Meteors
ATLAS OF THE UNIVERSE
M
eteors are cometary debris. They are very small,
and we see them only during the last seconds of
their lives as they enter the upper atmosphere at speeds
of up to 72 kilometres (45 miles) per second. What we
actually observe, of course, is not the tiny particles
themselves (known more properly as meteoroids) but
the luminous effects which they produce as they plunge
through the air. On average a ‘shooting-star’ will become
visible at a height of about 115 kilometres (70 miles)
above ground level, and the meteoroid will burn out by
the time it has penetrated to 70 kilometres (45 miles),
finishing its journey in the form of fine ‘dust’. Still smaller
particles, no more than a tenth of a millimetre across,
cannot produce luminous effects, and are known as
micrometeorites.
When the Earth moves through a trail of cometary
debris we see a shower of shooting-stars, but there are
also sporadic meteors, not connected with known comets,
which may appear from any direction at any moment.
The total number of meteors of magnitude 5 or brighter
entering the Earth’s atmosphere is around 75 million per
day, so that an observer may expect to see something of
the order of ten naked-eye meteors per hour, though
during a shower the number will naturally be higher.
It is also worth noting that more meteors may be
expected after midnight than before. During evenings,
the observer will be on the trailing side of the Earth
as it moves round the Sun, so that incoming meteors
will have to catch it up; after midnight the observer
will be on the leading side, so that meteors meet the
Earth head-on, so to speak, and the relative velocities are
higher.
The meteors of a shower will seem to issue from
one particular point in the sky, known as the radiant. The
particles are travelling through space in parallel paths, so
that we are dealing with an effect of perspective – just
as the parallel lanes of a motorway appear to ‘radiate’
from a point near the horizon.
The richness of a shower is measured by its Zenithal
Hourly Rate (ZHR). This is the number of naked-eye
meteors which could be seen by an observer under
ideal conditions, with the radiant at the zenith. Theseconditions are never met, so that the observed rate is
always appreciably lower than the theoretical ZHR.
Each shower has its own particular characteristics.
The Quadrantids of early January have no known parent
comet; the radiant lies in the constellation of Boötes (the
Herdsman), the site of a former constellation, the Quad-
rant, which was rejected by the International Astronomical
Union and has now disappeared from the maps. The ZHR
can be very high, but the maximum is very brief. The
April Lyrids are associated with Thatcher’s Comet of
1861, which has an estimated period of 415 years; the
ZHR is not usually very high, but there can be occasional
rich displays, as last happened in 1982. Two showers, the
Eta Aquarids of April–May and the Orionids of October,
come from Halley’s Comet, though they were not partic-
ularly rich around the time of the comet’s last return in- The October Draconids are associated with the
periodical comet Giacobini–Zinner, and are sometimes
referred to as the Giacobinids. Usually they are sparse, but
they produced a major storm in 1933, when for a short
time the rate of observed meteors reached 350 per minute.
Ever since then, unfortunately, the Draconids have been
very disappointing.
Two major showers occur in December: the Geminids
and the Ursids. The Geminids have an unusual parent –
the asteroid Phaethon, which is very probably a dead
comet. The Ursids, with the radiant in the Great Bear, are
associated with Tuttle’s Comet and can sometimes be rich,
as in 1945 and again in 1986.
Some showers appear to have decreased over the
years. The Andromedids, as we have seen, are now almost
extinct. The Taurids, associated with Encke’s Comet, are
not usually striking, though they last for well over a
month; reports seem to indicate that in past centuries they
were decidedly richer than they are now.
Probably the most interesting showers are the Perseids
and the Leonids. The Perseids are very reliable, and last
for several weeks with a sharp maximum on 12 August
each year; if you look up into a clear, dark sky for a few
minutes during the first fortnight in August, you will be
very unlucky not to see several Perseids. The fact that
the display never fails us shows that the particles have
had time to spread all round the orbit of the parent comet,
Swift–Tuttle, which has a period of 130 years and was
last back to perihelion in 1992. The comet was not then
conspicuous, but at its next return it will come very near
the Earth – certainly within a couple of million kilometres,
perhaps even closer – and there have been suggestions that
it might hit us. In fact the chances of a collision are many
hundreds to one against, but certainly Swift–Tuttle will
be a magnificent spectacle. It is a pity that nobody born
before the end of the 20th century will see it.
The Leonids are quite different. The parent comet,
Tempel–Tuttle, has a period of 33 years, and it is when
the comet returns to perihelion that we see major
Leonid displays; the particles are not yet spread out all
round the comet’s orbit. Superb meteor storms were
seen in 1799, 1833 and 1866. The expected displays of
1899 and 1933 were missed, because the swarm had
been perturbed by Jupiter and Saturn, but in 1966 the
Leonids were back with a vengeance, reaching a peak
rate of over 60,000 per hour. Sadly, this lasted for only
about 40 minutes, and it occurred during daylight in
Europe, so the observers in the New World had the
best view. The Leonids were rich in 1999, 2000 and 2001,
though there was no display comparable with that of 1866.
Leonid showers have been traced back for many centuries,
and indeed 902 was known as ‘the Year of the Stars’.
▲ The Leonid Meteor Storm
of 1833, when it was said
that meteors ‘rained down
like snowflakes’. Other major
Leonid meteor storms were
those of 1833, 1866, 1966
and 2000.▼ Great Meteor of
7 October 1868.Old painting
by an unknown artist. The
meteor was so brilliant that
it attracted widespread
attention, and seems to
have been as bright as the
Moon, lasting for several
seconds and leaving a trail
which persisted for minutes.D108-151 UNIVERSE UK 2003CB 7/4/03 5:19 pm Page 146