Sky & Telescope - December 2018 - 7

the ﬁrst time in a long life, thinking
seriously that this could be something
from another civilization. The peculiar
light curve, indicating an object very
far from spherical, would ﬁt a tumbling space probe of rod or pancake
shape. The orbit through our inner solar
system is what an alien scientist, curious about those small planets, might
choose. When its path to us is calculated back in time, it doesn't lead to
any nearby planetary system, but such
calculations assume that only gravity
was affecting it. A spacecraft would
have its own engines, which might
have modiﬁed its trajectory, particularly if it was an ionic or light-pressure
method, as has been proposed for long
space missions. That would cause the
apparent source to deviate considerably
from the real source when the orbit is
calculated back.
One scenario to investigate is that
this spacecraft got a boost to high velocity by the slingshot method shown in

the article, which would work well only
if the ﬁnal velocity vector was near
the orbital plane of the planet used.
A correction at a right angle to that
plane would then be necessary. I would
suggest looking for a nearby planetary
system whose orbital plane is perpendicular to the path of the object as
calculated back.

that `Oumuamua might be only three or four
times longer than it is wide - far less than
the factor of 10 that has been dominating
the discussion.
As the late Carl Sagan often emphasized,
"Extraordinary claims require extraordinary
evidence."

Jack Ullman
Bronx, NY

David Nakamoto has done an excellent
job describing the history and evolution of the science behind BL Lacertae
objects (S&T: Sept. 2018, p. 30). Several
are visible in large amateur telescopes,
and even more can be snagged by imaging. Readers, observers, and imagers are
encouraged to partake in the Astronomical League's Active Galactic Nuclei
Observing Program (https://is.gd/
AGN_observing).

Kelly Beatty replies: A few fastspinning asteroids do have light
curves as dramatic as `Oumuamua's, so it's
not impossible that a solid natural, rocky
object would have that shape. Also, asteroid
expert Alan Harris reminds us that effects
such as shadowing and irregular shape
can result in light-curve amplitudes that
mimic aspect ratios much greater than they
actually are. For example, Harris suspects

1968

1993

Al Lamperti
Oreland, Pennsylvania

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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1943

Open Invitation

º December 1943
Exoplanet Quest "The symposium
on Dwarf Stars and Planet-like
Companions was a most worthy
feature of the [American Astronomical Society] meeting. . . . Dr.
[Peter] van de Kamp [reported that]
Sproul Observatory is making a
systematic photographic study
of all stars within 10 parsecs (33
light-years) and within reach of the
24-inch refractor. Deviations from
uniform (proper) motion are already
indicated for several stars. . . .
"Discussion that afternoon was
even more active than . . . during
the morning, indicating that much
more work needs to be done."
More work, and 52 years. Not
until 1995 was a Jupiter-size planet
conﬁrmed orbiting 51 Pegasi, an
ordinary star like our Sun.
º December 1968
Wise Men's Star "We can readily
imagine the ancient scene: As the
sun set over the broad Euphrates

River, three Babylonian priests
made their evening climb up the
mammoth ziggurat temple . . . to
watch the stars come out.
"In the east the moon was rising, and in the west Venus shone
high above the sunset. Just beside
it Jupiter could be seen, dimmer
and yellower. The priests had
been watching the pair for some
weeks now, but tonight there was
a difference. The two planets were
closer together than any one had
seen them in many years. As the
sky darkened this brilliant 'double
star' sank lower, the planets drawing nearer and nearer. At last . . .
they fused into one, gleaming like a
great beacon over Judea. . . .
"The date was June 17, 2 B.C.,
and what these Wise Men saw may
have been the Star of Bethlehem."
My proposal proved sound in
its astronomical details, and many
planetariums have recreated it in
Christmastime shows. But recent
scholarship has veered away from
suggestions that the Star was a
true spectacle, favoring instead an

astrological alignment unnoticed by
the general public.
º December 1993
Pluto's Surface "A team of . . .
researchers has discovered that
the ices on Pluto's surface are
overwhelmingly dominated by
nitrogen, not methane. . . . Using
high-resolution spectra obtained in
May 1992 with the 3.8-meter
United Kingdom Infrared Telescope
on Mauna Kea, . . . Tobias C. Owen
(University of Hawaii) and his
colleagues conclude that . . . frozen
nitrogen must constitute about 98
percent of the surface ices. . . . The
researchers [did find some]
methane (CH4) ice [and] a trace of
frozen carbon monoxide (CO). But
no evidence exists for water ice or
frozen carbon dioxide (CO2)."
NASA's New Horizons ﬂyby of
Pluto in 2015 reﬁned this picture.
Along with widespread frozen nitrogen, Pluto has dunes of methaneice particles. Carbon monoxide
ice seems largely conﬁned to the
crater-free area of Sputnik Planitia.

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Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018