Sky & Telescope - December 2020 - 18

Meteor Watching

meteors of at least 2nd magnitude and about 10% reach zero
magnitude or brighter. A few become outstandingly bright
meteors (ﬁreballs) while others silently and spectacularly
explode (bolides).
Geminids can also be colorful. About 90% appear yellowish-white, but the rest can appear red, orange, blue, and
even green. Some observers report that the Geminids appear
brighter and more colorful before midnight.

Path of Earth
2300

2220

2140
2080
2060
2020

The Geminid "Rock"

1980

Geminid
meteor
stream
1900

1862
p A STREAM OF DEBRIS A theoretical cross-section of the Geminid
meteor stream. Imagine that this page represents the plane of the Earth's
orbit. The Earth moves from right to left and the zone of intersection
shifts over time, causing the Earth to pass through different parts of it.

In the November 2010 issue of The Astronomical Journal,
astronomers David Jewitt and Jing Li suggested that Phaethon
is essentially a "rock comet." Phaethon's short perihelion distance induces thermal fracturing of its rocky surface, which
leads to the production of dust that is subsequently ejected
into space.
Indeed, in 2009 and 2012 NASA's STEREO A spacecraft
imaged Phaethon emitting a stubby tail of gravelly debris. But
Jewett and Li also pointed out a problem: The amount of dust
ejected during the asteroid's solar encounters adds a meager
0.01% to the mass of the debris stream - far less than needed

q ECCENTRIC ASTEROID (Left) In 2009 and again in 2012, David Jewitt, Jing Li, and Jessica Agarwal (University of California, Los Angeles) used
NASA's STEREO A spacecraft to image Phaethon as it reached perihelion, and they detected a stubby dust tail pointing away from the Sun. Visible
in this STEREO image is the southeastward extension of Phaethon's dust trail. (Right) Asteroid 3200 Phaethon is the source of the Geminid meteor
shower. The object travels in a very eccentric orbit and passes closer to the Sun than any known asteroid. On December 16, 2017, it whizzed past
Earth at a distance of 10.3 million km (6.4 million miles). It won't come this close again until 2093.

Orbit of Phaethon
Orbit of Mars
Orbit of Venus

Sun
Orbit of Mercury
Orbit of Earth

DECEMBER 2 02 0 * SK Y & TELESCOPE

IM AG E OF PH A E THON: JE WIT T, LI, AG A RWA L / N ASA / STEREO; PH A E THON
ORBIT DIAG R A M: LE A H TISCIONE / S&T; ME TEOR STRE A M DIAG R A M: JOE R AO

Meteor streams in general are thought to comprise dusty
debris released by comets. However, for many years the source
of the Geminids was an enigma because no parent comet
could be identiﬁed. In fact, up until the early 1980s, astronomers assumed that the original Geminid comet had disappeared centuries ago.
Then, on October 11, 1983, the Infrared Astronomical
Satellite (IRAS) found an asteroid racing through the stars of
Draco. The object was provisionally designated 1983 TB and
turned out to be a most unusual ﬁnd in more ways than one.
First, traveling in a highly elongated orbit, the 5-km-wide
asteroid comes closer to the Sun than any other asteroid. Its
perihelion distance is only 20.9 million km in its 1.43-year
orbital period - nearly one-third Mercury's average distance,
and 7 million km closer than the previous asteroid record
holder, 1566 Icarus. The IRAS asteroid's surface heats up to
about 700°C (1,300°F) - hot enough to melt aluminum.
Appropriately, the object was christened 3200 Phaethon (pronounced "FAY-uh-thun"), after the son of Helios, the god of
the Sun in Greek mythology.
Two weeks after its discovery, highly regarded comet expert
Fred L. Whipple reported in IAU Circular 3881 that this
asteroid's orbit was practically identical with the Geminid
stream and was the long-sought parent body of these meteors.
But how could an asteroid produce meteoroids?

Sky & Telescope - December 2020

Table of Contents for the Digital Edition of Sky & Telescope - December 2020