Sky & Telescope - January 2021 - 35

23.45º

Ecliptic plane
N

Equator

5.15º

Equa

tor

's orbit

f Moon

Plane o

6.68º
S

S

Earth

Moon
To Sun
S LOCKED IN PLACE The near-vertical orientation of the Moon's spin
axis to the ecliptic plane creates pockets of permanent sunlight and
nighttime on the floors of craters near the lunar poles.

1961, Caltech scientists Kenneth Watson, Bruce Murray, and
Harrison Brown extended that idea, suggesting that "water
is actually far more stable on the lunar surface [than other
volatiles] because of its extremely low vapor pressure at low
temperatures and that it may well be present in appreciable
quantities in shaded areas in the form of ice."
Space missions eventually proved they were right. In 1994,
the Clementine orbiter conﬁrmed that PSRs do exist on the
Moon, pinging some with radio waves. The signals came back
strong enough to indicate that among the polar rocks were
likely deposits of water ice, which reﬂects radio energy more
strongly than rock does. Four years later, an orbiter called

Lunar Prospector found more evidence, indirectly detecting
hydrogen atoms (presumably in water molecules) on and
just below the surface. In a strange denouement, however,
when the team deliberately crashed the craft into a PSR, they
detected no water in the ejecta.
More than a decade would pass before the next leaps in
lunar-ice research. During ﬂybys of the Moon in 1999 and
2008, respectively, NASA's Cassini and Deep Impact spacecraft found tantalizing evidence for water on the lunar
surface as they sped by.
By November 2008, India's Chandrayaan 1 satellite had
arrived in lunar orbit, observing the terrain below with a suite
of 10 experiments. Two of those, including the NASA-funded
Moon Mineralogy Mapper (M3), detected the spectrum of
water-bearing compounds on the surface, and an instrumented probe released by the orbiter persistently detected
whiffs of water vapor before it slammed into the Moon.
Then, on October 9, 2009, a NASA team deliberately
crashed a Centaur rocket body into Cabeus, a large crater
situated close to the Moon's south pole. The Lunar Crater
Observation and Sensing Satellite (LCROSS) trailed the
empty rocket stage in order to observe the ensuing plume of
debris, which rose some 16 kilometers (10 miles). LCROSS
kept recording data until it too slammed into the crater's

M O O N CR ATE R: N ASA / GS FC / A R IZO N A STATE U N I V E R S IT Y; M O O N
O R B IT: LE A H TISCIO N E / S&T, SOURCE: PAUL SPUDIS

N

sk yandtelescope.org * JANUARY 2 021

35

http://www.skyandtelescope.org

Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
Sky & Telescope - January 2021 - 4
Sky & Telescope - January 2021 - 5
Sky & Telescope - January 2021 - 6
Sky & Telescope - January 2021 - 7
Sky & Telescope - January 2021 - 8
Sky & Telescope - January 2021 - 9
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Sky & Telescope - January 2021 - Cover3
Sky & Telescope - January 2021 - Cover4
Sky & Telescope - January 2021 - SA1
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Sky & Telescope - January 2021 - SA3
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Sky & Telescope - January 2021 - SA8
Sky & Telescope - January 2021 - SA9
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