Sky & Telescope - September 2023 - 49

ROLANDO LIGUSTRI
Neptune at
Opposition
pComet 103P/Hartley as imaged on November 1, 2010, during its most recent favorable apparition.
This time, the comet passes nearest to Earth on September 26th at a distance of 56.9
million kilometers (35.4 million miles), the last time this century it will come this close.
Comet 103P/Hartley is a Jupiterfamily
body - a group of short-period
comets with orbital periods of less
than 20 years whose orbits have been
modified by the planet's gravitational
influence. In Hartley's case, it circles
the Sun every 6.5 years. Curiously, it
rotates on two axes - spinning on its
short axis every 18.3 hours while also
rotating along its long axis.
Data gathered by the Deep Impact
(EPOXI) spacecraft flyby in November
2010 reveal that Hartley's 2.3-kilometer-long
(1.4 miles), peanut-shape
nucleus ejects " hyperactive " amounts
of water compared to other comets of
similar size. A team of astronomers
led by Michał Drahus also observed
that the comet's rotation rate was
rapidly slowing due to mass loss from
its highly active nucleus. In a 2011
article in Astrophysical Journal Letters,
they noted, " At the present rate, the
nucleus will stop rotating during the
fourth or fifth return from now (perihelion
in 2036 or 2043). " After that, it
will start to speed up again, but in the
opposite direction. Eventually 103/P
will spin fast enough to potentially
break apart around the year 2200.
In other words, catch Comet Hartley
while you can!
A Double Dose of Psi
LUNAR OCCULTATIONS are exciting to watch and timing them can
potentially provide scientifically valuable information. On the night of
September 27-28, the dark limb of the almost full (97%-illuminated)
Moon will cover 4.2-magnitude Psi1 (ψ1) Aquarii and 4.4-magnitude Psi2
(ψ2) Aquarii, one after the other. Observers across the contiguous U.S.,
Canada, Mexico, and Central America will see the first disappearance,
while the second occultation will be visible from the western two-thirds of
the U.S., most of Canada, and parts of Mexico.
The brilliant Moon will make this a telescope-only event. In Kansas City,
Missouri, the Moon's dark, southwestern limb occults Psi1 at 9:11 p.m. CDT
followed by Psi2 at 10:09 p.m. Just 6 minutes later at 10:15 p.m., Psi1 reappears
along the bright northeastern limb and Psi2 later, at 10:45 p.m.
You can simulate the occultations for your specific location with planetarium
software such as Stellarium.
THE BRIGHTEST PLANETS naturally
get a lot of telescopic attention because
most of them appear large enough to
display interesting surface or atmospheric
features. Neptune, on the other
hand, is our solar system's most distant
major planet and presents the tiniest
disk, despite being four times larger
than Earth. When it reaches opposition
on September 19th, it will be 4.3
billion kilometers away and shine at
magnitude 7.8. The ice giant looks like a
field star at low magnification - albeit a
blue-colored one - due to the methane
in its atmosphere, which absorbs red
light. But a 6-inch telescope and a magnification
of around 150×, combined
with good seeing, will turn this faint
" star " into a 2.4″-wide dot. Beyond that
little else is visible, though experienced
imagers occasionally tease out bright
bands and cloud formations. The fun
of tracking down Neptune is seeing a
world so very far away and imagining
what it might be like to be there.
A few facts give us a clue. Consider
its winds. Tearing around the planet
at 2,000 kilometers per hour, they're
nearly five times faster than Earth's
most powerful gusts and earn Neptune
the distinction as the windiest place
in the solar system. Because Neptune
isn't a solid, rocky body, its atmosphere
experiences differential rotation like
that on Jupiter and Saturn, but to a far
greater extreme. Neptune's equatorial
region spins once every 18 hours
compared to 12 hours at the poles. The
planet also radiates 2.6 times more
energy than it receives from the Sun,
likely fueling its ferocious winds and
occasional colossal storms.
Unlike many of the solar system's
biggest moons, Neptune's largest, Triton,
circles the planet at a steep angle
(157°) and orbits backward. Planetary
astronomers suspect Triton was once
a Kuiper Belt asteroid that Neptune
captured. When Triton is at its greatest
north or south elongation, which
sk yand tele scope .o r g * SEPTEMBER 2023 49

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Sky & Telescope - September 2023

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