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Apollo 12 Flag Still Aloft

by James Fincannon

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

Apollo 12 Flag
        shadow



Animation made from five LROC images of the Apollo 12 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) M131806467LC, Sun 8 degrees above the eastern horizon; (2) M114104917RC, 32 degrees, east; (3) M137699517LC, 59 degrees, west; (4) M117650516RC, 9 degrees, west; (5) M132983773RC , 6 degrees, west.  LROC images courtesy NASA/GSFC/Arizona State University.


The Apollo 12 moon landing occurred on November 19, 1969.  The crew spent 31.5 hours on the lunar surface before returning to lunar orbit.  A U.S. flag was erected early in EVA-1.  As indicated in an overall discussion of the six flags left on the lunar surface, there are questions as to whether the flag material has survived 40-plus years in the lunar environment 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 12.

detail from AS12-46-6896

Detail from AS12-47-6896, taken down-Sun early in EVA-1.  A rod is threaded through a hem sewn at the top of the flag and a latching hinge at the top of the pole was supposed to hold the flag out perpendicular to the pole.  The latching mechanism failed, so Pete Conrad is holding the flag out for the camera. (Click on the image for a larger version.)



Detail from 6983 showing the limp flag

Detail from AS12-47-6983, taken late in EVA-1 at about 1455 UTC on November 19, 1969.  Anne Platoff's article about the Apollo flags indicates that the flags were 5 by 3 feet (1.5 by 1.0 meters).  Because the supporting rod is hanging almost vertically, we can estimate that 6.8 feet (2.1 meters) of the pole is above ground and that the bottom of the flag is about 1.8 feet (0.5 meters) off the ground. The Sun was 9.2 degrees above the horizon and, on level ground the shadow would fall in a range from the pole of 3 to 13 meters.



View of the flag and shadow from LMP's window after
          EVA-1


Detail from AS12-46-6866, taken at about 16:32 UTC on the 19th, after EVA-1.  Solar elevation, 10.0 degrees. Shadow range from pole on level ground: 2.8 m to 12 m. (Click on the image for a larger version.)

Al Bean told Houston that he hammered the lower section of the pole about "a foot" into the ground.  The location is marked in Figure 10-50 from the Apollo 12 Preliminary Science Report. and is about 7 meters out from the MESA, halfway between the plus-Z (west) and plus-Y (north) struts.

Detail from PSR figure 10-50

Detail from Preliminary Science Report Figure 10-50, showing the flag location.  The LM landed rotated clockwise about 10 degrees.  The distance between the outer edges of footpads on opposite sides of the LM is 9.5 meters.  The flag is about 12 meters out from the MESA.

Detail from
        AS12--7152, taken by Al Bean on his way from Block Crater to the
        LM

Detail from AS12-48-7152, taken by Al Bean on his way back to the LM at the end of the EVA-2 traverse.  Pete Conrad went ahead of Bean to start the close-out.  The flag is on the right and the erectable S-band antenna in the center.  The antenna is connected to the MESA by a 20-foot (6 m) cable.  The dish is 3 meters across and the height of the supporting tripod is about 2 meters. 


During Apollo 12 preparations for LM liftoff, the RCS hot-fire check blew over the S-Band antenna:

140:12:11 Conrad: Here you go, Houston, with roll, pitch, and yaw (tests of the RCS).

140:12:14 (CapCom Gerry) Carr: Roger, Pete.  (Static; Long Pause)  Intrepid, Houston.

140:12:53 Conrad: (To Houston) Don't panic!   We just blew over our S-band erectable (with the exhaust from the RCS), and we're up on our steerable.<p>

140:13:02 Carr: Roger.  I was just going to tell you, Pete, we lost some of the data on that fire check.

Because of the tripod attached to the bottom of the dish and the transmitter mast at the top, the antenna probably ended up on its side, supported by the ends of two or three ribs and by either one or two of the tripod legs or the transmitter.  It may well have remained mechanically connected to the MESA by the cable.  Photo AS12-48-7163 was taken out the LMP's window after EVA-2 and shows most of the S-band shadow, with the densest part of the shadow cast by the hardware at the top of the tripod and the converging ribs at the center of the dish.  Further discussion can be found in the Apollo 14 flag page.

Site map and high-Sun LROC image

Detail from LROC image taken with the Sun 59 degrees up from the western horizon. 


Five Apollo 12 LROC images


This stack displays the five 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.  The vertical red lines approximate the east-west location of the S-band antenna.  With the antenna on its side, its shadow will be 'attached' to the antenna.

What is really being seen in these five images is the flag shadow itself and not the flag pole shadow.  The flag pole is only about 7/8ths of an inch in diameter. 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 .55 meters (1.8 feet) which is equivalent to one pixel of height.  This resolution is just good enough to see the drooped flag shadow (estimated to be from 6 inches to 1.5 feet).  Even if the drooped flag is narrower than the .45 meter resolution, since one pixel of the camera’s optics is an average over an entire area of coverage, the flag’s shadow can substantially dim a large fraction of this area and, on average, reduce the pixel illumination even though it cannot blacken it completely.

The shadows cast by the flag reach fairly far.  The astronauts indicate the Apollo 12 landing site is fairly flat.  If the slope was significant tilted toward East or West, the shadows would be elongated or shortened depending on the direction, as would be the case for slight craters and hills along the shadow path.  Assuming the flag is 2.1 m from the ground to the top of the pole and on a flat surface, for the near sunrise image (8 degree elevation), the shadow tip would be 15 meters from the flag pole base (which is not visible in these images due to the small diameter of the flag pole); for the near sunset image (6 degree elevation), the shadow tip would be 20 meters) from the flag pole base.  Due to the unsupported shape of the flag, a precise comparison of predicted versus actual shadow lengths is not possible, as well as derivation of flag height from shadow length.

Finally, note that in the morning images at the top of the LROC stack, the S-band dish shows up as a bright spot because sunlight reflecting off the mesh.  In the afternoon images, the dish is fainter, suggesting that the tripod is pointing east and the transmitter mast is pointing west.


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