Sky and Telescope - June 2015 - 54

OBSERVING

Exploring the Moon

More Lunar Landing Sites
Recent space missions have left their mark on the Moon -
by landing intact or crashing.

Ebb and Flow Crash Site
Mouchez

+
"Sally Ride ridge"
Anaxagoras
Philolaus

Mare Frigoris

50 km

RICHARD BOSMAN

NASA ﬂight controllers intentionally crashed the twin GRAIL spacecraft, Ebb
and Flow, into a broad ridge informally named for the late astronaut-educator
Sally Ride. Seen here in an image taken through a 14-inch telescope, the ridge
is observable near the distinct far-northern craters Philolaus and Anaxagoras.

June 2015 sky & telescope

Chang'e 3 Landing Site
Montes Recti

Sinus Iridum

Promontorium
Laplace

E
Helicon

Le Verrier

Mare Imbrium
25 km

The Chinese lander Chang'e 3 and its rover, Yutu, dropped
onto a nearly featureless stretch of northern Mare Imbrium in
December 2013. This is a composite of Lunar Reconnaissance
Orbiter images.

Atmosphere and Dust Environment Explorer (LADEE)
impacted on the farside on April 18, 2014. The fourth
spacecraft to reach the Moon lately was China's Chang'e 3,
which soft-landed with the self-propelled Yutu rover on
northern Mare Imbrium on December 14, 2013.
GRAIL was an inexpensive and conceptually simple
mission that provided some of the most important data
ever collected about the Moon. By tracking the exact distances between Ebb and Flow, mission scientists could
calculate minute variations in the pull of gravity all
around the Moon. These measurements led to detailed
maps of the thickness of the lunar crust, better understanding of the subsurface structure of impact basins,
and conﬁrmation that the upper few kilometers of the
highlands crust is hyper-fragmented.
When they ﬁnally ran low on fuel, these two small
satellites followed their polar orbit on a ﬁ nal pass south
to north over Mare Imbrium, across Mare Frigoris, passing near the crater Philolaus, and smashing at 6,100 km
per hour (just over 1 mile per second) into an unnamed
ridge. The mission team has proposed naming the ridge
after the late Sally Ride, who was an inspiring astronaut,

NASA / ARIZONA STATE UNIV.

Humans naturally want to see where their famous
predecessors have been. Some years ago, when traveling
in Europe, I made sure to visit the National Observatory
of Athens so I could view the location from which the
great 19th-century observer Julius Schmidt (see page 29)
observed the Moon. Lowell Observatory was likewise a
required stop when I ﬁrst visited Flagstaﬀ, Arizona.
My quest extends to the Moon as well. Even though
every landing or crash site for lunar spacecraft is too
small to detect from Earth, I like to closely observe the
landscapes of the landing areas.
In the ﬁve years since I described early Soviet (S&T:
July 2010, p. 51) and more recent (S&T: Feb. 2010, p. 51)
lunar landing sites, four additional spacecraft have
reached the surface of the Moon. Unlike early days of
the space race, when missions often ended with accidental impacts, now a spacecraft's ﬁnal orbits are usually
planned for the impact to probe the lunar surface or
simply to avoid historical landing sites.
Ebb and Flow, the two components of NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, collided with a ridge deep in the lunar northern
hemisphere on December 17, 2012, while the Lunar

Sky and Telescope - June 2015

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