2019-12-01_Astronomy

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QI

THE MOON IS SLOWLY MOVING

AWAY FROM EARTH. WHEN WILL

THE MOON LEAVE EARTH’S ORBIT?
Robert Byerly
Windsor, California

AI

Based on measurements made possible by

reflectors that Apollo astronauts left on the

surface of the Moon, we know that the Moon is moving

farther from Earth. Currently, our satellite is moving

about 1.5 inches (3.8 centimeters) away from the planet

each year.

This is happening because Earth and the Moon exert

a gravitational pull on each other, which causes the

ocean tides we observe each day. But because of Earth’s

rotation rate, the bulges — high tide — actually occur

at a point just ahead of the Moon, instead of directly

beneath it. The Moon, in turn, is attracted to the

increase in mass just in front of its current orbital posi-

tion, which slightly speeds up its orbit. And the faster

it moves, the larger its orbit around Earth becomes —

hence, the Moon’s recession from Earth.

But there’s another factor at play. The Moon is also

exerting a gravitational inf luence on Earth; the tides it

raises cause friction, which is slowly reducing the rota-

tional rate of our planet. Not only did the Moon start

out much closer to Earth in the past, but Earth was

rotating on its axis much faster at the same time. As the

Moon drifts outward, it will continue to slow Earth’s

rotation rate, until the amount of time it takes the Moon

to orbit Earth once equals the amount of time it takes

Earth to rotate once on its axis: 47 days. This will occur

in about 50 billion years, and at that point, the Moon

will stop moving away — so it will never completely

leave Earth’s orbit.

It’s worth noting that 50 billion years is a much lon-

ger time than our Sun has left to live, which is only

about 5 billion years. So, long before the Moon stops

moving away and slowing Earth’s rotation rate, the

entire Earth-Moon system may be swallowed by our

star as it swells during the later stages of its life.

Alison Klesman

Associate Editor

QI

THE APOLLO MISSIONS LEFT

SEISMOMETERS ON THE MOON.

HOW LONG DID THEY WORK? WHAT

CONCLUSIONS CAN WE MAKE ABOUT

MOONQUAKES?
João Miguel Matos
Setúbal, Portugal

AI

Every Apollo mission that landed on the

Moon left seismometers on the surface. Apollo

11's Early Apollo Surface Experiments Package was

simple, with a passive seismometer that functioned

only until August 27, 1969. Later missions all carried

passive seismometers, and Apollo 14, 16, and 17 carried

active seismic experiments as well; all were part of a

larger geophysical instrument suite called the Apollo

Lunar Surface Experiments Package (or ALSEP). These

instruments functioned for several years and were

permanently shut down due to budget cuts September

30, 1977, with the longest-lived – those from Apollo

12 – running about eight years.

During those years, the seismometers recorded thou-

sands of moonquakes. The majority of recorded events

occurred deep in the Moon’s interior, but some quakes

were shallow — the result of movement or changes in

the Moon’s crust.

Astronomers believe most moonquakes occur as the

Moon slowly loses heat from its initial formation. As

this happens, the Moon’s interior shrinks, causing the

surface to wrinkle and crack, generating faults that can

then slip and move. Based on 28 of the shallower quakes

recorded by the Apollo instruments, researchers

recently determined that at least some of the fault lines

visible on the Moon’s surface are still geologically

active, causing quakes when parts of the lunar crust slip

and rub against each other. (See “Apollo-era quakes hint

the Moon is still active,” on page 12 of the September

issue, to learn more.) Six of the 28 quakes also occurred

when the Moon was at or near the farthest point in its

orbit, which indicates that gravitational stresses

between Earth and the Moon may cause the portions

of the crust on either side of a fault to slide against each

other, prompting a shakeup.

Alison Klesman

Associate Editor

Apollo 16 astronauts

captured this view of

the Passive Seismic

Experiment (silver)

in front of the gold-

colored Apollo Lunar

Surface Experiments

Package Central

Station after

deployment. Such

seismometers

continued to function

until the Apollo

seismic experiments

were terminated in

late 1977. NASA