v1.3.0 / chapter 3 of 19 / 01 apr 10 / greg goebel / public domain
* In the 1990s, interest in Moon exploration revived, though activities were low-key compared to the frantic efforts of the 1960s and early 1970s. Now new spacecraft are being sent to improve our knowledge of this nearby world.
* The first spacecraft sent to the Moon following the long pause from the launch of Luna 24 in 1975 was the US "Clementine" lunar polar orbiter, sent to the Moon in 1994. Clementine was actually a NASA / Department of Defense collaboration. The US military had been working on a ballistic-missile defense technology development program known as the "Strategic Defense Initiative (SDI)" during much of the 1980s, and in 1990 NASA suggested building and launching a low-cost lunar probe as a means to testing SDI sensor technologies.
The military found the idea attractive, since such a mission would be a very thorough test exercise, and would be cheaper than launching a sensor spacecraft and then shooting target spacecraft towards it. The program was formally initiated in 1992, with the US Naval Research Laboratory performing the actual development with NASA support and backing.
Clementine was a stepping stone towards further US space missions with limited budgets and short schedules. Total mission cost was fixed at $150 million USD, and the development took only two years. The launch mass of the probe was 1,690 kilograms, though most of that was the solid-rocket stage needed for lunar orbit insertion. The fully fueled mass of the probe itself was 458 kilograms, about half of that mass being fuel. The probe was three-axis stabilized, and powered by a solar array with battery backup. Spacecraft systems were controlled by two processors, and the probe carried a 2-gigabit solid state data recorder.
Clementine's instrument payload was developed by the US Lawrence Livermore National Laboratory, and included:
The probe's cameras were able to provide complete coverage of the Moon in 11 narrow-wavelength bands, from the near infrared through the visible and into the ultraviolet regions of the spectrum. The bands were selected to provide information on the composition of the Moon's surface. Common minerals found on the Moon can be identified by their "color" in the visible and infrared parts of the spectrum. In particular, the major silicate minerals are recognized from their absorption of particular near-infrared wavelengths reflected from sunlight.
The probe was launched on a surplus Titan IIG ballistic missile on 25 January 1994, and entered lunar orbit on 19 February. The spacecraft's lowest approach to the lunar surface during its orbit was 425 kilometers. This gave a best resolution of about 100 meters per pixel in the visible and ultraviolet ranges, and about 150 meters in the near-infrared range. The LIDAR camera / ranging system could obtain images with a resolution of about 10 meters per pixel, and altitude measurements with a precision of plus or minus 40 meters. The probe left lunar orbit on 7 May 1994 for a planned flyby of the asteroid 1620 Geographos on 31 August 1994, but a software glitch caused an attitude-control thruster to be jammed ON, and the probe went into an unrecoverable tumble.
* Despite the unfortunate end of Clementine, the probe did return excellent data during its lunar observations. NASA followed up by the mission with a second lunar polar orbiter, the "Lunar Prospector", one of the agency's "Discovery" series of low-cost, fast-track science spacecraft. Project costs, including launch, were only $63 million USD, and development took only 22 months.
Lunar Prospector was essentially a follow-up to lunar mapping surveys performed by the Apollo missions. In the late 1980s, a group of space enthusiasts tried to promote the launch of a privately-funded lunar orbiter that would map the mineral composition of the lunar surface using a spare gamma-ray spectrometer left over from the Apollo missions. One of the more interesting schemes considered was launching such a probe as a "getaway special" payload on board the space shuttle, then boosting the probe on a slow journey to the Moon using a solar-electric engine.
NASA's approval of the mission led to a more conventional approach. The Lunar Prospector is a simple spacecraft, based on the LM-100 satellite bus Lockheed Martin had developed as a common basis for science spacecraft. The probe was a spin-stabilized drum 1.2 meters in diameter and 1.4 meters high, weighing 300 kilograms, with 45% of that mass in the form of fuel. It had no camera or on-board computer, though it had a hardwired digital controller that could interpret 60 control commands. It did not have any on-board processing capability, storing data in a solid-state data recorder for relay back to Earth. The spacecraft carried five instruments, including:
The spacecraft's radio system was also used as an element in a "radio science" experiment, with the Earth receiving station carefully observing changes in the signal frequency to track small shifts in the spacecraft's orbit, allowing the creation of a gravitational map.
The gamma ray spectrometer was the primary instrument. It could observe gamma rays emitted by the decay of radioactive elements, as well as pick up the gamma-ray signatures of iron, titanium, silicon, oxygen, calcium, and magnesium. These elements do not emit gamma rays naturally, instead producing them in response to excitation by cosmic rays, high-energy particles that originate in deep space.
* Lunar Prospector was launched on 7 January 1998 by a Lockheed Martin Athena 2 solid-fuel booster. The probe was put into lunar polar orbit at an altitude of 100 kilometers.
A particular item in the agenda of both Clementine and Lunar Prospector was the search for lunar polar ice deposits. Radar experiments performed by Clementine showed that there were highly reflective patches at the poles, hinting at such deposits.
Lunar Prospector's neutron spectrometer could pick up neutrons kicked up from surface material by cosmic-ray impacts. These neutrons could pass through concentrations of heavy mineral atoms without losing much energy, but they lost energy in collisions with hydrogen atoms that would be found in water and other ices. Lunar Prospector did in fact pick up bursts of such "slow" neutrons over the Moon's poles. The data indicated that there is about twice as much water ice at the north pole of the Moon as the south. The exact quantity of the ice remains somewhat uncertain. Radar observations of the poles performed later by the giant radio telescope at Arecibo in Puerto Rico suggest that the ice may be more in the form of permafrost than a thick sheet under a layer of moondust.
The formal mission lasted a year, and then probe was lowered to an orbital altitude of 30 kilometers. On 31 July 1999, it was sent into the Moon's surface near the south pole, while Earth-based telescopes observed for any evidence of water thrown up by the crash. No water was observed in the impact debris.
* Moon missions went quiet for a while after that, until October 2003, when the European Space Agency (ESA) launched a lunar orbiter, the first in the "Small Missions for Advanced Research In Technology (SMART)" series of technology-demonstration spacecraft.
"SMART-1" was mainly intended as a testbed for a solar-electric xenon-ion propulsion system to be used on the ESA "Bepi Colombo" Mercury probe, discussed later, with the engine driven by twin solar panels. However, the probe also carried a 15 kilogram suite of experimental instruments:
The payload also included a communications system designated "KATE" to support spacecraft telemetry and command uplink in the X and Ka communications bands. A radio-science experiment named "RSIS" was used to help track the performance of the xenon-ion engine, and to perform lunar gravitational mapping observations. SMART-1 was developed by the Swedish Space Corporation, with elements from 30 contractors in eleven European nations and the US.
SMART-1 was put into space as a secondary payload on an Ariane 5 booster on 27 September 2003. It had a launch mass of 367 kilograms, including 82 kilograms of xenon fuel, and featured twin solar arrays spanning 14 meters. It was placed in a high elliptical orbit, and the probe used its throttleable Hall-effect xenon-ion engine to spiral itself outward until it was captured by the Moon on 15 November 2004, after 332 gradually expanding orbits around the Earth. It then placed itself into a polar orbit that ranged from 3,000 kilometers over the lunar north pole and 300 kilometers over the lunar south pole.
The engine only produced 0.07 N of thrust, equivalent to the weight of a postcard, but it was continuously operational for the better part of the long trip to the Moon. SMART-1 finally ran out of xenon fuel in the summer of 2005. The probe impacted the Moon on 3 September 2006, with the flash of impact observed by an Earth-based telescope.
* Interest in Moon missions is currently strong around the world. On 14 September 2007, the "Japan Aerospace Exploration Agency (JAXA)" launched the "Selenological & Engineering Explorer (SELENE)" lunar orbiter on an H-2A booster. The probe was named after the Greek lunar goddess; It was renamed "Kaguya", the "Moon Princess", after a character in an old Japanese folktale once it got through space checkout.
Kaguya had a launch mass of 2,885 kilograms (6,360 pounds), with a payload including a high resolution camera; a stereo camera system; a radar sounder capable of penetrating the lunar surface to a depth of 50 kilometers (31 miles); a laser altimeter; a magnetometer; plus particle and fields detectors. The orbiter also carried two small auxiliary satellites, named "Okina" and "Ouna", again after characters in Japanese folklore. Both were eight-sided spacecraft with a mass of 50 kilograms (110 pounds), and were intended to support gravitational studies, as well as radio measurements of the Moon's tenuous atmosphere.
SELENE went into a highly elliptical Moon orbit on 3 October 2007 and was gradually put into a 100 kilometer (60 mile) high circular polar orbit. The two auxiliary satellites were released during different phases of orbital reduction. The mission was planned to last ten months but it was extended, with the spacecraft finally crashing into the Moon on 10 June 2009, returning striking imagery right up to the time of impact.
JAXA had planned another Moon mission, "Lunar-A", which was a lunar orbiter that was to drop two dartlike "surface penetrators" into the Moon's surface and then relay data from the penetrators back to Earth. However, the mission ran into technical and financial difficulties, and was finally killed off in 2007.
* The launch of SELENE was quickly followed by the flight of "Chang'e 1", China's first lunar probe -- in fact, China's first deep-space mission. It was sent into space from the Xichang launch center on 24 October 2007 by a Long March 3A booster and arrived safely in lunar orbit 12 days later.
Chang'e 1 was based on the DongFangHong 3 (DFH-3) weather satellite and had a launch mass of 2,350 kilograms (5,180 pounds). The spacecraft carried an instrument suite consisting of a CCD stereo camera, a laser altimeter, an imaging interferometer, a gamma-ray / X-ray spectrometer, a microwave radiometer, a high-energy particle detector, and a solar wind particle detector. It was placed in a 200 kilometer circular polar orbit to perform observations for a year, with the spacecraft obtaining maps and three-dimensional imagery of lunar surface features; analysing the distribution of 14 elements in the lunar surface; measuring the depth of the lunar soil; and monitoring space weather.
The spacecraft was named after a Chinese lunar deity. The Chinese National Space Agency (CNSA) announced plans for the mission in 2003. The ESA collaborated on the mission, providing tracking and mission support services; the joint effort followed Chinese assistance to the earlier ESA SMART-1 lunar orbiter mission.
* India followed China a year later. On 22 October 2008, the Indian Space Research sent the "Chandrayaan 1" lunar orbiter to the Moon on an ISRO Polar Satellite Launch Vehicle (PSLV), launched from Sriharikota Island in the Bay of Bengal. "Chandrayaan" means "Moon Voyager" in Hindi.
The orbiter had a launch mass of 1,380 kilograms (3,042 pounds). It was designed to provide a 3D atlas of the Moon's surface, as well as a map of the distribution of elements and minerals. It carried a payload of 11 instruments, five built by India and the others provided by foreign research organizations -- three by the ESA, two by NASA, and one by Bulgaria. The Indian-built instruments included a panchromatic camera, a hyperspectral imager, a laser altimeter, spectrometers, particle detectors, a miniature synthetic aperture radar (MiniSAR) provided by NASA, and an ISRO-built impactor probe.
Contact with Chandrayaan 1 was lost in late August 2009, which was a disappointment since the probe hadn't yet completed half of its operational mission. However, considerable data was returned in the time the spacecraft was operational -- the MiniSAR payload discovered over a half-billion tonnes of water ice deposits near the Moon's north pole -- and the mission was not regarded as a failure.
* NASA was slow to get in on the new Moon boom, but US President George W. Bush, in the course of his reelection campaign in 2003, announced that NASA would pursue a return to manned Moon landings as a stepping stone to a manned expedition to Mars under what was named the "Constellation" program. NASA began work on a new mission, a "Lunar Reconnaissance Orbiter (LRO)", to map the Moon in unprecedented detail to lay groundwork for new manned missions.
LRO was launched by a Atlas V booster on 18 June 2009, to be placed in a 50 kilometer high polar orbit for one year of observations. LRO had a launch mass of 1,916 kilograms and its payload consisted of:
The LRO mission was originally to use a medium booster, but NASA decided to use a larger Atlas V -- the 410 model, without any solid rocket boosters -- as insurance against any weight increases in the spacecraft. The bigger booster also permitted addition of a "crash lander" probe named the "Lunar Crater Observation & Sensing Satellite (LCROSS)".
The Centaur upper stage of the launch vehicle was directed to impact on the Moon's south pole on 9 October, with the LCROSS "Shepherding Spacecraft (S-S/C)" trailing behind. The impact of the Centaur upper stage, at a velocity of about 2.5 kilometers per second, released a plume of material and left behind a crater tens of meters across and several meters deep. The 901 kilogram S-S/C performed observations with a camera and spectrometers, relaying data through the LRO, before crashing itself, fifteen minutes after the first impact.
LCROSS carried an instrument payload of four infrared cameras, one visible-light camera, three spectrometers, and a photometer to observe the impact. LRO performed follow-up observations of the crater left by the upper stage. There was some disappointment that the impact of the Centaur stage didn't produce any activity that was visible from Earth even through the biggest telescopes, but NASA mission scientists were still pleased with the "take" from the experiment.
* However, the notion that LRO was paving the way for a manned return to the Moon did not long outlive the launch of the spacecraft. The Constellation program proved, like many NASA manned space programs, badly underfunded, with no progress made on hardware for a Moon landing. In early 2010, the Obama Administration recognized reality and canceled the Constellation program, with the US manned space program to be carefully rethought. However, the agency is still committed to robot missions.
* More lunar missions are being planned. A Chinese "Chang'e 2" flight is in the works, with the new orbiter to feature a different instrument suite and mission profile. Photographs of a lunar rover, with six wheels and a strong similarity to NASA Spirit & Opportunity Mars rovers (discussed later), were released in late 2007, with plans announced to perform the mission in the 2015:2017 timeframe. The rover design was solar-powered, but it was to be delivered to the Moon's surface on a lander with an RTG power source. The lander was planned to demonstrate technologies for a follow-on lunar sample-return mission.
ISRO is putting together a follow-up "Chandrayaan 2" mission, for launch no earlier than 2011. As currently envisioned, it will carry a Russian-built lander and rover. A "Chandrayaan 3" lunar sample-return mission is under consideration as the step beyond that, for launch no earlier than 2015. Work on Chandrayaan 3 won't get rolling until Chandrayaan 2 is launched.
The Russians have also considered new lunar missions of their own, focusing on a Moon shot named "Lunar-Glob". It will involve a Moon orbiter that will drop a network of ten "high speed penetrators (HSPs)", two "penetrator-landers (PLs)", and a single soft lander or "polar station (PS)" onto the Moon's surface. One of the PLs will be dropped on the old American Apollo 11 manned Moon landing site, while the other will be dropped on the Apollo 12 landing site. The PS, as its name implies, will set down on the Moon's south pole to look for water ice deposits.
The ten HSPs will each carry a seismometer and telemetry systems as a payload. They will be carried in a "cassette" that will be deployed from the orbiter as it approaches the Moon, with the cassette dropping the HSPs into two ring-shaped patterns in the Moon's Sea of Fertility. The two PLs will then be deployed from the orbiter; they will have a more sophisticated payload and will also have a retrorocket system to partly brake their descent, apparently to prevent damage to some of the instrument payloads.
Once the penetrators are deployed, the orbiter will go into polar lunar orbit, to then drop the PS soft-lander into a crater in the Moon's south polar region. The PS will land using a retrorocket / airbag system, and will carry a payload consisting of a mass spectrometer, neutron spectrometer, and seismograph. The orbiter will relay data from the thirteen landers; it is unclear if it will have any instrument payload of its own. Launch of Lunar-Glob is scheduled for 2009. The Russians are thinking about an improved generation of lunar rovers over the longer term, and have been discussing joint lunar exploration projects with the Chinese.
NASA JPL is working on another Discovery mission to the Moon, named "Gravity Recovery And Interior Laboratory (GRAIL)". It is more or less a follow-on to an Earth satellite system, the "Gravity Recovery And Climate Experiment (GRACE)", launched in 2002. GRACE consisted of two satellites, one following the other in the same orbit, connected by a precision microwave link; measurements of the variations in the flight path of the two spacecraft provided data for construction of an extremely precise gravitational map of the Earth, mostly to track the shifting of the Earth's oceans. GRAIL will similarly use a pair of spacecraft to obtain a precise gravity map of the Moon. Launch is expected in 2011.
NASA's Ames Center in California is working on a Moon orbiter, the "Lunar Atmosphere Dust Environment Explorer (LADEE)", a small spacecraft that will orbit low over the lunar surface to measure trace gases and dust. It will the first NASA spacecraft built using the "Modular Common Bus" scheme, in which a set of elements is being defined for assembling spacecraft for a variety of missions. Launch of LADEE is currently set for 2012.
Linkages between the different national lunar efforts have been generally informal, but in 2008 discussions began between NASA, the ESA, and seven other national space agencies on an "International Lunar Network (ILN)" to promote coordination and cooperation between all the parties interested in exploring the Moon. The immediate vision is the establishment of a network of surface stations across the lunar surface.
* A number of startups have attempted to promote commercial missions to the Moon, funded by corporate and private sponsors and sales of imagery. One company, Transorbital, did manage to launch a dummy spacecraft of their "Trailblazer" Moon orbiter on a Russian Dnepr booster in 2002, but the effort went into limbo after that. All such efforts have been plagued by funding difficulties; however, search engine giant Google and the X-Prize Foundation, which sponsors commercial space competitions, are now offering a $20 million USD prize to the first private company to successfully send a rover to the Moon by the end of 2012. Although there has been considerable skepticism that a Moon rover mission could be conducted for $20 million USD, several groups are already working on the competition.
* Lunar statistics:
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mean distance from primary 384,500 km (center to center)
orbital period (sidereal) 27.32 days
orbital eccentricity 0.055
orbital inclination 18-29 degrees
equatorial diameter 3,476 km
mass 734.9 x 10^20 kg (0.012 Earth)
mean density (relative to water) 3.34
albedo 0.11
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* In almost every source with any detail on the Moon, there's a discussion of
why it looks bigger closer to the horizon than it does directly overhead. I
didn't include that discussion here for the simple reason that I've never
noticed that it does, even when I've tried. It certainly looks more vivid
closer to the horizon, but it never occurred to me to think that it looked
bigger.
As another footnote, in 2002 astronomers spotted what appeared to be a tiny "moonlet" of the Earth, at a distance about twice that of the Moon proper. The object, catalogued as J002E3, was later determined to be spacecraft debris, apparently the third stage of the Saturn V booster that launched the Apollo 12 mission in 1969. It was in orbit around the Sun, but it was finally captured by the Earth's gravity. The orbit is highly elliptical, and the object is expected to either crash into the Moon, or fall into the Earth's atmosphere and burn up.
