NASA

: /k

¸

Technical

Memorandum

4271

.

u_

:i: i ¸

Lighting Lunar

Constraints Surface

Operations

i:

Dean Lyndon

B. Eppler B. Johnson

Houston,

Texas

i:_ _

_!ili:_

NASA National Aeronautics and Space Administration Office of Management Scientific Information

1991 : i•

•

and

Technical Division

Space

on

Center

CONTENTS

Section

Page

ABSTRACT

............................................................................................................................

INTRODUCTION STUDY THE

.................................................................................................................

ASSUMPTIONS

EFFECT

CALCULATION CASTING WORLD

TERMS

DURING APPLICATION CONCLUSIONS

ON

LUNAR

SURFACE

OF ILLUMINATION

ILLUMINATION

IN

EARTHSHINE

BY FULL

VALUES

IN

3

......................

EARTHSHINE

........

LUNAR

TO DAY

OF RESULTS

LUNAR

SURFACE

OPERATIONAL

SCHEDULES

..............................

.................................................................................................................... .....................................................................................................

........................................................................................................................

,°°

111

PRE'CED_NG

PAGE BLANK

4

6

OPERATIONS

.......................................................................................... TO

3

REAL

................................................................................................................

ACKNOWLEDGMENTS REFERENCES

OF

CONSTRAINTS THE

GEOMETRY

2

...................................................................................

OF VALUE

EARTHSHINE

LIGHTING

SYSTEM

ILLUMINATION

OBSERVATIONS

1

.....................................................................................................

OF EARTH-MOON

EARTHSHINE CREW

1

NOT

FILMED

8 8 9 10 11

TABLES i_ !!_ii_i Table ,:i,

Page

_

EARTHSHINE IN THE

'ii _i_

ili I

2

ILLUMINATION

APOLLO

VARIATIONS LOCATION

PROGRAM

IN EARTHSHINE ON

THE

LUNAR

VALUES

ILLUMINATION SURFACE

ii_!i_i_!ii!i _i_ !i _

' • ii__I

iv

il _i__

USED

........................................................................

12

WITH

....................................................

12

FIGURES Page

Figure 1

Variations phase

in the angle

and

level lunar

2

Apollo

17 photograph

3

Apollo Mare

17 photograph Procellarum

4

Variation phase

5

6

in the angle

of Earthshine surface of the

illumination

location crater

as a function

of Earth

..........................................................

Schl_iter

in Earthshine

..........................

of Reiner _,, a bright swirl feature on ..............................................................................................

level

of Earthshine

for the sub-Earth

illumination

point

as a function

of Earth

................................................................

15

under

Analog

under

scale

drawing Earthshine

of the

degree

available

of illumination

at the

sub-Earth down-Sun

7

Apollo

16 photograph

taken

90 ° to the

8

Apollo

16 photograph

taken

directly

as figure Operational the lunar

down-Sun

16

available point

at the

........................

same

showing

the range

of lighting

17 18

location

7 ............................................................................................................ diagrams surface

the

................................

direction

14

14

Analog scale drawing of the degree of illumination available full Earthshine ....................................................................................................

minimum

13

conditions

.........................................................................................

19 on

a) At sub-Earth

point

b)

At a western

limb

location

............................................................................

21

c) At an eastern

limb

location

...........................................................................

22

V

20

iilii_ ...............................................................................

ABSTRACT

An

investigation

that

for most

lunar

night The

similar

to the States

of the

throughout

Variations angle. tion

the

the

period

caused

activities

on a July

evening

night,

with

after

sunset).

consequent

the

Apollo

around

the

lunar

only night

Because

Program noon

suggests

may

that

be difficult

artificial in the

will be

southern

captured

length

rotation

EVA

angle.

of Earth

activities

to lack

con-

and

a function

due

illumi-

approximately

shadow

be solely

of the

Earthshine

of the

remain

constant

indicates most

minor from

8:00 p.m.

will

will

surface

throughout

with

lunar

of the Earth

illumination

during

by elimination

the

at approximately

the location

of Earthshine

on the lunar

(EVAs)

during

15 minutes

the lunar

lighting

will be adequate

available

Earth,

in the level Experience

during

lighting

level

about

of ambient

illumination

(approximately

Moon

stant

levels

extravehicular

maximum light

the

locations,

to conduct

nation. United

into

nearside

phase

conducted

of surface

defini-

of shadows.

INTRODUCTION

During lunar the

the Apollo

surface

operations

nominal

Due

landing

to vehicle

executed

The plans

Apollo

limit

necessary

be faced

during

developing surface

the

planning

Can the Can

be required?

proposed

and

constraints

execution

and

lunar

of crew surface

robotic

operations

operations

away

to mission departure

and

from

the lunar

surface

or

lunar

operations

George

beyond

the

Bush,

72- to 96-

Consequently, day.

would

Considerations on the

conditions

These

it

that

conditions these

in lunar

will have

constraints

of an outpost

outpost/lander in SEI are

planned

the

constraints

lighting

the effect

operations.

the

into

surface

Moon.

the lunar

in the vicinity

development from

surface

of the

the

and

morning, landing.

were

day

in 1989 by President lunar

during

include

lunar

lighting.

for, the operational

day,

limited

after

missions

for lunar

night

habitation

conducted

early

lunar

place

on the

to plan

operations

the =-655-hour

concern

affect

not

or lander,

as well.

Issues

but

that

are

as follows.

be executed

at any

time

during

day?

geologic

reflected

(SEI),

permanent

for routine

arrival lunar

Initiative

to eventual

for science

of significant

ambient

operational

and

planning

using

times

between

--328-hour

took

constraints

24 to 96 hours

no Apollo

the

planning

safety

day from

module

to consider,

these

lunar

morning

beyond

for stay

surface

and

on consumables,

capability

throughout

on planning

lunar

design,

of the

lunar

no significant

our

becomes

late

extended

Exploration

to extend

hour

only

have

outside Space

and

constraints

would

Consequently,

conducted

operational,

to the portions time,

design

that

night.

Program,

field

off the

work

Earth

and

other

(Earthshine)

surface

activities

be carried

during

the lunar

night,

out

or will

using artificial

only

light

lighting

i

Are

_il'_

nation

on the

during any _i ii i _

lighting

Earthshine

to "real

the •

the

in various

value

of Earthshine

Finally,

existing

lunar

photos

under

with

were

conditions

tion

is that

ment

the

study

of EVA

be adequate cal power

generation

The

form

third

of integral

EVA suits, and This is because still

be very

some

form The

landing program, between

at a habitat

is that lighting,

and

final

assumption

at a previously

data

that

would

from might

safety

differing

Earth-Moon

first apon the orbit on

Sun angles.

The

illuminaillumination

to understand

how

the

geometries.

to determine Sun

be

on illumi-

be experi-

lines. The recollections

in varying

what

lighting

angle.

for this study. or cooling

generation

thermal

The

questions inside

second

will be adequate to what

lighting, of ambient

will be required illumination,

considerations

will

dictate

assump-

in its assess-

the EVA

for lunar

is currently

first

equipment

assumption

of the conclusions

similar

The

life support

environment

regardless

of vehicular of the value

normal

that

and

rejection

future

or lander

assumption

of auxiliary

information

on Earth,

as a baseline

environment.

EVA

helmet

surface

values for full-Earth value of Earthshine

scrutinized

heat that

thermal

some form regardless

dark,

consider

an appropriate

ambient

surface to interpret

ASSUMPTIONS

used

assuming

to maintain of the

were

will not

operations,

regardless tions.

!,

of assumptions

the

of illumi-

the engineering several and

surface

of varying

STUDY

A number

with

illumination

on the lunar

changes

surface

exist

was

on the lunar

to relate

illumination

illumination

might

on the lunar study

of the

search to find existing necessary to relate the

to full-Moon

amount

geography visible on the lunar surface from of illumination, and also to collect information

in working a literature this, it was

(1) the

part

the reader

necessary

during

(2) lighting available

to a comparable

topography and as the sole source

surface

constraints

providing

conducted

afternoon?

conditions:

and

of illumination

it was

sources

involved

lunar

basic

An important

values

operations

lunar

approach to this task has followed information from crew observations

second approach was tion. To complement on the

two

to Earthshine,

amount

world"

on Earth. The was to collect

constraints

due

surface

to early

considered

To do this,

amount of surface using Earthshine

to lunar

morning

illumination.

about

found

enced proach

i!¸

surface

if so desired.

nation

lunar

this study

lunar

conclusions

constraints

of late

high-Sun-angle

tested

ii_i

any

Sun angles

For simplicity,

!_ii_i_ i_ii_i! _',

!i_ ¸

there

high

will suit,

is that night

of this used

electri-

EVA

study, on Space

opera-

some Shuttle

for normal operations. shadowed areas will the requirement

for

lighting. is concerned unprepared

mission rules dictated that 5 and 20 ° above the horizon

with location

the

surface

on the

lunar

lighting surface.

required During

for a first the Apollo

all lunar landings would occur when the Sun was and behind the lunar module as it made its final

2

approach

to the

terrain information cant terrain feature.

i_ i _I

eyes

during

be in effect

tion

will

ii

site.

conditions

maneuvering

for future not

by orbital

apply

mechanics

It is further

pared landing pads once the appropriate

landings

_ ::

,i_ q:

Moon

and

assumed

EFFECT

vehicle that

-_28-day

interval

that

causes

the

design

manned

to most

around

landing

sites.

mission This

which

rules

assump-

will likely

rather

than

outposts

be

lighting

eliminating

will have this

GEOMETRY

pre-

requirement

ON

ILLUMINATION

rotation

of its orbital

people;

similar

considerations aids,

the maximum

that

surface,

SYSTEM

or captured,

period

with

down-Sun from any signifinot be in the astronauts'

or man-tended

OF EARTH-MOON

of its rotation

is familiar

the lunar

and navigational is developed.

is in synchronous,

that

crew

It is assumed

from

EARTHSHINE

The

the

at unprepared

to departure

with lighting infrastructure

THE

situation

provided

for landing.

manned

probably

constraints.

These

in the form of long shadows extending It also insured that the sunlight would

the final

will

dictated "

landing

rotation

its own

the same

axis.

hemisphere,

with

respect

around This

the

to the

Earth

geometry

usually

Earth,

to equal

produces

referred

a the

an effect

to as the

lunar

nearside, always faces the Earth, while the lunar farside always faces effects, caused by the wobbling of the Moon's axis during its rotation,

away. results

Libration in _-60

percent visibility of the lunar surface from the Earth only one-half of the Moon's surface is visible at any

period,

although

The synchronous illumination

shine

will remain in a constant addition,

rotation has several (fig. 1). To begin with,

in the same location lighting angle, and

shadow

length

effects at any

over a 12-month one time.

that pertain to the question point on the lunar nearside,

of Earththe Earth

throughout the _655-hour lunar day.1 This will result shadow length and location after lunar sunset. In

will increase

away

from

the

point

at which

the

Earth

is directly

overhead (hereafter referred to as the sub-Earth point) to a maximum at the poles and limbs, resulting in a higher proportion of the ground in shadow as one moves toward either the poles or limbs. Also, the lunar farside will receive no illumination from

iii_::i_i_ii _ ,i _

i_ •

•

Earthshine, which will result in the surface on the farside being extremely dark lunar sunset. Finally, as the phase of the Earth changes, the value of Earthshine

after illumi-

nation

dawn

the

reflect

the

sub-Earth

western

different

point,

limb

location

CREW

During lunar

orbit,

1 To avoid The

prefix

the

on the lunar

will be a phase

will have

an Earth

OBSERVATIONS

the

Apollo

and

flight

potential

"lunar"

locations Earth

will

Program, crews

confusion, be added

OF LUNAR

the to refer

able

terms

"day" day

SURFACE

Apollo

to observe

to both

For example,

9, of 90 °, while

at lunar

the

lunar

dawn

at at a

q_of 180 ° (fig. 1).

six separate

were

surface.

angle,

and and

3

missions

the lunar

"night" night

IN EARTHSHINE

will periods

spent surface

refer on

to the the

up

to 6 days

in

in a variety

of

terrestrial

and

lunar

surface.

day

night.

illumination conditions. Captain John W. Young was one of three astronauts to fly two lunar flights, the first was as the command module pilot on Apollo 10, and the second was as Commander of Apollo 16. Many of the observations that follow were made by Captain Young. In lunar orbit, it was possible to seeall the features on the lunar surface in Earthshine as easily as they were seenon the sunlit portion of the Moon. Transition across the terminator from the sunlit portion to the Earth!it portion was rapid, and there was no time required for the eye to adjust to the Earthshine to pick out details on the lunar surface (personal communication with Captain John W. Young, Special Assistant for Engineering, Operations and Safety,Lyndon B. JohnsonSpaceCenter, 1990). Gross color differences, particularly the transition between the mare and the highlands, were visible, as were shadows castby topographic features (fig. 2). High contrast features, such as bright-rayed craters or the bright swirl features on the maria like Reiner y, were also visible and could be photographed with high-speed film (fig. 3), although the photographic film carried by the Apollo crews was never capableof recording a scene with the sameveracity as the human eye. At times, it was even possible to seedetails within the shadows castby Earthshine. On the basis of these observations, it should be possible to make a lunar module landing to an unprepared site using only Earthshine for illumination (personal communication with Captain John W. Young). Further, making a landing to a prepared landing site, using a minimum complement of landing lights and navigational aids, would be even lessdifficult than making a vertical night landing in a helicopter on Earth (personal communication with Captain JohnW. Young). Transition from the Earthlit portion of the Moon into the unlit lunar farside was dramatic, and there was no senseof the gradual diminution in Earthshine illumination as the terminator was approached. It was extremely dark beyond the terminator, and no surface features could be seen. The horizon was distinguishable only by the lack of stars; otherwise, it was invisible.

CALCULATION OF VALUE OF ILLUMINATION BY FULL EARTHSHINE The value of full-Earth illumination canbe approximated using two techniques. Both of thesetechniques make use of the ratio of the albedos of the full Earth and the full Moon, and both are sensitive to the values chosenfor these albedos. The first technique approximates the ratio of full-Earth illumination to full-Moon illumination, assuming that the reflectance of eachbody canbe approximated by a flat plate of equal radius with the same albedo. The calculations are asfollows: •

Earth radius = 6371km

°

Lunar radius = 1738km

4

_i!:_ _i_I_I_ _ii! '_

_iil!iiiiii_ _ _!i!_iiI!i!i_ _

:_i! _ _i_,

L

•

Area

of flat

plate

with

6371

km radius

•

Area

of flat plate

with

1738 km

•

Area

ratio

•

Average

Earth

albedo

= 0.4

•

Average

lunar

albedo

= 0.07

•

Albedo

•

Relative

intensity

(Albedo

ratio

radius

= 127.5

x 106 km2

= 9.5 x 106 km2

_:_i_! _

i_ •

i

ii_i_ d! _

of both

ratio

flat

The

of full Earthshine

also

the

unit unit

Moon

which

is also

the

to the units

i,ii:i _ _iiiii_ _

equivalent

BEarth

= BMoon

•

BMoon

= 0.25 cd/cm2

•

BEarth

values

the

area

be -_76 times

albedo

ratios

calculation,

for respective

a 2-dimensional unit

with the first

should

unit

In each

of these

is equivalent

unit

value

This

of

calcula-

of brightness

(lumen/steradianl/centimeter2),

is, lumen/meter2

to

the

albedos.

as

back

changes,

to an

the value

of

to candela/centimeter2, The

conversion

is necessary

to units of illumination by accounting for the solid when seen from the Moon. These calculations are as

• (AlbedoEarth/AlbedoMoon)

= 1.43 cd/cm2

from

from

along

As with

to lumen/steradianl/centimeter2.

•

Conversion

chosen

= 76.49

on the Moon

can be used

(lumen/meter2). that

to change from units of brightness angle, 0, subtended by the Earth follows:

_i_.

Earthshine

to a solid-angle same;

Earth:Moon)

of the full Earth.

changing

for illumination stays

ratio

B, of the full

(candela/centimeter2) area

(Area full

is sensitive

involves

=

calculations, on Earth.

the illumination

the approximation tion

= 5.7

Earth:Moon)

brightness,

approximate

= 13.42

of Earth:Moon

On the basis of these bright as a full Moon

ii

plates

brightness,

(ref. 3) = 1.43 lm/sr/cm2

B; to illumination,

_ ii!

E; uses

the equation:

E = _ B sin2@ @ = 0.95 °

:!,

_i_ _ ,:!

E = 1.24 _i

_ ii

_

10-3 lm/cm2

= 12.4 lm/m2

•

i_

i_i_ i_!i_

.......

:

,:

_

"

::: _

,

This

value

shown i_ _ iii_ : 2!u _i_ _

i _

can be compared

in table

ment

and

gram

standards

Physical

Standards used

of any

experiments

either

on the

unmanned

this

_

nation

as reading.

ii::

value

remain

pro-

for future

for Earthshine

illumination,

8 or 10, which

preceded

or on Apollos the value

for full-Earth

calculated

the

illumination

from

above.

situations which from Earthshine.

allow an understanding of the magniThe first of these is the value of illumi-

(2).

It is possible,

IN REAL

on full,

above,

full Earthshine

WORLD

Moonlit

without amounts

illumination

TERMS

nights,

to operate

lights; and to generally of visual acuity, such

is _-76 times

suggesting that a significant amount of activity can without the need for additional artificial lighting.

take

as bright

place

under

as a

full

• i¸

Another ,_

likely

NASA

The document does not indivalues. At present, no record

to move around outside on foot lighting that do not require great

As calculated

full Moon, Earthshine

the

Environ-

VALUES

0.25 lm/m2

agricultural machinery; conduct tasks without

will

Program

Natural

ILLUMINATION

physical available

full Moon,

values

the

the value

Apollo

(1) representing

These

Program

the

TM X-68863,

of this paper. or measured

As can be seen,

EARTHSHINE

of the

Program

measure

with

during

in NASA

planning.

actually

favorably

There are several of illumination

Apollo

balance values

Surveyor

agrees

CASTING

tude

that

for planning

published

for the

of this document.

document

used

were

in the Apollo

exists

publication

iiii_,i__

to values

figures

programs and will be used for the cate whether these were calculated

!i:

_,

1. These

_i_ _,

Civil

comparison

twilight,

sunrise

but

brightest

in nautical after

lights,

is the

In aviation,

civil twilight an aircraft

lights,

interior

cockpit

shown

540 lm/m2

(3).

m2.

these

Using

phase

angle,

point

(fig. 4).

values,

the value and

This

shows

of the lunar

night

after

prior

that than

before

and half

illumination

the minimum

has

illumination

into

further

sunrise,

lights,

(or just

The

after

lunar-night greater

value The

sunset),

lighting

is

is 100 lm/

as a function

illumination

when

navigation

lighting.

illumination

values

or before

of full Earthshine.

interior

in Earthshine

dark,

or before

taxiway

that

navigation.

still see the

for position

to sunrise

full-Moon

full and

sunset,

in typical

Earthshine

aeronautical

horizon

the need

(3), or about

higher

and

or runway

the range

graphically

sunset time

of illumination

plotting

is significantly

portion

is the

sky, just

to depict

and

to see the

without

is 6.5 lm/m2

it is possible

after

lights,

by the terrestrial

Finally,

point

substantial

time

nautical

it is possible

operate

civil twilight

illumination

sub-Earth

terms, when

terrestrial

to safely

landing

for terrestrial

from

full dark,

stars.

it is possible

comes

of Earth at the

sub-Earth

illumination on Earth, than

at the and

that

terrestrial

civil twilight. i_' ii_ _:_!_

To put

these

illumination

values

real

World

values,

they

can be com-

pared to the illumination given off by a 60-W incandescent light bulb, which 840 lm of light. The distance, r, at which a 60-W bulb produces an illumination 13.5 lm/m2

can be calculated

using

the

following

• ii ¸ii_ •

6

equation:

generates of

a

13.5lm/m2 = 840lm/4

n r2

r=2.2m

This

means

hine

will be roughly

from

the

able

that working

outside

Using

the

surface

working surface.

in July same

lighting

for the

on the western

and

0 ° longitude. east will earlier.

The

have

will

--164 hours sunrise.

sunset,

and

eastern

limb,

with

a "new''

just prior

to that

at the

midnight,

then

difference horizon,

at polar and

latitude

equatorial

the

2 Midnight

At pole point:

decreasing

These

chosen

bulb

time.

elevated

adequate

to scale

of these

two

night

dark

phase

and

at 80 ° north

at lunar "new"

at sunset, of 2.8 lm/m2

covered

in shadow

of illumination

are detailed

in table

will not 2.

lunar sunset and sunrise.

7

''2

just before

profile

of will be

to 13.5 im/m2

will be very

will be greatly

of

illumination

at sunrise.

the Earth

will

"midnight,

Earth

increasing

and

the level

to a maximum

illumination

or

discussed

a full Earth

progresses,

gradually

the

latitude,

effects

minimum

increasing

will

>81.4 ° west

will be the opposite: locations,

angle

(1) locations

libration

will be from

illumination

them:

longitudes

of 2.8 lm/m2

to full

the

Earth

(2) a location

the level

between

point,

the

because

will be that

variations

refers to the midpoint

avail-

on the lunar

light

among

As the lunar

to a minimum

of ground

sub-Earth

to the lunar

at sunset

2.8 lm/m2

however,

Consequently,

and

sky due

reduced

at sunset,

sub-Earth

light standard

provide

illustration

because

to a level

the profile

Earth

amount

locations.

further

central

a 60-W should

investigated,

Earthshine

to sunrise.

locations,

sites.

were

in the

be reduced

the

13.5 lm/m2

were

of 13.5 lm/m2.

gradually

the

at 80 ° longitude,

Earth

location,

after On

will occur

lower

limb

from

will be different

cases

the

be at a maximum

illumination

lunar

limb

under

A schematic

away

cases

80°-longitude

For a western

similar

eastern

not always

constantly

night

end-member

to the natural impinging

conditions

full Earths-

is 2.2 m away

6.

nearside

the lunar

Several

to working

under

which

at =8:00 p.m.

of Earthshine,

of these

EVAs.

5 and

point

bulb

is similar

Texas

value

Both

of lunar

on the lunar

throughout

be different.

minimum

in figures

a 60-W

of illumination

of Houston,

surface.

conduction

at the sub-Earth under

_) of 90 °, is equivalent

is shown

At locations profile

the

the working

conditions

level

latitude

formula,

surface

to working

This

at the

at an Earth

4.9 m above

on the lunar

equivalent

at

One

significant

low

on the

increased be as useful

from as in

LIGHTING CONSTRAINTS TO LUNAR SURFACE OPERATIONS DURING THE LUNAR DAY Determination of slope on the lunar surface is primarily a function of two different sourcesof information. Within -_100m of a particular location, shadows castby positive features, such as rocks; and by negative features, such as craters,provide the most information on centimeter to meter-scalevariations in local topography. For topography located at greater distances from an observer,broad-scale topographic changesare discernible by variations in brightness. Assuming a uniform surface color, the closer a surface is oriented at right anglesto the direction of sunlight, the brighter it will be. An examination of panoramic photographs from Apollos 15, 16, and 17 indicates that as an observer gets closer to looking directly down-Sun, the amount of information available on local topography diminishes to virtually zero (figs. 7a and 7b). In particular, the problem appears to becomecritical within _-20° of directly down-Sun. This suggeststhat when the Sun is within 20° of the zenith, topography may be sufficiently subdued to make EVA operations on unfamiliar terrain difficult. The underlying assumption is that conditions near down-Sun under low-Sun-angle conditions can be extrapolated to high-Sun-angle conditions. A plot of Sun angle as a function of mission day (fig. 8) indicates that the Sun will be within 20° of vertical between _-130and 200 hours after lunar sunrise. The difficulty in distinguishing surface topography may require some restrictions in EVA surface operations during this time, such as operations only in familiar terrain, or operation of rovers only along known tracks that are marked. Similar operational restrictions are implemented in Antarctica during periods of reduced visibility. APPLICATION OF RESULTSTO OPERATIONAL SCHEDULES The results of this study have been used to develop a seriesof figures that graphically depict the lighting conditions throughout the lunar day, and possible lighting constraints to lunar surface operations (figs. 9a, 9b, and 9c). The figures were created with several conditions in mind. The first is that a minimum illumination of I lm/m2 will be necessaryfor safeEVA operations. This figure is chosen to follow conservative guidelines; 1 lm/m2 should full-Moon carry

is likely be pointed

to supply out,

illumination

out EVA

a sufficient

level

however,

that

lighting

on Earth,

should

of light

to carry

levels

as low

be sufficient

under

operations.

8

out

all EVA

as 0.25 lm/m2, emergency

activities.

It

equivalent conditions

to to

The second condition is that the mission rules requiring low Sun elevation with a backlit landing area, aswas used for the Apollo missions, will continue to be in force for the initial landings at previously unexplored locations, although the specific restrictions of from 5 to 20° Sun elevation may not be necessary. Lessrestrictive Sun angle conditions were suggested by discussions with Captain JohnW. Young. His observations during the lunar landing on Apollo 16indicate that terrain scaleis as important as terrain form, and that almost any degree of backlighting will be sufficient to highlight the terrain. What is lacking on the lunar surface, however, is an adequate referencefor scalemeasurement. The most useful information on the topographic scaleduring the Apollo landings became the shadow of the lunar module, which was out ahead of the landing point during the transition from powered descentto hovering flight. Using the width acrossthe landing legs of the lander, =10m, it was possible to gauge the scaleof the topography at the landing site, and determine whether the chosentouchdown point would be safe. On the basis of this experience,it appearsthat while the restriction to a particular range of Sun angles may not be necessary,backlighting will be extremely useful during future landing approachesto the lunar surface (personal communication with Captain John W. Young, 1990). The charts lay out the suggested times during which lighting conditions will be optimal for lunar EVAs in central equatorial regions in the vicinity of the sub-Earth point, in western limb locations, and in easternlimb locations. The diagram also shows the approximate phase of the Earth at sunset, sunrise, midnight, and the portion of the lunar night when the ambient lighting is

Figure

8. Apollo

16 photograph

figure 7. Notice the diminution shadows. NASA photograph

ki:

taken

directly

of topographic AS16-110-17952.