NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) just achieved a very important milestone: 100 days of prime science! Performing science observations over this period of time was one of the more important of the mission objectives, and is a significant achievement. The LADEE mission launched Sept. 6, 2013 from the NASA's Wallops Flight Facility on the Eastern Shores of Virginia, and after some phasing loop passes by Earth, made it into lunar orbit Oct. 6. After performing the successful Lunar Laser Communications Demonstration experiment and checking out the science instruments, the LADEE observatory dropped down into its planned science orbit on Nov. 20. The goal for the next 100 days was to maintain the science altitude between the closest approach to the moon’s surface between 20 and 50 km, and farthest point between 75 and 150 km. The Mission Operations Team at NASA’s Ames Research Center in Moffett Field, Calif., commanded the spacecraft to perform Orbit Maintenance Maneuvers (OMMs) every week or so in order to maintain the science altitudes. The lunar gravity field is so lumpy that frequent maneuvers are required or the spacecraft will impact the moon’s surface. LADEE performed a total of 16 OMMs during the 100 days of prime science, which kept the observatory in the proper altitudes for the three instruments to make their measurements.
The three science payload instruments have been working full-time to unravel the mysteries of the lunar atmosphere and dust environment.
Ultraviolet/Visible Spectrometer (UVS): has acquired more than 700,000 ultraviolet/visible spectra of the exosphere, measuring both gas and scattered light due to dust. The UVS has carried out “extinction” measurements, looking for an extremely subtle reduction in measured sunlight due to obscuration by tenuous dust near the lunar surface. The UVS has mapped systematic variations of sodium as the moon orbits Earth; there is a clear increase in sodium abundance as the moon progresses from new to full, followed by a decrease approaching a new moon. Besides sodium, UVS routinely follows potassium and other species.
Neutral Mass Spectrometer (NMS): An Apollo surface experiment identified argon-40 (40Ar) more than 40 years ago. More recently, the Lyman Alpha Mapping Project (LAMP), a far-UV spectrometer on NASA's Lunar Reconnaissance Orbiter, has detected argon emissions, but with much lower density. This suggests argon changes with time. Now, NMS has positively identified 40Ar and has mapped out its diurnal variations. The NMS also has detected neon-20, a solar wind constituent, as well as helium. These species are all noble gases, but they each behave differently at the moon: argon freezes out on the cold lunar nightside, then springs off the surface in a burst at lunar dawn. Helium is not permanently bound to the moon, but is lost to space over the hot lunar dayside, where temperatures can approach 248 degrees Fahrenheit (120 degrees Celsius). Helium is re-supplied to the moon by the solar wind. This supply is cut off and the abundance on the moon rapidly dwindles when the moon is in Earth’s geomagnetic tail.
Lunar Dust EXperiment (LDEX): As soon as LADEE entered its high-altitude commissioning orbit, and LDEX was turned on, it saw the outer fringes of a “dust exosphere." LDEX has recorded more than 11,000 impacts from dust particles since arriving at the moon in October 2013. These dust particles are part of a dust cloud engulfing the moon, created as micrometeoroids continually bombard the surface and knock this dust into the atmosphere. The LDEX has mapped the spatial and temporal variations, and has found that dust density increases dramatically at lower altitudes. The dust is most abundant over the morning (sunrise) sector of the moon, in the direction of the motion of the Earth/Moon system about the sun. The LDEX also has observed intense bursts of particles, likely generated by impacts that hurl ejecta particles from the surface minutes before LADEE passes by.
Now that the prime science phase is complete, LADEE moves into an extended science phase. This is made possible by the spectacular performance of the Minotaur V launch vehicle, which put LADEE into the optimum trajectory after launch, and also the precise performance of the LADEE main engine, which performed all of the maneuvers to get the observatory to the moon and into the science orbit. As we begin LADEE's extended science phase, we will continue to make science observations and increase the amount of data we obtain. As we approach the end of March, we will allow the observatory to fly very close to the lunar surface in order to obtain science measurements as low as possible. These science measurements are extremely valuable, but also come with a big risk to the spacecraft because it will be flying so low. After we attempt these high-value low-altitude observations, we plan for LADEE to intentionally impact the moon in mid-April and conclude the mission.
Butler Hine and Rick Elphic
Ames Research Center, Moffett Field, Calif.