Sky & Telescope - USA (2020-01)

T

he most challenging observing tar-

gets in the solar system are located

right in our cosmic backyard, yet they

can’t be seen through any telescope.

The path that led to their discov-

ery six decades ago can be traced back

almost two and a half centuries to

attempts to solve the vexing “three-

body problem” in celestial mechanics.

In 1772 the mathematician Joseph-

Louis Lagrange determined that a

relatively small body that shares the

orbit of a planet and lies 60° ahead

or behind it will maintain its position

for an indefi nitely long period of time.

These Lagrangian points are designated

L4 (leading) and L5 (trailing). Forming

a pair of equilateral triangles with the

planet and the Sun, they are exception-

ally stable because the gravitational

attraction of the planet and the Sun on

the small body just balance the centrip-

etal force of its orbital motion.

The Lagrangian points remained

an abstract notion until the dawn of

the 20th century, when astronomers

discovered the brightest members of

an entire family of asteroids that circle

the Sun near the L4 and L5 points in

Jupiter’s orbit. Following a suggestion

by Austrian astronomer Johann Palisa,

these objects were named after famous

heroes of the Trojan War, and to date

more than 7,000 “Trojan” asteroids

have been cataloged. They comprise

almost one-fi fth the mass of the main

asteroid belt.

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52 JANUARY 2020 • SKY & TELESCOPE

In 1951 the Polish astronomer

Kazimierz Kordylewski, a specialist in

celestial mechanics and the photoelec-

tric photometry of variable stars, began

a systematic visual search for “Trojan”

bodies located at the L4 and L5 points

of the Earth-Moon system using the

8-inch refractor of the Kracow Observa-

tory. He looked for objects with a stellar

appearance that would betray their

nature by moving at the lunar rate —

about ½° every hour — relative to the

background stars.

Despite many nights at the eye-

piece, Kordylewski failed to detect any

suspects brighter than 12th magni-

tude, corresponding to objects about

20 meters in diameter with the same

refl ectivity as the lunar surface. Deep

photographic surveys using a 20-inch

Schmidt camera and a 24-inch refl ector

also proved disappointing.

Professor Josef Witkowski of Poznan

University suggested to Kordylewski that

he search for swarms of dust particles

rather than discrete bodies. Far too tiny

to be seen individually, if present in suf-

fi cient numbers these dust motes would

be visible as a very faint nebulous patch.

Without a contrasting expanse of sur-

rounding dark sky to reveal its outlines,

however, any cloud of suffi ciently large

dimensions would elude detection in

The Kordylewski Clouds

Join the hunt for these enigmatic lunar attendants.

JANUARY 2020 OBSERVING

Exploring the Solar System by Thomas A. Dobbins

tReported to appear similar to but consider-

ably fainter than the gegenschein seen at left,

Kordylewski’s clouds of dust reside 60° ahead

of and behind the Moon in its orbital path.

(40,750 miles

beyond Moon)

L 2

L 4 L 5

Moon

Earth

L 1

(36,000

miles inside

Moon’s orbit)

60 ° 60 °

Five Lagrangian points

exist for each three-body

system. L 1 , L 2 , and L 3

(not shown, but on the

far side of Earth at the

Moon’s distance) lie on

a straight line drawn

through the two large

masses. Weak perturbing

forces will knock objects

at these points out of

orbit. The stable L 4 and

L 5 are the third points of

two equilateral triangles

drawn in the plane of the

two large objects.