In 2004, Lunar Enterprise Corporation contracted the services of SpaceDev Inc. to continue its International Lunar Observatory (ILO) study, the initial step in the realization of a multi-function observatory on the Moon. SpaceDev is a Poway , CA-based company that creates and sells affordable and innovative space products and solutions to government and commercial enterprises. Their ILO Phase B Study, delivered on 19 November, focused on what was recommended in its 2003 "top level mission design” ILO Phase A Study – to research an accurate lunar landing navigation system that can deliver the ILO to a ‘Peak of Eternal Light’ (PEL).
"The challenge for this study is to determine how to be able to begin the descent (~ 4 km altitude above target) and land precisely on a target that may be ~100 meters². No deep space mission has ever required or achieved this level of accuracy." (SpaceDev)
This study focuses on the landing part of the mission and on navigation in particular.
The landing phase is defined to begin at a 200-km-altitude circular lunar orbit. The study assumes that PELs were located by SMART-1 and the target area size and topography is known to a high level of accuracy (several meters).
The study researched the specific possibilities of millimeter, submillimeter and optical (Figure 2) astronomy for the ILO (as opposed to radio, which was covered in Phase A). Both were deemed feasible through slight modifications, and these are only a few of the numerous wavelength options that are still open to the ILO.
Getting to the Peak of Eternal Light
"The science of navigation is always based on establishing a known reference point and position extrapolation integrating from there on. The accuracy of our navigation estimates will be based on our ability to accurately establish this reference point, coupled with our ability to resolve how much we have traveled from that point on. This has been the principle of navigation for centuries .” (SpaceDev)
The report details all of the options available within the ‘Navigation Tree’ (Figure 3). Both external and internal aids will be needed for the ILO. Manual / remote controlled navigation is ruled out due to the unreliable reflectivity of the lunar South Pole’s surface, especially near suspected PELs, due to the area’s low solar illumination angle. It is also ruled out because of the 2.56-second roundtrip communications delay. Figure 4 shows the target field of view as the ILO’s altitude decreases from 4,000 m to landing (target diameter 100 m).
The ILO mission will likely utilize the NASA Jet Propulsion Laboratory Deep Space Network’s (DSN) 34-meter-sized dishes as its ‘Earth Reference.’ SpaceDev suggests using the pseudo-noise regenerative ranging method.
Creating a lunar Global Positioning System (GPS) is impractical for the scope of the mission. Instead, ground reflectors or ground beacons could be placed onto the surface by a separate orbiter. Either of these systems could then be utilized by following landing missions. The reflectors are more practical due to their heat insensitivity and ability to withstand a freefall impact, whereas beacons require a soft landing. Also, only three reflectors are needed as opposed to four functioning beacons. Automated ground pattern recognition is ruled out due to the same reflective problems inhibiting remote controlled descent.
Below 1,000 meters, the ILO will have to employ an altimeter / landing radar antenna. LIDAR, which is like radar, but uses laser light instead of radio waves, would be more accurate, especially Optech, Inc. of Canada’s LAPS. If flight qualified by the time of the ILO’s construction, LAPS can provide 1-meter ground resolution.
For inertial navigation, mechanical gyros are too inaccurate. The ILO will have to use the highly accurate Fiber Optic Gyro, a new technology based on laser.
The ideal celestial navigation instrument for the ILO would be a "Star Tracker,” preferably Ball Aerospace’s CT-632, to be used prior to descent.
Descent will be controlled by a combination of a cold gas and mono propellant motor. Cold gas thrusters (Figure 9), which entail several valves attached to various places on the ILO, will be responsible for maintaining altitude and bringing the ILO above its target area. The twenty times more powerful (1000N) mono propellant motor will counteract the lunar gravity and bring the ILO to a soft landing.
SpaceDev examined the best and worst case landing scenarios, determining that they are both acceptable for the mission.
(Overall) Mission Concept
Once the ILO stack is launched into a 300 km circular Earth orbit, the Star Tracker will be used for attitude determination and standard NORAD ephemeris data will be used for orbital determination. After proper attitude had been confirmed, the Star 48 motor will begin the phasing orbits by firing during perigee, raising the ILO’s orbit to 258,000 km x 250 km (5.4 days). When the motor has shut down, the Star 48 will automatically separate from the stack.
Once everything is checked out, a short firing of the SpaceDev hybrid motor (47 m/s) will place the stack into a trans-lunar trajectory. The flight to the Moon will take four days from this point.
After the four days, the paths of the Moon and the ILO stack will come together. After a 905 m/s burn by the hybrid motor, the ILO stack will be captured into a 200 km circular polar lunar orbit. The stack can then remain in lunar orbit for a while, possibly performing optical imaging of the lunar surface and Earthrise.
"Once mission control decides to begin the descent, a final Inertial Measurement Unit (IMU) calibration procedure will be performed. A short 38 m/s burn will reduce the orbit’s periapsis to 4 km…
When the final descent is initiated, a 1721 m/s burn will be performed to zero out the orbital velocity and the hybrid motor will be ejected. At this point, the ILO will be guided solely by the IMU, main descent control motor and lateral/ACS thrusters. The radar or LIDAR altimeter will come into play once the altitude of the ILO is below 1000 meters. The altimeter will be used to shut the descent engine off to prevent the ILO from bouncing and from raising dirt. The descent control system will quickly bring the observatory to above the desired target so that maneuvering around a rough terrain will not be needed. The quicker the ILO zeros out lateral errors, the safer the descent will be…
Once the ILO is several meters above the ground, the descent motor is shut off. The reaction control system, IMU and Star Tracker will be shut off immediately following impact. Following landing, the observatory will switch to the surface operations mode. Depending on the fuel reserve and landing location, mission control could consider future skipping maneuvers or purging the tanks rendering the propulsion system inert." (SpaceDev)
Power budget and details of the above-summarized information are also included in the study’s report.
"The first and second International Lunar Observatory Studies together demonstrate that a private commercial Lunar mission is feasible in the near future…
Landing the ILO at a PEL would be a significant challenge granted that it is anticipated that PELs will be small areas in a topographically, rugged terrain. Nevertheless, the scientific rewards of landing on such a sight will be great. In addition to science, renewed international motivation to explore the Moon and inhabit it raises the need for affordable and reliable means of navigating to specific sites with pinpoint accuracy. Past Lunar and Mars missions provided considerable experience pertaining to the landing aspect of space missions. The triumphs and the failures observed in recent missions allowed us to conceptualize a private, low-cost Lunar landing mission.
Two main conclusions emerged from this study:
First, navigation beacons placed on the Lunar surface are perhaps the single most cost effective way to assist future robotic and manned missions to land at sites of interest. A Lunar-based navigation beacon system will reduce the lander’s complexity to that comparable with a GPS-type receiver, which could be easily integrated onto every mission. This will significantly reduce the cost, time to build and mission risk of every future Lunar lander mission. Such technology could act as a significant enabler to Lunar exploration as much as Earth based GPS did for many applications on Earth. With that, we recognize that the cost of such a system will be too high to allow implementation in the short term.
Second, flying and landing the ILO to a specific target with the accuracy of about 100 meters is possible using currently available commercial technology (DSN is available for use on a commercial basis). The ILO will utilize a mix of leading-edge propulsion, inertial, celestial navigation together with established Earth based deep space tracking to achieve the required accuracy…" (SpaceDev)