Video caption: A series of star tracker images taken by LADEE on April 12, 2014. The lunar horizon is ahead, a few minutes before orbital sunrise. Click to see gif animation. Credit: NASA Ames
Mere days before its planned mission ending crash on April 17, scientists directing NASA’s wildly successful LADEE lunar orbiter sought to accomplish another key objective by photographing Zodiacal light near the lunar horizon in an attempt to replicate mysterious observations stemming back over four decades to the Apollo moonwalkers.
The probe was skimming barely a mile above the pockmarked lunar surface as the team commanded one of the star trackers to shoot near the horizon and look for eerie zodiacal glow just before sunrise on April 12. See images above and below.
The LADEE dust explorer was tasked with scanning for horizon glow thought to be caused by sunlight scattering off tenuous bits of levitated lunar dust it has been investigating the past few months from ultra low lunar orbit.
The science team diligently sought to extract every possible bit of science data from LADEE to the bitter end. And they succeeded!
Indeed the probes last data transmission including all of the final science observations took place only moments before the April 17 suicide plunge.
“In an incredible race with time, LADEE’s Real Time Operations team queued and downloaded all science files just minutes prior to LADEE’s impact,” said Rick Elphic, LADEE project scientist at NASA Ames Research Center, Calif.
“We used one of the star tracker cameras to gaze out over the Moon’s horizon, while LADEE was in the deep darkness of the lunar night and over the far side where no Earthshine can reach.”
The goal was to try and reproduce from orbit a famous sketch from Apollo 17 moonwalking astronaut Eugene Cernan during NASA’s final lunar landing mission in December 1972.
See Cernen’s sketch herein showing the horizon glow as well as “streamers” and “crepuscular rays.”
“What if we could see a horizon glow just before orbital sunrise? Might we see levitated dust, illuminated by sunshine while LADEE is in the dark shadow of the moon?”
So LADEE mission control at NASA Ames transmitted commands for the probe to snap a special sequence of images as the couch sized probe emerged from shadow into sunlight on April 12.
“We commanded the spacecraft to take a series of images. We wanted to see the same scene the astronauts saw, with the sun just below the horizon. In this configuration, we could view anything that might scatter sunlight.”
Back here on Earth, aerosols and dust particles suspended in the atmosphere scatter the sunlight resulting in a sunrise glow in the background shadow.
But the mystery on the Moon is that measurement from LADEE’s Lunar Dust Experiment (LDEX) indicated that the dust density is so low that a horizon glow would not be expected to be seen.
So what happened?
A zodiacal glow was indeed detected in the star tracker images.
“In fact, the glow becomes so bright, parts of the image are saturated,” Elphic reported. “This sequence is the closest thing to the astronaut’s orbital viewpoint that LADEE could provide!”
Only the streamers and rays were not seen.
So what’s the source of the glow?
Elphic says its likely from comets.
“The origin of this dust appears to be comets, which shed gas and dust in their orbital progress around the sun. The lenticular shape of the zodiacal light, seen in the LADEE star tracker images, results from the tendency of the dust to be more concentrated near the orbital plane of the planets.”
The team is now poring through all the science data returned by LADEE.
Perhaps something related to levitated lunar dust will emerge, concludes Elphic.
“But it sure looks like sunrise is just as impressive from LADEE’s vantage point as it is to us on Earth.”
LADEE was launched during a spectacular night time blastoff on Sept. 6, 2013, from NASA’s Wallops Flight Facility along the eastern shore of Virginia. It was visible to tens of millions of spectators up and down the coast due to exceptionally clear viewing conditions on a remarkably cloud free night.
The science goals were to investigate the composition and properties of the Moon’s pristine and extremely tenuous atmosphere, or exosphere, and untangle the mysteries of its lofted lunar dust dating back to the Apollo Moon landing era.
All those objectives and more were fully accomplished during over five months investigating Earth’s nearest neighbor. Which endede as planned on April 17 with a crash into the Lunar far side. Read my account here.
It entered lunar orbit on Oct. 6, 2013, amidst the idiotic government shutdown that negatively affected a host of US science missions funded across the federal government.
The 844-pound (383-kg) robot explorer was assembled at NASA’s Ames Research Center, Moffett Field, Calif., and is a cooperative project with NASA Goddard Spaceflight Center in Maryland.
The $280 million probe is built on a revolutionary “modular common spacecraft bus,” or body, that could dramatically cut the cost of exploring space and also be utilized on space probes to explore a wide variety of inviting targets in the Solar System.
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