v1.2.3 / chapter 4 of 19 / 01 jan 09 / greg goebel / public domain
* Mercury, the closest planet to the Sun, was only visited once in the 20th century and even then only partially mapped, despite the fact that it is relatively near to the Earth. This neglect was partly due to the fact that at first sight Mercury appears to be an uninteresting ball of rock, not too different from our Moon. However, on closer inspection Mercury proves to have a number of unique features that make it a more interesting target for new missions, and in the 21st century new missions are in progress or development. This chapter provides a history of the exploration of Mercury.
* The planet Mercury is one of the four planets known since ancient times, since it can be seen by eye at various times of the year, low over the horizon before sunrise and after sunset. Its rapid motion through the sky from night to night led the ancient Greeks to name it after the god Mercury, the "fleet-footed" messenger of the gods.
Up to the beginning of the Space Age, little was known about Mercury. As far as any telescope could see, it was a ball of rock with a diameter of 4,878 kilometers, or 38% the diameter of the Earth and 1.4 times the diameter of the Earth's Moon. Mercury's mass, which could be estimated from its orbit around the Sun, was only 5.5% that of the Earth, and its surface gravity was only 38% that of the Earth. It had neither moons of its own nor any atmosphere worth speaking of.
Mercury turns out to have an average density of 5.44 grams per cubic centimeter, which is greater than that of any major object in the Solar System except the Earth, which has an average density of 5.52 grams per cubic centimeter. However, Earth's internal density is increased by the compression of its larger mass. If this compression did not occur, the Earth's average density would 4.4 grams per cubic centimeter, while Mercury's would be 5.3 grams per cubic centimeter. What this implies is that Mercury has a very large iron core relative to its size, about 75% of the planet's diameter and 42% of its volume. In contrast, the Earth's iron core is only 54% of its diameter and 16% of its volume.
* Mercury is the closest planet to the Sun, performing an orbit once every 88 days. Its orbit has an average diameter of 57.9 million kilometers, or 0.39 AU. However, this orbit is unusually eccentric, with an eccentricity of 0.206, and the distance between the Sun and Mercury varies from 0.31 to 0.47 AU. The planet also has an orbital inclination of 7 degrees. The orbit of Mercury is more eccentric and has a higher inclination than the orbit of any other planet.
Mercury's elliptical orbit slowly "precesses" around the Sun at a rate of about 1.56 degrees of arc per century. This puzzled astronomers for a long time, since analyses of its orbit using Newtonian mechanics that took into account all known influences on the planet showed that rate of precession should be 43 arc-seconds per century slower. One explanation that was proposed was that there was another planet, named "Vulcan", which was even closer to the Sun and very hard to detect. Some observers claimed they spotted Vulcan just after sunset or before sunrise, but the observations were never confirmed.
In 1915, Albert Einstein published his General Theory of Relativity, which can be described as "relativistic" theory of gravity. General Relativity gives different results than Newton's theory of gravity, with the differences between the two theories becoming more significant as the gravitational forces become more extreme, and a new analysis of Mercury's orbit using General Relativity was able to account for the observed precession of the planet.
* Telescopic observations of Mercury are very difficult, since Mercury never moves farther away from the Sun in the Earth's sky than 27 degrees. This means observations always have to be made at low inclination, with the image passed through an extended thickness of atmosphere that distorts it badly.
Astronomers had concluded from their observations that Mercury was "tidally locked" to the Sun, meaning the length of its day was exactly the same as the length of its year, or 88 Earth days, and it always kept one side facing towards the Sun, to be eternally seared, and the other side facing away from the Sun, to be eternally frozen. This made for an exotic environment for science-fiction novels, but it turned out to be a half-truth, or more precisely a two-thirds truth.
In 1962, radio astronomers measured the thermal radiation emitted by Mercury's "dark side" and concluded that it was too warm to be "eternally frozen in darkness". In 1965, Gordon Pettengill and Rolf Dyce, then of Cornell University, bounced ultra-long-range radar signals off of Mercury, using the Arecibo radio telescope. They discovered that Mercury actually rotates once in a little under 59 Earth days. More precise radar observations performed in the early 1970s pinned down the rotation rate to 58.646 days.
The significance of this value is that Mercury rotates three times for every two orbits around the Sun. This means that it is in a "tidal resonance" with the Sun. To get out of that orbital state would require adding energy, say by a collision with a large asteroid, and so it is trapped in this 3:2 resonance, even though a 1:1 lock would be a lower energy state. It's somewhat analogous to a stone that has fallen into a valley but ended up being trapped on a ledge, and it won't fall to the bottom unless something knocks it off the ledge.
It's not surprising that astronomers thought that Mercury was in a 1:1 lock, since if they made observations on every other orbit they would see the planet in the same orbital position and displaying the same features. Images of the planet were bad to begin with and so discrepancies could be chalked up to mistaken observations. Interestingly, it appears that the mistaken idea that Mercury was tidally locked to the Sun was established in the 1880s by the Italian astronomer Giovanni Schiaparelli. Schiaparelli also did much to establish the presence of "canals" on Mars, which turned out to be another error.
Mercury's rotation rate means that a sunrise-to-sunrise "day", as opposed to the "sidereal" day relative to the stars, lasts two years, or 176 Earth days. The eccentric orbit also has the interesting effect that at perihelion, the closest approach to the Sun when the planet's orbital velocity is the highest, the motion of the Sun across Mercury's sky due to the planet's rotation is more than balanced by the motion of the Sun due to the planet's orbital velocity. This means that, depending on the vantage point, there will be a double sunrise, a double sunset, or the Sun will backtrack across the sky.
* That was all that was known about Mercury until the 1970s. However, in 1973, NASA launched the Mariner 10 probe to perform three flybys of Mercury. Mariner 10 weighed 503 kilograms. Like other Mariner spacecraft sent to Venus and Mars, it was built around a octagonal bus. Unlike Mariners sent to Mars it used two solar panels, not four, since there was plenty of sunlight where Mariner 10 was going. Spacecraft systems were protected by thermal insulation and a sunshade. The payload suite included:
Data was also obtained with a "radio science" experiment that involved careful monitoring of changes in the probe's radio communications beam.
The probe was launched by an Atlas-Centaur booster on 3 November 1972 and placed into a "retrograde" orbit around the Sun, in a direction opposite of that of the orbits of the planets. The probe flew past Venus on 5 February 1974, using the encounter to adjust its orbit toward Mercury. This made it the first planetary probe to use a "gravity assist" trajectory, and performed measurements of Venus as it flew past.
Mariner 10 performed its first flyby on Mercury on 29 March 1974, with a closest approach of 705 kilometers. The probe flew past the planet a second time on 21 September 1974, with a closest approach of 47,000 kilometers. The probe's third and final flyby was on 16 March 1975, with a closest approach of 327 kilometers. The mission was formally ended eight days later.
The dynamics of the probe's orbit meant that all three flybys were over the same side of the planet. Mariner 10 returned 2,700 images, photographing 45% of Mercury's terrain at a resolution of 1.5 kilometer per pixel. A small fraction of the terrain, less than 1%, was imaged at higher resolutions ranging from 100 to 500 meters. The detail and extent of the map obtained by the Mariner probe was comparable to that of maps made of the Moon before the Space Age.
No other detailed images have ever been obtained of the planet. While the Hubble Space Telescope might be able to make detailed maps of most of the planet's surface, the telescope cannot be pointed in the direction of the Sun because its sensors would be ruined. Given the limited resolution of Mariner 10's images and the fact that less than half of Mercury was imaged, much remains to be learned about the planet.
* Most of Mariner 10's images of Mercury present a world that looks very much like the Moon. Both worlds have heavily cratered upland regions, and large and relatively smooth plains that surround and fill impact basins.
Mariner 10 also performed infrared measurements of Mercury that showed its "soil" is a good thermal insulator. Although the surface temperature varies between about 100 to 700 degrees Kelvin, at a meter below the surface the temperature is a constant 350 degrees Kelvin. This is consistent with a surface covered with powdered rock, much like the lunar "regolith", created by endless impacts from space.
However, Mercury differs from the Moon in many ways. Mercury's cratered regions are separated by widespread rolling plains, and in fact the plains are the planet's dominant surface features. Mercury also seems to be crisscrossed by a network of "thrust faults" or "lobate scarfs" known as the "Mercurian Grid", caused by the planet's shrinkage as it cooled; no such network is seen on the Moon. Another difference is that Mercury is significantly more reflective than the Moon. This implies a different surface composition, with much less iron and titanium than the Moon, which seems odd given the planet's large metal core.
The most prominent feature observed by Mariner 10 was an impact crater 1,300 kilometers in diameter named the "Caloris Basin". It is filled and surrounded by smooth plains that resemble the flat lunar regions known as "maria", but it differs from the maria in that it has a closely-spaced network of ridges and fractures arranged in both concentric and radial patterns, apparently caused by the lowering of the floor of the basin in the impact, and then the raising of the floor due to its rebound.
The Caloris Basin was on the edge of the planet's disk observed by Mariner 10, and so the region opposite to it on the other side of the planet was visible as well. This "antipode" area is covered with a pattern of hills and valleys, apparently created when the seismic shock waves of the impact met up in a focus. Studies of the density of cratering in the Caloris Basin suggest that the impact that formed it occurred about 3.6 billion years ago.
Coarse radar observations of Mercury made after the Mariner 10 mission hint that the unseen half of the planet is similar, with cratered terrain and lobate scarfs.
* One of Mariner 10's most significant discoveries was that Mercury does have a magnetic field, like Earth's but much weaker, though still strong enough to deflect the Solar wind. This implied that the outer layers of the iron core are still molten, which is puzzling since such a small planet cannot retain heat very well and a purely metal core should have solidified completely long ago. Calculations showed that a concentration of 7% sulfur would be adequate to lower the melting point enough to keep the upper layers of the core liquid, and eventually radar observations of variations in Mercury's rotation rate demonstrated that the planet in fact had a molten core.
Mariner 10 did detect a trace atmosphere around Mercury, a trillion times thinner than the Earth's atmosphere. Mercury's atmosphere contains hydrogen, helium, and oxygen, all apparently derived from the solar wind. Later Earth-based measurements also identified small proportions of sodium and potassium, elements that were not detectable by Mariner 10's ultraviolet spectrometer as it could not pick up their spectral lines. The sodium and potassium appear to be derived from Mercury's surface, kicked up by micrometeorite impacts or solar-wind sputtering of the surface.
* Much about Mercury remains mysterious. There is of course the half of the planet not observed by Mariner 10. In addition, current models of the creation of the planets also suggest that it shouldn't have been able to acquire such a large iron core. This implies that a better knowledge of Mercury would lead to a better understanding of the origins of the other planets of the Solar System.
In the 1990s, researchers at the California Institute of Technology performed radar observations of Mercury that hinted at the existence of water ice at the poles. Mercury's spin axis is almost perfectly perpendicular to the plane of its orbit, and this means that volatiles could be trapped in shadowed areas near the poles. Such observations and a revival of planetary science missions in the 1990s led to two new Mercury missions, the NASA "Mercury Surface, Space Environment, Geochemistry, & Ranging (MESSENGER)" mission and the ESA "BepiColombo" mission.
MESSENGER was originally to be launched in May 2004, to go into orbit around
Mercury in July 2009. Various delays forced a schedule slip, and the launch
didn't actually take place until 3 August 2004, when the probe was put into
space by a Delta II 7925H from Cape Canaveral. The delayed launch will
result in almost a two-year slip to arrival at Mercury. The probe will
perform a complicated series of flybys:
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August 2005: first Earth flyby
October 2006: first Venus flyby
June 2007: second Venus flyby
January 2008: first Mercury flyby
October 2008: second Mercury flyby
September 2009: third Mercury flyby
March 2011: insertion into orbit around Mercury
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The first Earth flyby took place on schedule on 2 August 2005, with the
spacecraft's instrument suite obtaining readings and images to provide test
and calibration, as well as some publicity pictures. The first Venus flyby
followed on 24 October 2006, with the second on 5 June 2007. The first
Mercury flyby took place on 14 January 2008, with MESSENGER passing the
planet at a minimum distance of 200 kilometers, taking 1,300 images during
the flyby, covering the hemisphere of Mercury unseen by Mariner 10. The
second flyby took place on 6 October 2008, with the probe taking more images
of the unseen part of the planet.
Following the third flyby, it will then be placed into a highly elliptical orbit, ranging from a maximum of 15,000 kilometers to a minimum of 200 kilometers, with an orbital inclination of 80 degrees from the planet's equator. The orbital mission will last twelve Earth months, equivalent to four Mercurian years, six Mercurian sidereal days, or two Mercurian solar days. The lengthy flight and six gravity assists were required because inserting a probe into Mercury orbit is a difficult problem in orbital mechanics. The probe carried an unusually high proportion of propellant, 55% of launch mass, to allow it to perform the proper course adjustments.
The probe had a launch mass of about 600 kilograms and is built around a graphite-epoxy composite bus, with titanium fuel tanks. It is protected by a sunshade with dimensions of 2.4 x 1.8 meters, composed of layers of Kapton plastic insulation wrapped in Nextel ceramic cloth, as well as heat pipes and radiators to dump heat off the "dark side" of the spacecraft. The solar arrays are covered with a mirror surface that reflects two-thirds of the sunlight to reduce heating. All the instruments are located on a "science deck"; most of the instruments are on fixed mounts, meaning that they have to be pointed by adjusting the orientation of the spacecraft.
The instrument suite weighs a total of only 50 kilograms and includes:
The probe's communications system will also be used for radio science experiments, with subtle shifts in the signal being used to map out Mercury's mass distribution. The communications system was an innovation, including two electronically-steered "phased array" antennas, the first time such a technology was used on a planetary probe. Data will be stored in twin 1-gigabit solid-state data recorders. Mission scientists hope to get the probe down to an altitude of 20 kilometers at the end of the mission, before the spacecraft crashes into the planet.
MESSENGER was built by the Johns Hopkins University's Applied Physics Laboratory in Maryland, which also is performing operational management of the mission.
* The ESA BepiColombo project was approved in 2000 as a "Cornerstone" mission for the agency's "Horizon 2000 Plus" program. BepiColombo was originally planned as a Mercury sample-return mission, but that goal turned out to be too ambitious, and now the plan is to launch a pair of orbiters, including the "Mercury Planet Orbiter (MPO)" and the "Mercury Magnetospheric Orbiter (MMO)", in 2013. A small lander, the "Mercury Surface Element (MSE)", was also included in the original definition, but was eliminated due to funding limitations in late 2003.
The MMO will be provided by the Japanese. The ESA will be responsible for the other orbiter, as well as launch services for all the spacecraft using two Russian Soyuz Fregat boosters. The assembly will use a solar electric Hall-effect xenon-ion propulsion system for interplanetary cruise, with the engine based on that tested on the ESA SMART-1 lunar orbiter. The cruise stage will be discarded on entry into Mercury orbit and deployment of the two orbiters.
The mission is named after Giuseppi "Bepi" Colombo (1920:1984) of the University of Padua in Italy, who had designed the flyby trajectory that took Mariner 10 to Mercury, and made other significant contributions to space technology, including studies of "space tethers".
* Statistics for Mercury:
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mean distance from Sun 0.39 AU (57.9 x 10^6 kilometers)
orbital period (sidereal) 87.97 days
orbital eccentricity 0.206
orbital inclination 7.0 degrees
equatorial diameter 4,878 km (0.38 Earth, 1.4 Moon)
mass (relative to Earth) 0.055
mean density (relative to water) 5.43
gravity (relative to Earth) 0.38
escape speed 4.3 kilometers per second
rotation period 58.65 days
solar day 176 days
oblateness 0
inclination of equator 0.0 degrees
albedo 0.11
max surface temperature 430 degrees Celsius
atmosphere (major constituents ) none
atmospheric pressure at surface negligible
number of known moons none
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