Sky & Telescope - December 2024 - 16

Meteor Science
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1998 1965
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1998
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1932
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2001
Nov 15.0
NEAR MISS OR A HIT Looking down on the ecliptic plane (the page is the plane), we see where each meteor stream intersects the plane (gray ellipses).
In 1998, none of Comet Tempel-Tuttle's streams crossed Earth's orbit when the planet was passing by (left). But in 2001, Earth flew right through
three streams (right). The X symbol marks where the comet crossed the plane in 1998, and ellipse years are for the outburst that created each stream.
have turned Phaethon into a " comet " of magnitude −5, on par
with some of the greatest comets seen in history! An event of
such magnitude would have caught the attention of ancient
stargazers. Unfortunately, the event probably happened before
civilizations began systematically recording celestial events,
which would explain why no surviving historical records indicate
its observation. Without the Geminids, we wouldn't have
known that this dramatic event occurred.
Another strong annual meteor shower, January's Quadrantids,
is also an interesting case. Astronomers long
thought that the parent comet of the Quadrantids was either
96P/Machholz 1 or a bright comet observed in 1490-91. But
neither was a perfect match to the Quadrantids' orbit. Then,
in 2003, the faint asteroid 2003 EH1 was found and emerged
as a convincing candidate.
But the answer turns out to be more complicated than
that. Years of work in meteor, comet, and asteroid sciences
have gradually revealed the presence of a gigantic, complex
system of parent bodies and debris now known as the
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Orionids
Perseids
Taurids
Quadrantids
Geminids
Machholz Interplanetary Complex, which comprises Comet
Machholz 1, asteroid 2003 EH1, hundreds of smaller comets
or perhaps asteroids, at least four annual meteor showers
including the Quadrantids, and perhaps even the 1490 comet.
These all progressively formed over a timescale of 12,000 to
20,000 years, presumably from the same parent object. The
Quadrantids is the youngest component of the system, with
its core formed perhaps only 200 to 500 years ago.
Sample-Return Missions by Mother Nature
No two meteors are the same: Some flare, some break into
pieces, some are more colorful than others. These colors can
range from orange and yellow to blue or just plain white,
depending on the composition of the meteoroids. In a way,
our atmosphere is a huge laboratory floating above us, autonomously
processing meteoric samples that are delivered to
us with the help of the Sun and planets. With specially built
instruments, astronomers can learn about these cometary
materials without leaving our planet.
Let us talk about the Geminids again. If you have seen the
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Entry speed (km/s)
LOOK UP Meteoroids start burning up in the atmosphere at different
heights and surface temperatures. Those from comets tend to be more
fragile and ablate at higher temperatures than asteroidal ones. Slower
particles also dive deeper before they're hot enough to light up.
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Geminids, you might notice that they are rarely orange in
color. This is because they tend to miss an important contributor
to the orangish hue: sodium. About 80% of spectrally
observed Geminids are free of sodium, far higher than many
other showers. Astronomers have long suspected that this is
due to the unique, Sun-approaching orbit of the Geminids:
Intense solar heating vaporizes rock on Phaethon and releases
sodium atoms, which help carry dust away as vapor escapes
from the asteroid to space. In 2022, the space-borne Solar
and Heliospheric Observatory was temporarily repurposed to
observe Phaethon, and its data convincingly demonstrated
that Phaethon's recurring activity close to the Sun is, indeed,
caused by escaping sodium atoms. The dots line up nicely!
You might remember the Draconids that Davies and
Turski first modeled in 1962. Astronomers have long known
that the Draconid meteors tend to appear at unusually high
Beginning height (km)
METEOR INTERSECTIONS: BEATRIZ INGLESSIS / S&T, SOURCE: D. J. ASHER / PROCEEDINGS OF THE INTERNATIONAL
METEOR CONFERENCE 1999; ABLATION: BEATRIZ INGLESSIS / S&T, SOURCE: P. JENNISKENS ET AL. / ICARUS 2016
Low temperatures
Medium temperatures
High temperatures
Earth's path
Earth's path

Sky & Telescope - December 2024

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