NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft has impacted the Moon, capping an extremely successful operational mission. Science analysis will continue for months, as the science teams churn through the data and write papers about their findings. So LADEE is gone, but its science legacy lives on!
LADEE ran its science instruments almost non-stop right up to impact the evening of April 17, 2014, in an effort to gather as much low-altitude data as possible. Further study of the returned data will reveal what the instruments saw at these amazingly low orbits, just a few kilometers above the surface. Early results suggest that LADEE was low enough to see some new things, including increased dust density and possibly new atmospheric species. 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.
As the clock was running out on the LADEE mission, we took advantage of an opportunity to replicate observations by the Apollo astronauts more than 40 years ago. (We hinted at this in an earlier update). 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.
In the minutes before orbital sunrise, when LADEE emerged from shadow into sunlight, 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. On Earth, "rosy-fingered dawn" paints the sky prior to sunrise because aerosols and dust particles suspended in the atmosphere scatter the sunlight. With the sun below the horizon, the reflected and scattered light makes the sunrise glow for an observer to see in the dark shadow beyond. However, the very low dust densities that LADEE's Lunar Dust Experiment (LDEX) measured should not produce such a sunrise glow – there were just too few particles along the line of sight to scatter measurable light. Yet some Apollo astronauts reported a pre–sunrise glow and even rays of light, as if the sun was shining through notches of the lunar mountains, and the light was scattered by…something. Could LADEE spot this?
Shown here is a series of images taken on one such occasion, Saturday, April 12. The series begins with LADEE viewing the lunar horizon ahead, a few minutes before orbital sunrise. At this position, there is already a glow in the sky above the completely dark surface of the Moon (right), though the sun is many degrees below the horizon. LADEE’s orbital motion makes the stars appear to move to the left. The same motion brings the sun closer to the horizon ahead and the glow gets brighter. In fact, the glow becomes so bright, parts of the image are saturated. Finally, sunrise fully saturates the camera image. This sequence is the closest thing to the astronaut's orbital viewpoint that LADEE could provide!
The shape of this glow is familiar; we've seen it before. The images reveal a phenomenon that even we can see during a dark, clear night after sunset here on Earth – the zodiacal light. What is zodiacal light? It is the scattered sunlight from billions upon billions of dust grains, not at the Moon, but in the innermost reaches of the solar system. 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.
Unlike Eugene Cernan's sketches from his vantage point in the Apollo 17 command module, America, LADEE saw no rays or other strange features - just good old zodiacal light, plus possibly the outer fringes of the sun's corona. LADEE took several sequences of these orbital sunrise images, and for now, nothing has shown up that clearly is lunar in origin. In the coming weeks and months, we'll carefully analyze these images, and perhaps something related to levitated lunar dust will emerge.
But it sure looks like sunrise is just as impressive from LADEE's vantage point as it is to us on Earth.
NASA's Ames Research Center, Moffett Field, Calif.