NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE), pronounced “Laddie,” was a 2013 robotic mission that was launched to orbit the moon and collect detailed information regarding the composition and structure of the thin lunar atmosphere. It was also the first-ever Lunar Laser Communications Demonstration (LLCD).
Artist Concept - NASA Lunar Lasercomm LLCD transmitting to Earth Ground Terminal. Image Credit: NASA
The LLCD used lasers rather than radio waves, which are typically used on other spacecraft that travel beyond close-Earth orbit, to send messages to controllers on Earth, enabling broadband-speed communications between the spacecraft and the ground.
The probe measured approximately 7.7 ft x 4.75 ft x 4.75 ft and weighed 383 kg (844 lb). It took approximately 30 days for the probe to reach the moon. LADEE then completed a checkout period of an additional 30 days. Following that period, the probe conducted a 100-day science mission, which was extended an additional month.
Once LADEE’s mission was completed, NASA intentionally impacted the probe onto the far side of the moon, avoiding historically important sites such as the Apollo landing zones.
LADEE Mission Patch. Image Credit: NASA
Demonstration of the Lunar Laser Communication
The LLCD project was conducted by MIT Lincoln Laboratory, NASA's Goddard Space Flight Center, and the Jet Propulsion Laboratory. It served as NASA’s inaugural attempt to exhibit optical communications between a lunar orbiting spacecraft and Earth-based ground receivers.
All previous communications involving spacecraft beyond close Earth orbits demanded spacecraft with small, low-mass, low-power radio transmitters and massive satellite dishes on the ground. LLCD offered an alternative process.
The new approach was capable of replacing traditional radio communications with uniquely designed lasers and laser detection units to transmit information between lunar orbit and three receiving stations in the US and Spain.
LLCD exhibited error-free communication from the moon, including in broad daylight and when the moon was positioned within 3° of the sun, as seen from Earth.
LLCD successfully operated without errors even when the Moon was less than 5° above the horizon, as observed from the ground station. This performance demonstrated that atmospheric turbulence and wind had minimal impact on the system's functionality. Additionally, LLCD was capable of transmitting signals through thin cirrus clouds.
The 30-day test offered a more comprehensive check of its potential use in all-purpose communication. Instead of downloading a pre-arranged file, NASA attempted real communication with LADEE with real data collection.
LADEE’s science data was routed to the ground via radio frequency (RF) link, with the data transmitted by the LLCD test being verified against the RF data. This demonstrated the feasibility of using laser communication uplink/downlink on future missions.
The LLCD utilized a pulsed laser beam to send data over 239,000 miles between the Earth and the moon at a record-breaking data-download speed of 622 Megabits per second (Mbps). This was over 6x faster than previously achieved speeds of the best radio system ever flown to the moon, and it would typically require several days to download.
LLCD demonstrated a 20 Mbps uplink, which could send error-free high-definition video to and from the moon, key for future human exploration missions. Simultaneous centimeter-class precision ranging to the spacecraft was also provided, which is useful in improving the gravity models of planetary bodies.
Artist Concept Image NASA's Lunar Atmosphere and Dust Environment Explorer. Image Credit: NASA
The testing examined more than just raw download speed, with priorities placed on signal accuracy and reliability, as well as potential distance effects.
LLCD demonstrated excellent performance even at very oblique angles, such as when the satellite approached the Earth’s horizon, and its signals had to pass through the densest parts of the atmosphere. The laser signal remained error-free, performing reliably in broad daylight and through thin clouds.
As well as limited error, the LLCD was capable of switching between ground stations as the Earth rotates, similar to how a mobile phone network functions according to NASA, and did so without human assistance. The system was able to lock on to the ground stations without the use of a radio signal.
The most notable breakthrough of the LLCD demonstration was the incredible success of what turned out to be faultless operations, which enabled the system to relay definitive, high-value scientific data obtained through LADEE’s investigations of the moon’s environment. The relay of data showed the great potential of laser communication in future missions.
The downside of the system for future research is its range. Simply increasing the laser's power to maintain coherence over greater distances is not a practical solution. A more effective approach, which NASA is actively pursuing, involves the Lunar Communications Relay Demonstration (LCRD).
This initiative aims to deploy laser routers in space to receive and retransmit laser signals from a primary transmitter, extending their reach and reliability. LCRD is housed aboard the U.S. Department of Defense’s Space Test Program Satellite 6 (STPSat-6), which launched in December 2021.
Line of Sight Jitter Testing
In laser communication systems, maintaining the dynamic stability of the laser beam is crucial for achieving high data rates. Typical design requirements range from 1/10 to 1/5 of the beam's angular extent. For the Lunar Lasercomm Space Terminal (LLST) system—a secondary payload and part of LLCD—the specified stability requirement was 4.2 µrad RMS. This exceptionally small and challenging target demanded extensive modeling and testing to ensure the system could meet the stringent metric.
Achieving this precision relied on the magneto-hydrodynamic inertial reference unit (MIRU), which actively stabilized the optical telescope to enable fine laser pointing during space operations. Testing included careful evaluation of LOS jitter to confirm the system’s performance.
LADEE Jitter Testing on Minus K BM-4 Vibration Isolator. Image Credit: Minus K Technology
A detailed finite element model was developed to include all major components of the optical module assembly. The purpose of this system model was to capture the structural dynamics behavior of both the overall system and each of the individual optical elements and predict the system’s performance.
The structural dynamics of the system were accounted for by the model, as was active stabilization control. The LOS jitter prediction was generated through the integration of the results from the structural, optical, and control models.
A test bed was created to measure the jitter using the prototype and flight hardware. For initial jitter testing, a telescope mass mock-up was built and mounted to a real panel gimbal and flexure.
The mass mock-up closely imitated the mass and moment of inertia of the optical head. A stiff frame served as the mounting for the panel, which was connected to a Minus K BM-4 negative-stiffness vibration isolation table.
Since testing could not be conducted under actual flight conditions, a system model was developed to verify the optical module's ability to meet the stringent jitter requirements. This model needed to align closely with test data, particularly in the frequency ranges that had the greatest impact on residual LOS jitter.
Test equipment limitations prevented the hardware from being driven with the simultaneous 6-axis input spectrum defined for the system. Three stingers were used to excite the Minus K table, and angle rate sensors on the stiff frame were used to measure the input into the panel. Instead, the system could be excited in several, but not all, axes simultaneously.
The data gained from this testing served to validate the LOS jitter finite element model (FEM) through direct laser pointing comparisons. The initial step in validating the model involved building a FEM representative of the hardware to be tested, as well as the excitation method. Then the modal frequencies and mode shapes from the test were matched to the FEM.
The initial attempts at model correlation displayed a 25 % difference in frequencies from test to FEM predictions at the modes of interest. A 5 % difference is considered ideal. The stiffness of the bolt was altered until FEM predictions from the configurations landed within the desired 5 % of measured values.
Further simplification of the models verified that the modal frequencies matched and that the mode shapes aligned with the test data. Even after several modifications, the LOS jitter FEM was not correlating well with the test data.
The LOS Jitter FEM finally agreed with the test data after the mass of the fixturing (stiff frame, Minus K table, etc.) was included. In addition, the model needed to be loaded with the identical 3-point force excitation used in the test. With the inclusion of these elements, the first four modes of the LOS Jitter FEM displayed a correlation within 5 % of the test data.
What’s in the Moon’s Exosphere?
For decades, scientists have speculated about the presence of neon in the Moon's atmosphere. The LADEE Neutral Mass Spectrometer (NMS) instrument confirmed that the Moon's exosphere is primarily composed of helium, argon, and neon, with their relative abundances varying depending on the lunar time of day.
While most of the exosphere originates from the solar wind, the NMS also revealed that some gases come directly from lunar rocks. For example, argon-40 is produced by the decay of radioactive potassium-40, a naturally occurring element found in the rocks of all terrestrial planets, dating back to their formation.
The NMS also discovered an unexpected source of helium in the lunar exosphere. The moon itself creates approximately 20 % of the helium, likely originating from radioactive thorium and uranium decay, substances found in lunar rocks.
The atmosphere of the moon is very thin, making it easier for rocket exhaust and outgassing from spacecraft to change its composition. It is crucial to study the lunar exosphere before sustained human exploration substantially alters it.
End of LADEE’s Mission
The LADEE spacecraft impacted the Moon approximately 0.5 miles from the eastern rim of the larger Sundman V crater. This was just 0.2 miles north of the location mission team members had predicted based on precise tracking data.
The final impact occurred on April 18, 2014. At the time of impact, LADEE was traveling at a speed of 3600 mph. A photo of the crater created by the impact was taken by NASA’s Lunar Reconnaissance Orbiter.
The image was created by a ratio of two images, one taken before the impact and one after. The areas of brightness in the image highlight the changes between the two images, particularly the impact point and the ejecta.
While the mission was relatively short, LADEE accomplished a lot and has created a foundation for quicker communication between space and the Earth.
LADEE Impact Site. Image Credit: NASA/Goddard/Arizona State University
This information has been sourced, reviewed and adapted from materials provided by Minus K Technology.
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