 1811 had an even greater perihelion distance (just outside the earth's
orbit), but still displayed a tail at least 25 degrees long (or about a
quarter of the way from the horizon to the zenith in the night sky).
An intrinsically fainter comet, C/1983 H1 (IRAS-Araki-Alcock), became
brighter than the stars of the Big Dipper in May 1983 but showed no
dust tail, and thus appeared merely as a large fuzzy ball in the sky.
We do know that comet C/1995 O1 is quite dusty as comets go, and its tail had
higher surface brightness in 1997 than did the tail of C/1996 B2 in March 1996,
being visible even from the largest cities on earth in March and April 1997.
The tail of C/1995 O1 was tens of millions of miles long in space,
corresponding to 10 or more degrees long to the naked eye in very-dark-sky
- 8) Do we know how large this comet is?
- No, not really -- at least not in terms of the nucleus.
Note that one must distinguish between the size of the nucleus
(which has not yet been directly visible, due to significant
"contamination" of light by the bright inner-coma material),
the size of the coma or atmosphere surrounding the nucleus,
and the size of the comet's tails. The coma size translated into well over
1 million km already in August 1995, though this size varies over time. This
is very large for a comet this far from the sun, but we know that there is
activity from sublimated carbon-monoxide (CO) ices (and probably from
dust, as well). The activity varies, generally increasing (but sometimes
decreasing) as a comet approaches the sun. 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).
Astronomers have assumed that the large amount of coma activity (and
corresponding total brightness) meant a larger-than-usual comet
nucleus for comet C/1995 O1. But calculations by comet scientist Zdenek
Sekanina of the NASA's Jet Propulsion Laboratory suggest that the
activity need not require a nucleus larger than 10-15 km in size.
The comet's tail length was tens of millions of miles long when the
comet was nearest the sun. Some professional astronomers have estimated that
the nucleus of C/1995 O1 has a diameter of 40 +/- 10 km, but it is
possible that we will never know the true size of this comet's nucleus.
- 9) How frequently are comets discovered?
- 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
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). In 1998, comet discoveries dramatically increased as a result of
increasing coverage in scanning the night sky each month with dedicated,
sensitive CCD cameras by programs in the southwestern United States
such as LINEAR and LONEOS (which added to already-existing programs such
as Spacewatch). Since 1998, the average number of new comet discoveries
has been more than 30 per year (not counting hundreds of recently detected
faint apparent comets close to the sun detected only by a spaceborn
coronograph named SOHO, and not seen from the ground due to the sun's glare).
- 10) How frequently do 'spectacular' comets become visible?
- 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. The increase in
light pollution will make any
comet such as
C/1995 O1 (Hale-Bopp) 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.
- 11) What are the IAU Circulars?
- 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 email@example.com 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:
Contact: Daniel W. E. Green
(Associate Director, Central Bureau for Astronomical Telegrams).
E-mail firstname.lastname@example.org. 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 2000 October 20]