NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) performed its final orbital maintenance maneuver (OMM-22) last Friday evening, April 11. This was the 22nd maneuver during the science mission, designed to keep the spacecraft at the desired altitudes for the three science instruments to obtain their best data. After the primary mission concluded in March, we have been allowing the spacecraft to go much lower in altitude, in order to obtain the really high-value science data, accepting the risk of flying barely above the lunar terrain. We flew very close – approximately two miles (3.2 kilometers) above the surface – on April 5, and made it through just fine.
After safely flying through our close approach on April 5, we barely had time to breathe a sigh of relief when we had to face a most nerve-wracking moment on April 14: the day of a unusually long lunar eclipse, which the spacecraft was not designed to survive. The launch date in September 2013, and the primary science mission were chosen so that we could complete everything prior to this long eclipse. However, with our extended mission, not only are we able to gather valuable extra science data, but also conduct an engineering test by flying LADEE through the eclipse.
An eclipse like the one April 14-15, is challenging because there is no sunlight to power the spacecraft and heaters, or recharge the battery. The low temperatures mean the spacecraft needs power to keep itself from freezing. LADEE normally experiences a period of eclipse-like darkness that lasts about an hour every time it orbits the moon, but this extended eclipse lasted four hours. We watched telemetry as the spacecraft lost sunlight and then began to cool down. The battery discharged as the heaters kicked in, and we started receiving yellow and red alarms from the spacecraft as the power dropped and everything got colder. Once the eclipse ended, and the spacecraft started charging up again, the alarms gradually cleared and everything returned to normal. Aside from a couple of sensors getting too cold, everything looked good and we were once again able to gather science data. LADEE had survived the eclipse!
The OMM-22 maneuver was designed to get LADEE as low as we could for science measurements, but also placed the spacecraft on a trajectory that will naturally decay to the planned impact on the moon's far side on April 21. Now LADEE is in the final low-altitude passes prior to its planned impact. These altitudes are very low to the surface and so close to the walls of lunar craters and mountain ridges that there is a good chance the spacecraft may impact a few days ahead of that on April 17 or 18. This risk is definitely worth it, however, as it gives us the chance to collect really valuable science data at low altitudes that normally are impossible to safely achieve. If LADEE gets through these low altitudes by the evening of April 18, then the spacecraft will stay on its final path to for a planned impact in the night on Sunday, April 20 or early morning on Monday, April 21.
Ames Research Center, Moffett Field, Calif.