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Apollo 16 Flag Still Aloft

by James Fincannon

Copyright © 2012 by Eric M. Jones
All rights reserved.
Last revised 21 April 2012.

Animation from
        LROC images


Animation made from seven LROC images of the Apollo 16 landing site, ordered from sunrise to sunset, and showing the changing length and location of the shadow cast by the U.S. flag erected by the crew.  The frames are: (1) M116215545RC, Sun 8 degrees above the eastern horizon; (2) M131548593RC, 10 deg. east; (3) M144524996LC, 43 deg. east; (4) M142164190RC, 69 deg. east; (5) M122108795LC , 61 deg. west; (6) M117392541LC, 7 deg. west; and (7) M132732855RC, 2 deg. west. LROC images courtesy NASA/GSFC/Arizona State University.


The Apollo 16 moon landing occurred on April 21, 1972, with the crew spending 71 hours on the lunar surface before returning to lunar orbit.  During that time, the U.S. flag was planted on the surface.  As indicated in an overall discussion of the six flags left on the lunar surface, there are questions as to whether the flag material disintegrated over time or whether any of the flag poles had fallen, either during the Lunar Module liftoff or anytime afterward).  This article resolves this issue for Apollo 16.

The following three images show the Apollo 16 flag before and soon after the Lunar Module ascent.

Detail from AS16-113-18339 showing John Young leaping
          salute

Detail from AS16-113-18339, taken early in EVA-1. View to the south toward Stone Mountain.  The LM landed on level terrain but, beyond 25 meters from the spacecraft, slopes could be 6-10 degrees.  The flag is pointing somewhat north of west.  Note that John Young jumped about 0.42 meters off the ground.  (Click on the image for a larger version.)


Photo taken by John Young on his way back from the
          Rover's final parking place

Detail from AS16-116-18723, taken by John Young on his way back to the LM from the Rover's final parking place. The view is to the west.  The camera was tilted about 4-5 degrees to the right when Young took the photo.  It is possible that Young took the photo with the camera handheld, almost certainly at chest height. The spacecraft landed tilted back to the east by 2.3 degrees and rolled left (south) by 0.4 degrees. (Click on the image for a larger version.)


Rover TV view to the west with the flag on the
        right

Frame from the Rover TV taken after LM liftoff.  The TV camera is tilted to the left by about 9 degrees because the Rover was parked about 50 meters east of the LM on uneven ground.  Note that the flag is pointing more to the north than it was in Young's photo (above) and that the flagpole is tilted to the right.


Based on a comparison of pre- and post-ascent images, the flag did not fall, but the pole seems tilted by 30 degrees off the local vertical.  Since the rover took video from the East, the orientation of the flag suggests it offers a shadow perpendicular to the Sun’s rays at dusk and dawn.  At LM liftoff (about 01:26 UTC on 24 April 1972, the Sun was at an azimuth  of 77 degrees and an elevation of 50 degrees.  Consequently, the dark spot on the ground near the flag is the flag's shadow.  It seems unlikely that either the flag or the support rod that runs along the top edge of the flag is touching the ground.  Rover TV images show the rest of the flag below this edge fairly well which would not be the case if the top support rod was touching the ground (the flag on the bottom edge would be bent and crinkled).  A TV frame taken moments after John Young erected the flag shows its condition at that time.  Although no TV coverage is available of the actual deployment, it is evident that Young pushed the flagstaff into the ground by hand.  He did not comment on the depth of penetration.  The Apollo 17 crew chose to hammer the lower section of the flagstaff into the ground before fitting the upper section.  A comparison between the Apollo 16 and 17 flags, as deployed, indicates that the lower section of the Apollo 17 flagstaff (below the gray joint section) is about 10-20 cm farther in the ground than the Apollo 16 flagstaff.  Seeing the obvious tilt of the Apollo 16 flagstaff in the post-liftoff TV may have convinced the Apollo 17 crew that they should hammer in the lower section of theirs.
 
The following animation made from a detail from Figure 6-13 in the Apollo 16 Preliminary Science Report and a detail from an LROC image M142164190RC, taken with the Sun only 31 degrees east of the zenith, shows the layout of the site near the LM.  North is up. 

LROC image versus site map


With the Sun near the zenith, variations in surface brightness in the LROC image are small.  Contrast adjustments made to bring out details makes the soil the astronauts disturbed around the LM nearly black.  The flag shadow northwest of the LM is well defined.  The crater immediately east of the LM has a diameter of about 25 meters.


In the LROC animation and the following figure, we are seeing the shadow of the flag, but not of the flag pole. The flag pole is only 0.876 inches in diameter and its shadow will be about the same. LRO cameras can at best see down to 0.45 meters (1.5 feet) which is the equivalent to one pixel width in the images and 0.55 meters (1.8 feet) which is equivalent to one pixel of height.  This resolution is good enough to see the tilted flag’s shadow which, because the flag is pointing north, is about 0.9 m (3 feet) across from north to south.  Importantly, clear separations of the entire shadow from the flagpole locations before mid-morning and after mid-afternoon (following figure) demonstrates that (1) we are seeing shadows of the flag and not the pole; and (2) the flag is not touching the surface.

Seven LROC stills

This stack displays the seven LROC images details used to make the animation at the top of the page.  The vertical yellow lines approximate the east-west location of the flagpole. In the early morning images at the top of the stack, the flag shadows start from points west of the flagpole and end farther west.  The situation is reversed for the late afternoon images at the bottom of the stack.


Flag assembly before stowage on the LM

Flag assembly prior to deployment.  The upper part of the flagpole is at the top, with the support rod for the top of the flag attached by a latching hinge at the right.  The bottom section of the pole is below those two items.  The portion of the bottom section that is pushed or hammered into the surface on on the right, with two short knurled sections indicating how far the pole should be pushed in for adequate stability.


The flag height depended upon how far the pole could be placed into the regolith, which varied from mission to mission. Assuming vertical deployment of a flat surface, with the flagpole 87 inches from the ground to the top of the pole and the bottom hem of the flag 50 inches above the ground, for the near sunrise image (8 degree solar elevation), the nearest part of the shadow would be 356 inches (9.0 meters) from the flagpole and the farthest part would be 619 inches (15.7 meters) away.

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