Prospector Orbits the Moon

by John E. Gruener
Hernandez Engineering

It took 25 years, but NASA is back in the business of sending spacecraft to the Moon. Lunar Prospector is now in lunar orbit and sending back long sought after scientific data every 32 seconds (except, of course, when the spacecraft is occulted by the Moon). Below is a brief summary of Prospector’s first month of operations.

Lunar Prospector, the first spacecraft NASA has sent to the Moon since Apollo is being assembled.
The Lunar Prospector, the first spacecraft NASA has sent to the Moon since Apollo is being assembled.
Lunar Prospector was launched on January 6 atop a Lockheed Martin Athena II rocket at 9:28:44 p.m. EST (an attempt to launch the previous night was canceled due to a range safety tracking radar failure). This was the first launch from pad 46 of Spaceport Florida’s new commercial launch complex. All four stages of the Athena II performed as planned and after about 15 minutes of powered flight placed Prospector and its trans lunar injection (TLI) stage, a Thiokol Star 37 rocket motor, into a ‘parking orbit’ around Earth at an altitude of 200 km. As they passed over Australia about 55 minutes after launch, Prospector was spun up to 60 rpm and the Star 37 was ignited. The TLI stage burned for 64 seconds, successfully propelling Prospector out of earth orbit and on it’s planned 105 hour trajectory towards the Moon.

Though delayed by a minor communications problem with getting Prospector’s telemetry signal to mission control at Ames Research Center, the science instruments were all turned on and the three 2.5 m booms were all fully extended. This resulted in Prospector’s spin rate decreasing to about 12 rpm, its final operational spin rate. The electron reflectometer, neutron spectrometer, and gamma ray spectrometer were all allowed to ‘degas’ before being commanded to high voltage. All of the science instruments, including the alpha particle spectrometer and magnetometer, were successfully calibrated enroute to the Moon.

Three trajectory correction maneuvers were initially planned for Prospector’s trip to the Moon, but the accuracy of the first two made the third unnecessary. On January 11, at 6:45 a.m. EST, the first , and most critical, lunar orbit insertion (LOI) burn began when Prospector was only 71 km above the lunar surface. It lasted about 32 minutes and resulted in an elliptical orbit of roughly 70 km by 8,500 km. The inclination of the orbit was about 89.7° and the period was about 11.6 hours, very close to the targeted 12 hour orbit. The second LOI burn occurred on January 12 and lasted 27 minutes. The resulting elliptical orbit was about 80 km by 1870 km, and the orbital period was reduced to 3.5 hours, as planned. The third LOI burn occurred on January 13 and also lasted about 27 minutes. This resulted in an orbit close to circular, with dimensions being roughly 90 km by 150 km and with an orbital period of 2 hours. Finally, on January 15, two orbit trim maneuvers were performed, resulting in a 99 km by 100 km orbit with an inclination of 90° and a period of 118 minutes, Prospector’s planned mapping orbit.

Prospector DeployedLunar gravity, of course, will continually affect Prospector’s orbit, and rarely will it be in a ‘perfect’ circular orbit. As Prospector’s orbit degrades, orbital adjustment maneuvers will periodically be made to get the spacecraft back near the nominal 100 km orbit. Since one of the primary science objectives is to provide more information about the lunar gravity field and improve the current gravity models, the time intervals between the orbit adjustments are not set and will be determined as the mission evolves.

The science instruments are in excellent health, continue to perform superbly, and are sending data back that will help accomplish the other primary science objectives. The three spectrometers will measure the elemental chemistry of the lunar surface, record any out-gassing episodes emanating from the lunar surface, and possibly detect water ice in permanently shadowed areas at the polar regions of the Moon. And though the Moon lacks a global magnetic field, the magnetometer/electron reflectometer will map the distribution of the many small magnetic fields on and around the Moon. This magnetic data, along with the gravity data, will also help answer questions about the size and characteristics of the lunar core. Prospector has no onboard computer,and science data is sent to earth as it is collected. When Prospector is occulted by the Moon, the command and data handling system has enough memory to store up to 53 minutes of data. This delayed data stream is then sent with the real time data stream once Prospector is back in communications with earth.

All in all, the Lunar Prospector mission has begun without a ‘hitch’ and it looks like we’re in store for a successful year of lunar data collection. The mission can be followed from the Lunar Prospector website at Ames Research Center (http://lunarprospector.arc.nasa.gov). The people at Ames have done a great job on the data visualization section of the website. Prospector’s location and position are updated real time using images from the Clementine mission, and raw science data can be seen every 32 seconds as it comes in from the Moon. Dr. Alan Binder, Prospector’s Principal Investigator, should be congratulated on his perseverance during the past 10 years, guiding Prospector through it’s various ‘lives’ and finally getting NASA back to the Moon.