The landing of Chang’e 3 on the Moon on December 14, 2013 rightfully received a lot of attention not only because it was China’s first landing on the Moon, but because it was the first landing on the Moon by anyone in over 37 years. In addition to the Yutu rover, Chang’e 3 also carried a modest suite of instruments including the EUV (Extreme Ultraviolet camera) and LUT (Lunar-based Ultraviolet Telescope) designed to make astronomical observations from the lunar surface. In addition to sparking fear of Chinese technical advancements in some, this mission has also rekindled interest in others of using the Moon as a base for astronomical observation.
The EUV is designed to make observations of Earth’s plasmasphere at a wavelength of 30.4 nm (corresponding to emissions from He+ ions) to see how it is affected by solar activity. The LUT is a 150 mm Ritchey-Chretien telescope designed to observe astronomical sources at a wavelength range of 245 to 340 nm. These telescopes have been advertised as being the first long-term observatory to be deployed on the Moon. Of course maybe this depends on how one defines “long term” since there have already been instruments that have been deployed on the lunar surface, as well as in orbit, during the early 1970s specifically tasked with making astronomical observations (as opposed to lunar-oriented experiments that by accident or design also made measurements of an astronomical nature). These included missions that acquired data over months from the lunar surface and over years from orbit.
The first astronomical “observatory” on the lunar surface was also the first extraterrestrial rover. The 756-kilogram Lunokhod 1 was built by NPO Lavochkin who had been responsible for the development and construction of the Soviet Union’s increasingly successful lunar and planetary probes since 1965. Lunokhod 1, launched using the Proton-D on November 10, 1970, was delivered to Mare Imbrium by Luna 17 on November 17 after two days of maneuvering in low lunar orbit. In addition to various cameras and other instruments meant to study the lunar surface during its nominal three-month roving mission, Lunokhod 1 also carried an astronomical instrument: an x-ray telescope called RT-1 (Roentgen Telescope-1 where “roentgen rays”, named after German physicist Wilhelm Röntgen, are Russian for “x-rays”). Since the Moon does not possess an atmosphere, it is a perfect site to perform x-ray astronomy. Part of the rationale for including an x-ray instrument like the RT-1 on Lunokhod was to assess the ability to use the Moon as an observatory during future missions.
Like many early x-ray telescopes, the RT-1 did not use difficult-to-manufacture grazing incidence optics to form images. Instead it was essentially a single-pixel x-ray photon counter whose field of view was restricted to about 3° by use of a metallic grid in front of the detector called a collimator. The RT-1 had two detectors each with an effective area of 6.5 square centimeters with one employing a filter to attenuate the x-ray flux, especially at lower energies, and another without a filter. The unfiltered channel was sensitive to x-rays in approximately the 0.2 to 0.8 nm wavelength range (equivalent to “soft” x-rays with energies in the 1.5 to 6 keV range). The RT-1 incorporated a second detector that was used to measure background radiation and remove its effects from the x-ray detections.
The RT-1 was mounted on Lunokhod 1 so that it would point vertically towards zenith. A typical “observation” used an integration time of six hours which could be made even during times when the rover was out of contact with the Earth. Using the slow 0.5 degree per hour rotation rate of the Moon, the RT-1 could make a 3°-wide swath of x-ray measurements in 3° steps across the sky during the course of a lunar day.
The RT-1 made observations during at least three different lunar days creating a narrow swath of measurements across about half the sky. The swath included the Cygnus Loop, the galactic equator and several astronomical x-ray sources with measured fluxes as great at 0.8 photons/second/cm2. Lunokhod 1 was last heard from on September 14, 1971 after covering 10.5 kilometers over 11 lunar days of operation far exceeding its design goal. Mission operations officially ceased on October 4 which was the 14th anniversary of the launching of Sputnik 1. With several months of published astronomical observations made by the RT-1, I believe Lunokhod 1 beat Chang’e 3 for the title of “first long-term observatory to be deployed on the Moon” by 43 years.
The next attempt to use the Moon as an astronomical observatory site was during the American Apollo 16 manned lunar mission launched on April 16, 1972. Among the experiments performed by astronauts John Young and Charles Duke during their three days on the lunar surface was an experiment officially designated the S201 Far Ultraviolet Camera/Spectrograph. While this experiment was primarily astronomical, all readily admit that with only three days on the lunar surface, it does not qualify as “long term”. Built by the Naval Research Laboratory, the S201 was meant to image ten specific star fields with a diameter of 20° each at two different far ultraviolet wavelengths covering about 7% of the sky. Since the far ultraviolet is absorbed by the Earth’s atmosphere, astronomical observations must be performed in space. Unfortunately, the sensitivity of past surveys such as those performed by NASA’s Orbiting Astronomical Observatory 2 (OAO 2) launched in 1968 were limited in part by Lyman-α emissions at 121.6 nm from the hydrogen in Earth’s extended geocorona as well as atomic oxygen emissions at 130 nm. The airless Moon, which lies well outside the geocorona, is a perfect site to perform a sensitive survey in this important wavelength range.
The S201 Far Ultraviolet Camera was a film-based electrographic Schmidt camera. The first optical element of the S201 camera was the corrector plate at the entrance aperture designed to help minimize the optical distortions in the wide field of view images. This 75-mm corrector plate also served as a spectral filter with one corrector composed of lithium fluoride being transparent to wavelengths in the 105 to 160 nm range and the other composed of calcium fluoride meant for the 125 to 160 nm range that excluded Lyman-α emissions. After passing through the corrector plate, ultraviolet light reflected off of the primary mirror and formed an optical image on a photocathode composed of potassium bromide. The electrons emitted by this image on the photocathode were accelerated by an electrical potential of 25 kilovolts and focused by a magnet to form an electron image on 35 mm photographic film. These images produced by the S201 had a resolution of 3 arc minutes and could detect objects with a stellar magnitude at 140 nm of 11 with a 30-minute exposure (the equivalent of a hot O or B-type star with a visual magnitude of 16). Mounted beneath the camera was a separate ultraviolet spectrograph employing an objective grating and sensitive to the 50 to 155 nm wavelength range.
The S201 experiment was delivered to lunar surface by the Apollo 16 LM Orion on April 21, 1972. Deployment of the S201 by astronaut John Young started less than an hour into the first of three EVAs. With the camera placed in the lunar module’s shadow and its battery pack in the sunlight to maintain proper temperature, the S201 was periodically repointed manually by the astronauts over the course of the mission’s three EVAs towards various areas of interest in the lunar sky including towards the Earth to get the first clear images of the geocorona. The S201 operated normally and near the end of the last EVA, its film cassette was retrieved to return to Earth for analysis by astronomers. This unique set of far ultraviolet images would be the subject of numerous astronomical papers for over a dozen years to come.
The Soviet Union’s second lunar rover, Lunokhod 2, was launched on January 8, 1973 and was delivered to Lemonnier Crater by Luna 21 a week later. The 840-kilogram Lunokhod 2 sported a number of improvements based on the experience with its successful predecessor. Like Lunokhod 1, the new Soviet rover also carried a telescope to study solar and galactic x-rays that beat Chang’e 3 to the claim of “the first long-term observatory to be deployed on the Moon” by almost 41 years. But probably more important to those interested in using the lunar surface as a base for future astronomical observatories, Lunokhod 2 also carried an important instrument that can be argued was non-astronomical.
Designated the AF-3L astrophotometer, this device was designed to make measurements of the lunar sky’s brightness at wavelengths centered at 544 nm in the yellow part of the visible spectrum roughly equivalent to astronomers’ V band and 268 nm in the mid-ultraviolet. Baffles and a series of optical stops were used to minimize the amount of scattered light and restrict the field of view of the photomultiplier detectors to 12.5° in the visible and 17.4° in the UV. Astrophotometers of similar design were flown in Earth orbit on Kosmos 51 launched in 1964 and Kosmos 213 launched in 1968.
Between January 16 and March 20, 1973, the astrophotometer on Lunokhod 2 made a total of a dozen measurements of the lunar sky brightness: nine during the lunar day, two at night and one during “twilight” when the Sun was one degree below the local horizon. During the lunar day, the measurements in the visible band were off the scale because the sky was so much brighter than anticipated. Only during the single twilight measurement on January 24 produced a strong, on-scale reading in the visible band that was the equivalent of ten times larger than the brightest part of a dark sky observed from Earth. The sky brightness in the ultraviolet produced seven on-scale measurements with the sky brightness being 2 to 16 times higher than expected from stars and zodiacal light alone. The two nighttime measurements, made difficult by the cold lunar temperatures, were consistent with measurements made by Kosmos 51 and 213 indicating only astronomical sources of light.
The conclusion from these observations was that sunlight scattering from dust suspended above the lunar surface was making the sky up to 13 to 15 times brighter than the Earth’s nighttime sky with a full Moon present. This glow would certainly hinder daytime observations in the visible and UV from the lunar surface casting some doubt on the idyllic view of using the Moon as an observatory site at these wavelengths. Data from the NASA’s LADEE (Lunar Atmosphere and Dust Environment Explorer) mission completed in April 2014 should be able to shed more light on the role of lunar dust in brightening the lunar sky.
Although the last published astrophotometer measurement was made in March 1973, Lunokhod 2 continued its mission until May 11 when contact was lost after the rover overheated because of dust covering its radiator from a driving incident two day earlier. During its almost four months of operation on the lunar surface, Lunokhod 2 had covered 39 kilometers which stood as an extraterrestrial roving record until NASA’s Opportunity Mars rover surpassed it on July 27, 2014. While a further improved “Lunokhod 3” (presumably with more astronomy payloads riding piggyback) had been built for launch in 1977, funding for the Soviet Luna program was cut and, after the last lunar sample return mission of Luna 24 in August 1976, eventually cancelled.
No discussion of lunar observatories from the 1970s would be complete without Radio Astronomy Explorer 2 (also known as Explorer 49 or simply RAE-2) which was the first dedicated astronomy mission sent to the Moon (albeit in orbit). In addition to high energy ultraviolet and x-ray radiation, the Earth’s atmosphere can also block electromagnetic radiation with much longer wavelengths of interests to astronomers. Among these are radio waves with frequencies less than about 27 MHz (equivalent to wavelengths longer than about 11 meters) which tend to reflect off of Earth’s ionosphere. As in other areas of astronomy, the only way to detect signals at these frequencies is from space which is precisely what NASA did with RAE-2.
The predecessor of RAE-2, RAE-1 or Explorer 38 launched in 1968, attempted to monitor radio emissions in the 200 kHz to 9.18 MHz frequency range from a 5,800 kilometer high Earth orbit. While it did return useful astronomical data, half of its data were rendered unusable because of strong terrestrial emissions caused by the interaction of various charged particles with our magnetic field, thunderstorms and even man-made sources. To get around this problem, RAE-2 was placed into lunar orbit. In addition to being farther away from terrestrial interference, the Moon would periodically block terrestrial signals allowing more distant radio sources to be detected. The occultation of astronomical radio sources by the Moon would also allow the position of these sources to be determined much more accurately than could be done with the simple dipole antenna arrangement used by the RAE satellites.
RAE-2 was a squat cylinder with a diameter of 92 centimeters and a height of 79 centimeters. A total of 38.5 watts of power were supplied by four curved solar panels fixed to the exterior that fed rechargeable nickel-cadmium batteries. Lunar orbit insertion was performed using a 128-kilogram solid rocket motor that was jettisoned after firing. RAE-2 had a mass of 200 kilograms once in lunar orbit and was normally gravity gradient stabilized so that one end always faced towards the Moon. Hydrazine thrusters were used for trajectory corrections and attitude adjustments.
RAE-2 had a set of receivers that scanned frequencies from 25 kHz to 13.1 MHz in 32 discrete steps over 144 seconds. Two pairs of V-shaped antennas, one pair on the top of RAE and another on the bottom, were made of beryllium-copper alloy strips 5 centimeters wide and would be unreeled once in lunar orbit to a maximum length of 229 meters. A second pair of 37-meter dipole antennas on the satellite’s mid-section were aligned parallel with the lunar surface. A 129-meter long boron boom was used to damp out librations from the stable gravity-gradient orientation. RAE-2 could transmit its data live or record it on a pair of tape recorders when it was out of view on the farside of the Moon for subsequent transmission.
RAE-2 was launched on a Delta 1913 (number 95) on June 10, 1973 into a direct-ascent trajectory towards the Moon. RAE-2 performed a single course correction the day after launch and entered lunar orbit after a flight of 113 hours. Subsequent firings of the hydrazine thrusters placed RAE-2 into a 1,053-by-1,063-kilometer orbit with a period of 221 minutes and an inclination of 58.7° – one of the “magic” inclinations that minimizes growth in orbital eccentricity due to the Moon’s lumpy gravitational field.
During the first three weeks of operation which began on June 20, RAE-2 was spin-stabilized with its spin axis parallel to the ecliptic and a spin rate of 4 RPM. During this time, only its 37-meter dipole antennas were deployed to monitor solar radio bursts. The 37-meter dipoles were then reeled back in and the spacecraft used its thrusters to stop the spin of the spacecraft and reorient it for gravity-gradient stabilized operation. At this point, the 37-meter dipoles were redeployed along with the other two pairs of long dipoles and the stabilizing boom. Initially the long dipole antennas only reeled out to a length of 183 meters and were not extended to their full length until November 1974 but this minor setback did not affect the mission.
For the next two years, RAE-2 sent back data on a range of astronomical targets as part of an ongoing campaign to map the sky at long radio wavelengths. As expected, Earth was readily detected in the RAE-2 data especially during full Moons when Earth’s noisier night side was turned towards the Moon. RAE-2 successfully completed its mission in June 1975 and continued its transmissions until they were terminated in August 1977. In addition to returning useful radio data on the Sun, planets and astronomical sources beyond, RAE-2 proved the utility of using the Moon as a site for performing radio astronomy well away from the natural and artificial radio noise of the Earth. RAE-2 would also prove to be the last spacecraft to operate in lunar orbit until the Japanese Hiten mission reached the Moon in October 1991.
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“Vintage Micro: The Original Gamma Ray Observatory”, Drew Ex Machina, June 13, 2014 [Post]
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