v1.2.2 / chapter 3 of 19 / 01 feb 07 / 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 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 an extension was expected.
JAXA had planned another Moon misson, "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 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. The Chinese are already working on follow-on missions. 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 announed 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.
* The Indian Space Research Organization (ISRO) is also planning to perform a lunar polar orbiting mission, designated "Chandrayan 1" and approved in late 2003, for launch by an ISRO Polar Satellite Launch Vehicle (PSLV) in early 2008. "Chandrayan" means "Moon Voyager" in Hindi. The probe will have a launch weight of 529 kilograms and will observe the Moon for at least two years from an altitude of 100 kilometers. The instrument suite will include:
Chandrayan 1 will also carry ESA and Bulgarian payloads, and will drop a surface impactor with a weight of 20 kilograms to allow the orbiter to observe the dust it kicks up. The orbiter mission is expected to lead to landers, rovers, and sample return missions. ISRO is now building a deep-space communications network near Bangalore to support Chandrayan 1.
* A follow-up "Chandrayan 2" mission is now in planning, for launch no earlier than 2011. As currently envisioned, it will carry a Russian-built lander and rover. 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 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. NASA began planning of a new mission, a "Lunar Reconnaissance Orbiter (LRO)", to map the Moon in unprecedented detail to lay groundwork for new manned missions.
LRO will be launched by a Atlas V booster in 2008, to be placed in a 50 kilometer high polar orbit. Its payload will include:
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 will be directed to impact on the Moon's south pole, 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, will release a huge plume of material, estimated to be as much as a thousand tonnes, leaving behind a crater 30 meters across and almost 5 meters deep. The 534 kilogram S-S/C will perform observations with a camera and spectrometers, relaying data through the LRO, before crashing itself, fifteen minutes after the first impact. LRO will perform follow-up observations of the crater left by the upper stage.
* It is hoped that LRO will arrive in lunar orbit while Chandrayan 1 is still operating so the two spacecraft can perform collaborative observations. NASA is now planning 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.
* Renewed manned Moon landings are not expected to begin until 2018. The current plan is to develop a flight system something like a scaled-up Apollo spacecraft, with a "Orion" crew exploration vehicle (CEV) like the conical Apollo command module, but bigger and more capable, and a "Lunar Surface Access Module (LSAM)" with a configuration like that of the Apollo LM, but also bigger and more capable. The CEV would be launched into orbit with a crew using a "Ares I" crew launch vehicle based on the solid rocket booster now used with the NASA space shuttle, and the LM launched on a big "Ares V" booster derived from the space shuttle itself.
Prolonged stays are planned, with the astronauts making use of lunar resources such as water, as a stepping stone to a manned Mars expedition. NASA is even thinking of buying two small commercial communications satellites and putting them in polar orbit around the Moon to provide communications support for lunar missions. How far such plans actually go remains to be seen.
* 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. 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.
