Astronomy

The following table lists statistical information for the Sun and planets:

* The Sun’s period of rotation at the surface varies from approximately 25
days at the equator to 36 days at the poles. Deep down, below the convective
zone, everything appears to rotate with a period of 27 days.

Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of “space” comprises the interplanetary medium. It includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons — plasma — which stream from the Sun, called the solar wind.

The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 kilometers per second in the vicinity of Earth’s orbit. The point at which the solar wind meets the interstellar medium, which is the “solar” wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun’s influence perhaps 100 AU from the Sun. The space within the boundary of the heliopause, containing the Sun and solar system, is referred to as the heliosphere.

The solar magnetic field extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields.

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%.

The following table is a list of the mass distribution within our Solar System.

• Sun: 99.85%
• Planets: 0.135%
• Comets: 0.01%
• Satellites: 0.00005%
• Minor Planets: 0.0000002%
• Meteoroids: 0.0000001%
• Interplanetary Medium: 0.0000001%

The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth’s. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. The following diagram shows the approximate distance of the terrestrial planets to the Sun.

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter - like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. The following diagram shows the approximate distance of the Jovian planets to the Sun.

Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes: the satellites of the planets (Moons); numerous comets, asteroids, and meteoroids; and the interplanetary medium.

The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun’s nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away.

The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way.

The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the Andromeda Galaxy. It is a spiral galaxy like the Milky Way but is 4 times as massive and is 2 million light years away. Our galaxy, one of billions of galaxies known, is traveling through intergalactic space.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun’s north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic.

Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elciptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune.

The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.

Viking was the culmination of a series of missions to explore the planet Mars; they began in 1964 with Mariner 4, and continued with the Mariner 6 and 7 flybys in 1969, and the Mariner 9 orbital mission in 1971 and 1972.

Viking was designed to orbit Mars and to land and operate on the planet’s surface. Two identical spacecraft, each consisting of a lander and an orbiter, were built.

NASA’s Langley Research Center in Hampton, Virginia, had management responsibility for the Viking project from its inception in 1968 until April 1, 1978, when the Jet Propulsion Laboratory assumed the task. Martin Marietta Aerospace in Denver, Colorado, developed the landers. NASA’s Lewis Research Center in Cleveland, Ohio, had responsibility for the Titan- Centaur launch vehicles. JPL’s initial assignment was development of the orbiters, tracking and data acquisition, and the Mission Control and Computing Center.

NASA launched both spacecraft from Cape Canaveral, Florida — Viking 1 on August 20, 1975, and Viking 2 on September 9, 1975. The landers were sterilized before launch to prevent contamination of Mars with organisms from Earth. The spacecraft spent nearly a year cruising to Mars. Viking 1 reached Mars orbit on June 19, 1976; Viking 2 began orbiting Mars on August 7, 1976.

After studying orbiter photos, the Viking site certification team considered the original landing site for Viking 1 unsafe. The team examined nearby sites, and Viking 1 landed on July 20, 1976, on the western slope of Chryse Planitia (the Plains of Gold) at 22.3 degrees North latitude, 48.0 degrees longitude.

The site certification team also decided the planned landing site for Viking 2 was unsafe after it examined high-resolution photos. Certification of a new landing site took place in time for a Mars landing on September 3, 1976, at Utopia Planitia, at 47.7 degrees North latitude, and 48.0 degrees longitude.

The Viking mission was planned to continue for 90 days after landing. Each orbiter and lander operated far beyond its design lifetime. Viking Orbiter 1 exceeded four years of active flight operations in Mars orbit.

The Viking project’s primary mission ended November 15, 1976, 11 days before Mars’s superior conjunction (its passage behind the Sun). After conjunction, in mid-December 1976, controllers re-established telemetry and command operations, and began extended-mission operations.

The first spacecraft to cease functioning was Viking Orbiter 2 on July 25, 1978; the spacecraft had used all the gas in its attitude-control system, which kept the craft’s solar panels pointed at the Sun to power the orbiter. When the spacecraft drifted off the Sun line, the controllers at JPL sent commands to shut off power to Viking Orbiter 2’s transmitter.

Viking Orbiter 1 began to run short of attitude-control gas in 1978, but through careful planning to conserve the remaining supply, engineers found it possible to continue acquiring science data at a reduced level for another two years. The gas supply was finally exhausted and Viking Orbiter 1’s electrical power was commanded off on August 7, 1980, after 1,489 orbits of Mars.

The last data from Viking Lander 2 arrived at Earth on April 11, 1980. Lander 1 made its final transmission to Earth Nov. 11, 1982. Controllers at JPL tried unsuccessfully for another six and one-half months to regain contact with Viking Lander 1. The overall mission came to an end May 21, 1983.

With a single exception — the seismic instruments — the science instruments acquired more data than expected. The seismometer on Viking Lander 1 would not work after landing, and the seismometer on Viking Lander 2 detected only one event that may have been seismic. Nevertheless, it provided data on wind velocity at the landing site to supplement information from the meteorology experiment, and showed that Mars has very low seismic background.

The three biology experiments discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in soil near the landing sites. According to mission biologists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open.

The landers’ gas chromatograph/mass spectrometer instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements. The X-ray fluorescence spectrometers measured elemental composition of the Martian soil.

Viking measured physical and magnetic properties of the soil. As the landers descended toward the surface they also measured composition and physical properties of the Martian upper atmosphere.

The two landers continuously monitored weather at the landing sites. Weather in the Martian midsummer was repetitious, but in other seasons it became variable and more interesting. Cyclic variations appeared in weather patterns (probably the passage of alternating cyclones and anticyclones). Atmospheric temperatures at the southern landing site (Viking Lander 1) were as high as -14°C at midday, and the predawn summer temperature was -77°C. In contrast, the diurnal temperatures at the northern landing site (Viking Lander 2) during midwinter dust storms varied as little as 4°C on some days. The lowest predawn temperature was -120°C, about the frost point ofcarbon dioxide. A thin layer of water frost covered the ground around Viking Lander 2 each winter.

Barometric pressure varies at each landing site on a semiannual basis, because carbon dioxide, the major constituent of the atmosphere, freezes out to form an immense polar cap, alternately at each pole. The carbon dioxide forms a great cover of snow and then evaporates again with the coming of spring in each hemisphere. When the southern cap was largest, the mean daily pressure observed by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were 7.3 and 10.8 millibars. (For comparison, the surface pressure on Earth at sea level is about 1,000 millibars.)

Martian winds generally blow more slowly than expected. Scientists had expected them to reach speeds of several hundred kilometers an hour from observing global dust storms, but neither lander recorded gusts over 120 kilometers an hour, and average velocities were considerably lower. Nevertheless, the orbiters observed more than a dozen small dust storms. During the first southern summer, two global dust storms occurred, about four Earth months apart. Both storms obscured the Sun at the landing sites for a time and hid most of the planet’s surface from the orbiters’ cameras. The strong winds that caused the storms blew in the southern hemisphere.

Photographs from the landers and orbiters surpassed expectations in quality and quality. The total exceeded 4,500 from the landers and 52,000 from the orbiters. The landers provided the first close-up look at the surface, monitored variations in atmospheric opacity over several Martian years, and determined the mean size of the atmospheric aerosols. The orbiter cameras observed new and often puzzling terrain and provided clearer detail on known features, including some color and stereo observations. Viking’s orbiters mapped 97 percent of the Martian surface.

The infrared thermal mappers and the atmospheric water detectors on the orbiters acquired data almost daily, observing the planet at low and high resolution. The massive quantity of data from the two instruments will require considerable time for analysis and understanding of the global meteorology of Mars. Viking also definitively determined that the residual north polar ice cap (that survives the northern summer) is water ice, rather than frozen carbon dioxide (dry ice) as once believed.

Analysis of radio signals from the landers and the orbiters — including Doppler, ranging and occultation data, and the signal strength of the lander-to-orbiter relay link — provided a variety of valuable information.

Other significant discoveries of the Viking mission include:

• The Martian surface is a type of iron-rich clay that contains a highly oxidizing substance that releases oxygen when it is wetted.
• The surface contains no organic molecules that were detectable at the parts-per-billion level — less, in fact, than soil samples returned from the Moon by Apollo astronauts.
• Nitrogen, never before detected, is a significant component of the Martian atmosphere, and enrichment of the heavier isotopes of nitrogen and argon relative to the lighter isotopes implies that atmospheric density was much greater than in the distant past.
• Changes in the Martian surface occur extremely slowly, at least at the Viking landing sites. Only a few small changes took place during the mission lifetime.
• The greatest concentration of water vapor in the atmosphere is near the edge of the north polar cap in midsummer. From summer to fall, peak concentration moves toward the equator, with a 30 percent decrease in peak abundance. In the southern summer, the planet is dry, probably also an effect of the dust storms.
• The density of both of Mars’s satellites is low — about two grams per cubic centimeter — implying that they originated as asteroids captured by Mars’s gravity. The surface of Phobos is marked with two families of parallel striations, probably fractures caused by a large impact that may nearly have broken Phobos apart.
• Measurements of the round-trip time for radio signals between Earth and the Viking spacecraft, made while Mars was beyond the Sun (near the solar conjunctions), have determined delay of the signals caused by the Sun’s gravitational field. The result confirms Albert Einstein’s prediction to an estimated accuracy of 0.1 percent — 20 times greater than any other test.
• Atmospheric pressure varies by 30 percent during the Martian year because carbon dioxide condenses and sublimes at the polar caps.
• The permanent north cap is water ice; the southern cap probably retains some carbon dioxide ice through the summer.
• Water vapor is relatively abundant only in the far north during the summer, but subsurface water (permafrost) covers much if not all of the planet.
• Northern and southern hemispheres are drastically different climatically, because of the global dust storms that originate in the south in summer.

The human exploration of Mars will be an enterprise that confirms the potential for humans to leave their home planet and make their way outward into the cosmos. Though just a small step on a cosmic scale, it will be a significant one for humans, because it will require going away from Earth with very limited capability to return. Once committed to a journey to Mars, astronauts will not be able to return until the alignment of the planets allows their return. This is the most radical difference between this exploration and all previous explorations. There is a very narrow window within which return is possible, and the commitment to launch is a commitment to three years in space.

Mars is an intriguing and exciting planet, and there are many adventures and findings that await explorers. We must prepare for these before we go, providing the tools that the explorers will use, anticipating as much as possible the situations they will encounter and preparing them for the unexpected. For the first time in a space exploration mission, it will be up to the crew to solve their own emergency problems. At the distance of Mars from the Earth, it can be as much as 40 minutes from the time a message goes out from Earth to the time an answer is received back on Earth. The crews and their systems must be able to accomplish their objectives in a highly autonomous manner.

Exploration Program Objectives

Science

Mars is an intriguing planet, for what it can tell us about the origin and history of planets and of life. Visible to the ancients, and distinctly reddish in the night sky, the next planet has always been an attractive subject for imaginative science fiction. As the capability for space exploration grew in the 1960’s, it became clear that Mars is not, like Earth, a planet teeming with life, and is now a hostile environment for humans. The images of Mariner 4 showed a Moon-like terrain, dominated by large impact craters. This terrain now is believed to an represent ancient crust, similar to the Moon’s, formed in an initial period of planetary differentiation. Mariner 9 showed for the first time that Mars was not totally Moon-like, but exhibits later volcanic and tectonic features. Large volcanos of relatively recent activity and large crustal rifts due to tensional forces demonstrate the working of internal forces. The absolute time scale is not accurately calibrated, however, by analogy with the Moon, the initial crustal formation may have occurred between 4 billion and 4.5 billion years ago, and the apparent freshness of the large Martian volcanos suggests their formation within the last billion years.

Mars’ atmosphere consists largely of carbon dioxide, with a typical surface pressure of about .01 Earth atmospheres, and surface temperatures that may reach 25°C on the equator in mid-summer, but are generally much colder. At these pressures and temperatures, water can not exist in liquid form on the surface. However, Mariner 9 and the subsequent Viking missions observed features which indicate that liquid water has been present on Mars’ surface in past epochs. Evidence of both running water and standing water has been noted. The interpretation is that the atmosphere of Mars was thicker and warmer in former times, and perhaps much like the Earth’s early atmosphere before the appearance of oxygen.
Three questions arise:

1. What was the reason for the change of atmospheric conditions on Mars?
2. What are the implications of such changes for environmental changes on Earth?; and
3. Is it possible that life arose in the early Earth-like history of Mars (and, if it arose, can it still be found somewhere on Mars)?

These three scientific questions are at the core of the Mars scientific exploration defined by the Reference Mission. They can all be addressed principally by understanding the geological characteristics of the planet - the types of rocks present, their absolute and relative ages, the distribution of subsurface water, the history of volcanic activity, the distribution of life-forming elements and compounds, and others. These attributes all have to be understood in the context of what we know about the Earth, Moon and other bodies of our solar system.

Human Expansion

The dream of human exploration of Mars is intimately tied to the belief that new lands create new opportunities. In human history, migrations of people have been stimulated by overcrowding, exhaustion of resources, the search for religious or economic freedom, competitive advantage, and other human concerns. Rarely have humans entered new territory, then completely abandoned it. In the past, there have always been a few people who were adventurous enough to adopt a newly-found territory as their home. Most of these settlements have eventually become economically self-sufficient, and have enlarged the genetic and economic diversity of humanity. The technological revolution of the 20th Century, with high speed communication and transportation and integrated economic activity, has reversed the trend toward human diversity; however, settlement of the planets can once again enlarge the sphere of human action and life.

The settlement of Mars presents new problems and challenges. Principal among these is the absence of a livable natural environment. That, and the current high cost of transportation are the main barriers to human expansion there. The fact that humans, once on Mars, can not easily return to the Earth, and then only at specified times approximately 26 months apart, makes it necessary to develop systems with high reliability and robustness. The creation of a livable, artificial environment, is technically feasible. The high cost of transportation will ultimately be reduced. The Reference Mission is not a program to settle Mars; however, the objectives of the Reference Mission are to establish the feasibility of and the technological basis for human settlement of the planet.

International Cooperation

The space age gained its start in a period of intense technical and social competition between east and west, represented by the Soviet Union and the United States. Competition during the International Geophysical Year resulted in the Soviet Union being the first to launch a satellite into Earth orbit, and served as a challenge and reminder to the United States that technological supremacy was not solely the province of the United States. The start of the Apollo program was a political decision based more on the perception of the political and technological rewards to be gained by attacking a truly difficult objective in a constrained time period. The space race began, the United States won it, and a few years later, the Soviet Union had collapsed.

Fortunately, the Russians did not view the Apollo success as a reason to terminate their program, and they continued to develop capabilities that are fully on a par with United States capabilities in many areas. Also, during the post-Apollo time frame, space capability grew in Europe, with the formation of the European Space Agency, in Japan, China and other countries.

The basis has been laid for a truly international approach to Mars exploration. The exploration of Mars should be an international enterprise.

It would exhibit a great vanity for any country to undertake human exploration of Mars alone, particularly when others, who may not now have the required magnitude of capability or financial resources, do have the underlying technological know-how. Mars should be an objective in which all humanity can share. An underlying requirement for the Reference Mission is that it be implemented by a multinational group of nations and explorers.

Deimos [DEE-mos] (panic) is a moon of Mars and was named after an attendant of the Roman war god Mars. Deimos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Some scientists speculate that Deimos and Phobos (the other martian moon), are captured asteroids; however, other scientists present arguments counter to this theory. Both Deimos and Phobos are saturated with craters. Deimos has a smoother appearance caused by partial filling of some of its craters. Asaph Hall discovered Deimos in 1877.

Deimos Statistics

Views of Deimos

Mosaic of Deimos

Measuring 16 by 12 kilometers, Deimos circles Mars every 30 hours. Craters of varying age dot its surface, which is somewhat smoother than the surface of Phobos.

Deimos

This image was taken by the Viking Orbiter spacecraft in 1977.

Deimos

This image shows a slightly different view of Deimos. It was acquired by the Viking Orbiter spacecraft.

Map of Deimos

This image is a photomosaic of Deimos, the outer satellite of Mars. The leading side faces forwards in the orbit of Deimos. The trailing side faces backwards along the orbit. Longitude 0 is at the blunter end with the most prominent craters, and faces Mars. As with all conformal (true shape) projections, the scale in these maps varies, increasing from the center to the outer edge.

Phobos [FOH-bohs] (fear) is a moon of Mars and was named after an attendant of the Roman war god Mars. Phobos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Some scientists speculate that Phobos and Mars’ other moon, Deimos, are captured asteroids. However, other scientists point to evidence that contradicts this theory. Phobos shows striated patterns which are probably cracks caused by the impact event of the largest crater on the moon. Asaph Hall discovered Phobos in 1877.

Phobos Statistics

Views of Phobos

Phobos

This image was taken by the Viking Orbiter spacecraft in 1977. Striated patterns can be seen in this image. These are probably cracks caused by the impact event of the Stickney crater shown below.

Stickney Crater

One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only 28 by 20 kilometers, the moon must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact. Near the crater, the grooves measure about 700 meters across and 90 meters deep. However, most of the grooves have widths and depths in the 100 to 200 meters and 10 to 20 meters ranges, respectively.

Stickney Crater Another View

This image shows a slightly different view of the Stickney crater. A crater within the Stickney crater is visible.

Conformal Projection of Phobos

This shows two different views of Phobos in a Morphographic Conformal Projection. One view shows the leading side and the other the trailing side.