6th Grade SI Notes Earth in the Universe Astronomy Unit 6.E.1

The Solar SystemThe solar system is made up of a central star, eight planets, moons, asteroids, comets, meteoroids, dust, gases, and empty space.

The SunThe Sun is a massive, glowing ball of very hot gas that is located at the center of the solar system. The Sun is made up mostly of helium and hydrogen. The hydrogen undergoes a nuclear reaction, called fusion, to produce the helium. The Sun is the only star in the solar system. However, light from other stars can be seen in the night sky.

The solar system's central star—the Sun.

The Sun is the major source of heat and light for the solar system. And everything in the solar system is under the direct influence of the Sun's gravitational pull.

PlanetsIn a planetary system, the largest bodies orbiting the central star—such as the Sun—are planets. The planets in the solar system include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Planets can be made mostly of rock, like Earth, or mostly of gas, like Jupiter.Planets revolve around the Sun along different orbital paths and at different distances and frequencies. The diagram below shows the relative distances of the planets from the Sun.

The Sun, Mercury, Venus, and Mars are all relatively close to the Earth. Jupiter, Saturn, Uranus, and Neptune are progressively farther away from the Earth. In general, the distance between planets increases as the distance from the Sun increases.

MoonsMoons are bodies of rock that orbit planets. Moons are smaller than the planets they orbit, but are usually larger than asteroids.

Earth and its moon

Some planets have a variety of moons, and some of these moons show evidence of geologic activity.

AsteroidsAsteroids are small bodies of rock and metal. They can be small or large, and they can be almost any shape. While moons orbit planets, asteroids orbit stars.A region between Mars' and Jupiter's orbits contains a large number of asteroids and is known as the asteroid belt. The picture below is an artist's interpretation of how the asteroid belt might appear.

Image courtesy of NASA/JPL-Caltech

CometsComets are smaller than planets, moons, or asteroids. They are made up of dust particles, frozen water, and frozen gases. They orbit stars, such as the Sun, and they have long, narrow elliptical orbits. When comets pass close to a star, heat from the star partially melts the comet. The frozen gas and liquid that composes the comet streams off and forms a "tail."

Comet Hyakutake

MeteoroidsMeteoroids are small bodies orbiting the Sun. They are formed when pieces are knocked off of larger objects, such as asteroids and moons. When a meteoroid enters a planet's atmosphere, friction between the meteoroid and the particles in the atmosphere create heat and light along the meteoroid's path through the sky. The streak of light seen along the meteoroid's path is called a meteor. It is also sometimes called a shooting star. Any portion of a meteoroid that remains intact through the atmosphere and strikes the Earth is called a meteorite.

Planets & MoonsThe solar system is comprised of various objects, including planets, dwarf planets, and moons, which orbit the Sun and are classified based on their characteristics.

If you look at the night sky regularly during a year, you will notice small disks that change in brightness and position against the background of other stars. The Greeks called these wandering stars planets.

Arrangement of the PlanetsWe know that there are 8 official planets in the Solar system: The inner planets (Mercury, Venus, Earth, Mars) and the outer planets (Jupiter, Saturn, Uranus, Neptune). The inner planets each orbit the Sun in less than 2 years, but the outer planets take from 12 to 164 years each to fully orbit the Sun.

The diagram above shows the order of the planets from the Sun and their relative sizes. The sizes of their orbits are not

drawn to scale.

The Inner PlanetsThe inner planets, also known as the terrestrial planets, are small, dense, and made of rock. Their orbits are close to the Sun.Mercury is the smallest planet. It is a little larger than the Moon. Mercury has no significant atmosphere, so its surface is extremely hot in the sunlight but cold in the shade. Its surface is covered with craters.Venus is about the size of the Earth. Venus has a thick atmosphere of carbon dioxide and sulfuric acid, and the surface is hot enough to melt lead. When Venus is closest to Earth, it is about 25 million miles away from Earth.Earth is mostly covered by water, has a nitrogen-oxygen atmosphere, and is the only planet known to have life.Mars is about 7 times smaller than Earth. Mars has a thin atmosphere rich in carbon dioxide. The Martian surface is extremely cold (below the freezing point of water). Scientists believe that Mars may once have been

warm enough for liquid water and possibly life. When Mars is closest to Earth, it is about 35 million miles away from Earth.

The Outer PlanetsThe outer planets (also known as the gas giants) are extremely large, cold, and made of gases such as hydrogen and helium. Their orbits are farther out and spaced widely apart.Jupiter is the largest planet—over 1,000 times the size of Earth. It has colorful cloud bands and a large storm, the Great Red Spot.Saturn has three large sets of rings surrounding it, which are visible in small backyard telescopes. Both Jupiter and Saturn have many moons (also called satellites) and are like mini solar systems. Some of these moons could support life.Uranus has smaller, thin rings, has 21 moons, and is tipped on its side.Neptune has eight moons including one large moon, Triton. Triton has active cold nitrogen geysers that erupt frequently.

Distances from the SunEach planet in the solar system has a unique orbit around the Sun and a unique average distance from the Sun. The diagram below shows the relative average distances of the planets from the Sun.

The table below shows the average distance from the Sun, orbital speed, and surface temperature of each planet.

Planet

AverageDistance from Sun

(km)

AverageOrbital Speed(km/s)

Orbital Period(Earth years)

AverageSurface

Temperature(° C)

Mercury 58 million 47.9 0.24 77

Venus 108 million 35.0 0.62 457

Earth 150 million 29.8 1.0 14

Mars 228 million 24.1 1.9 -63

Jupiter 778 million 13.1 11.9 -149

Saturn 1,427 million 9.65 29.4 -176

Uranus 2,871 million 6.80 83.8 -215

Neptune 4,499 million 5.43 164 -214Notice that planets closer to the Sun tend to move at greater orbital speeds and have shorter years. Thus, the farther a planet is from the Sun the slower it moves. And since the planets farther from the Sun have farther to travel to complete one orbit, they also have much longer years.A planet's distance from the Sun is also one of the most important factors influencing its surface temperature. In general, the farther away from the Sun a planet is, the lower its surface temperature tends to be. However, there are some important exceptions. Venus, for example, has the highest surface temperature of all the planets but is actually farther from the Sun than Mercury. This difference is due to the very different atmospheres of Venus and Mercury.

Planetary AtmospheresThe thickness of a planet's atmosphere is, in part, due to the strength of the planet's gravitational field. A very massive planet with a very strong gravitational field is likely to have a very thick atmosphere. Jupiter, Saturn, Uranus, and Neptune, for example, have strong gravitational fields and thick, dense atmospheres. Mercury, on the other hand, has a weak gravitational field and an extremely thin, low-density atmosphere. In fact, the particles in Mercury's atmosphere are so far apart that they are more likely to collide with the planet's surface than with each other.

Venus has a very thick, dense, cloudy atmosphere that hides the planet's surface from view. The atmosphere of Venus is made mostly of carbon dioxide (96.5%), with a lesser amount of nitrogen (3.5%) and trace amounts of other gases, including sulfur dioxide, argon, and water vapor. The main source of Venus's atmospheric gases is volcanic eruptions that occurred earlier in its history, although impacting objects may have added some materials as well. Because of the high concentration of carbon dioxide, Venus has a "runaway" greenhouse effect that is much stronger than Earth's greenhouse effect. Because of its powerful greenhouse effect and its distance from the Sun, Venus has a surface temperature of about 465°C (870°F), which is the highest of all the planets.Although not as thick and dense as Venus' atmosphere, Earth's atmosphere is thicker and denser than the atmospheres of the other rocky, inner planets. Earth's atmosphere developed from volcanic eruptions, biological processes, and early impacts by comets and asteroids.

Mars has a thin, low-density atmosphere. The atmospheric pressure on the surface of Mars is only about 1/150 the atmospheric pressure on Earth's surface. Mars' atmosphere came mainly from past volcanic eruptions, with additional contributions likely from impacting bodies.Past Mars, the planets of the solar system have very large and thick atmospheres. Jupiter, Saturn, Uranus, and Neptune are giant balls of gas and compressed liquids (with very small rocky-icy cores). Their atmospheres formed very early in the history of the solar system and have changed very little. These planets are much larger than the inner planets and are much more massive.

In general, when planets or moons have thick atmospheres, they tend to experience a relatively narrow range of surface temperatures from day to night. Planets or moons with thin atmospheres often experience extremely large surface temperature ranges. For example, the Moon is much less massive than the Earth and has almost no atmosphere. Because it lacks a thick atmosphere to regulate its temperature, the Moon has a much wider surface temperature range than Earth. The same is true of Mercury, which has a very thin atmosphere and whose surface temperature varies by about 600°C from day to night.

Comparison of Planetary Masses and GravityThe eight planets of our solar system vary greatly in mass, diameter, and surface gravity. The table below summarizes some of the characteristics of the planets.

PlanetMass

(number of Earths)

Diameter(number of

Earths)

Surface Gravity(compared to

Earth)Mercury 0.055 0.38 0.38

Venus 0.95 0.814 0.903

Earth 1.00 1.00 1.00

Mars 0.109 0.53 0.380

Jupiter 318 11.2 2.53

Saturn 95.3 9.44 1.07

Uranus 14.5 4.00 0.86

Neptune 16.7 3.88 1.14

Dwarf PlanetsPluto was once considered the farthest planet. However, part of its orbit brings it closer to the Sun than Neptune, and it is about as small as the largest asteroid in the Solar System, Ceres. The status of Pluto was changed in 2006 to "dwarf planet", so it is no longer considered one of the major planets. Pluto is very cold and dim. It has a moon called Charon which is almost as big as Pluto itself.Ceres is another celestial body that is considered a dwarf planet. Before the "dwarf planet" category was established, it was classified as the largest known asteroid in the Solar System.

MoonsThe moons in the Solar system differ greatly in size. The largest of the moons in the Solar system is Ganymede, which orbits Jupiter. The second largest is Titan, which orbits Saturn. Both of these moons are actually larger in size than the planet Mercury. The smallest moon is Trinculo, a satellite of Uranus, which has a radius of 5 km. The table below summarizes some of the characteristics of the two largest moons in the Solar system and the Earth's moon.

The moons in the Solar system also differ greatly in their composition. The most volcanically active body in the Solar system is Io, a moon orbiting Jupiter. Another of the moons orbiting Jupiter, named Europa, is the most likely place in the Solar system to find liquid water and possible life. It is believed that there is a huge ocean of liquid water below the icy surface.Only two of the planets in the Solar system, Mercury and Venus, do not have any moons. Earth only has one moon, and all other planets have more than one moon. Jupiter and Saturn are orbited by more than 60 moons each. Most, but not all, of these satellites orbit their planets in the same direction as the planet's rotation. In other words, if the planet is rotating

counter-clockwise, then the planetary satellite will usually orbit the planet counter-clockwise.By far, the Moon is the closest large body in the Solar system to the Earth. The average distance between the two is 384,400 km. The closest planet to the Earth is Venus. At its closest approach, the distance between the two is 41,440,000 km. By contrast, the outer planet Neptune is more than 4 billion kilometers from the Earth. Compared to that, the Moon is a very close neighbor indeed.

Surface Features of Terrestrial Planets

Our solar system has several objects known as terrestrial bodies. The word "terrestrial" refers to the Earth or to land in general. Celestial bodies are classified as terrestrial if, like the Earth, they are large, spherical, made largely of rock, and have surfaces with geologic features.

Terrestrial bodies include the inner planets, the Earth's Moon, and several moons of the outer planets. This lesson focuses on surface features of the inner planets (Mercury, Venus, Earth, and Mars).

MercuryMercury is the smallest planet and the nearest planet to the Sun. This planet's ancient surface is covered in large impact craters, which formed when asteroids and comets struck the planet long ago.

A mosaic of Mariner 10 images showing Mercury's surface (light-colored band near top right represents regions not

imaged)

This typical terrain of Mercury shows several circular, rimmed areas, which are impact craters.

Mercury also has some surface features that are unique to the planet. These include a global system of curved scarps (cliffs) and a region known as "Weird Terrain." The leading hypothesis for the scarps is that they are large faults that formed in the crust when Mercury shrank in size sometime in its past. The Weird Terrain is wrinkled crust believed to have formed in response to a giant asteroid impact on the opposite side of the planet. The impact, which created the huge Caloris Basin, is believed to have sent shockwaves around the planet, which all converged on the opposite side to wrinkle Mercury's crust.

VenusVenus is the second planet from the Sun and is about the same size as the Earth. The planet's surface is hidden from view by a thick, cloudy atmosphere. This thick carbon dioxide atmosphere traps heat and causes higher temperatures on Venus in a process known as the greenhouse effect.

This ultraviolet image, which was taken by the spacecraft Pioneer Venus, shows the cloudy atmosphere that hides

Venus' surface from view.

Fortunately, scientists have been able to penetrate the clouds of Venus with radar waves. Observations from Earth-based telescopes as well as radar instruments aboard spacecraft have revealed that Venus has a great variety of features, most of which are volcanic or tectonic in origin. Scientists have also learned that Venus lacks large impact craters like those seen on many other terrestrial bodies. This suggests that Venus' surface is relatively young and that some major volcanic-tectonic event resurfaced the planet in the past.

This image of Venus' surface, which was produced from Magellan orbiter data, shows how different surface textures

reflect radar signals differently. The brighter areas represent rough surfaces, which greatly scatter the radar signal. The

darker areas represent smoother surfaces, which reflect the signal more uniformly. The bright, pancake-shaped features

are lava domes. The large dark region is a smooth volcanic plain.

Image courtesy of JPL/NASA

EarthEarth is the third planet from the Sun, and it is the largest terrestrial body in the solar system. It is the only body in the solar system known to have life forms and permanent bodies of liquid water at the surface. Earth also has the greatest variety of geologic features of any body in the solar system. At the broadest scale, Earth's surface can be divided into continental and oceanic crust. The continental crust is thicker, less dense, and reaches elevations above the Earth's liquid oceans. The oceanic crust is thinner, denser, and lies at the bottom of the oceans.

A view of Earth's western hemisphere showing continents, oceans, and clouds

Image courtesy of NASA

Earth has a unique, complex plate tectonics system that produces volcanoes, rift valleys, and many types of mountain chains. The presence of water and a dynamic atmosphere create and transform many surface features as well, such as canyons, rivers, glaciers, islands, and sand dunes.

This satellite image shows part of a volcanic island arc that is located along the boundary between the Pacific and

North American tectonic plates. These islands were created by volcanic activity that resulted directly from interactions

between the plates. Although tectonic activity does occur on other solar system bodies, no other body has a complex

system of plate tectonics like Earth's.

NASA image courtesy of Jeff Schmaltz, MODIS Rapid Response Team, NASA-Goddard Space Flight Center

MarsMars is the fourth planet from the Sun, and it is the third-largest terrestrial planet. Mars has many surface features that are also found on Earth, including volcanoes, glaciers, ice caps, sand dunes, dry river beds, canyons, and many more. In most cases, these features are significantly larger than their cousins on Earth. For example, Mars has the tallest volcano in the solar system—Olympus Mons towers about 22 km above the surrounding martian plains.

Mars has many surface features that are similar to Earth's. Visible near the equator in this global view of Mars is Valles

Mariners, a canyon that extends for a distance greater than the width of the United States. The brown circular spots near

the left edge of the image are giant volcanoes.

Overall, Mars is cold and dry at present. But the patterns of its geologic features suggest it was warmer, wetter, and more Earth-like in its past. For example, features called valley networks on Mars are very similar to branching networks of river valleys on Earth. Valley networks were likely carved by running water during Mars' past.

This image, which was taken by one of the Viking Orbiter spacecraft, shows valley networks on Mars. Valley networks

are one of the strongest lines of evidence that Mars was more Earth-like in its past.

Movement of Objects in the Solar System

Most objects in the solar system orbit the Sun along elliptical paths.

Since the Sun is the center of the solar system, all of the asteroids, comets, dwarf planets, and planets (including their associated moons) in the solar system orbit the Sun.

Objects in the Solar System Orbit the SunThe Sun is the only object in the solar system that is essentially stationary relative to the solar system as a whole. As the Sun slowly orbits the center of the galaxy, the entire Solar System comes along with it. So, relative to the planets, moons, asteroids, and comets of the solar system, the Sun does not move.Objects move in orbits around the Sun or other objects due to their gravitational attraction and inertia. Without the Sun's gravity, a planet's inertia would send it traveling in a straight line off into space. Without a planet's inertia, the Sun's gravity would pull the planet straight toward it, and the two would collide. These two factors combine to form an elliptical planetary orbit and determine the planet's orbital speed.All eight planets in our solar system orbit the Sun along elliptical orbits. An ellipse is a shape that is very similar to an oval. The paths that the planets follow, however, are only barely elliptical. In fact, the planets move around the Sun in very nearly circular paths.The diagram below shows the planets and their relative orbits around the Sun. The sizes of their orbits are not drawn to scale.

Each moon in the Solar System orbits a planet and also orbits the Sun. As a moon orbits its planet, the planet moves around the Sun. So a moon orbits the Sun at the same time as it orbits its planet.

Solar Days & Solar YearsThe time that it takes a planet in our solar system to make one full revolution around the Sun is called a solar year. Planets that are located closer to the Sun have shorter solar years because they have a smaller distance to travel (i.e. their orbits have a smaller diameter). For example, Mercury takes 88 Earth days to revolve around the Sun, the Earth takes 365 days, and Neptune takes 165 Earth years.All of the planets in our solar system also rotate on their axes. The time that it takes a planet to make one full rotation is called a solar day. Jupiter has the shortest solar day; it equals only 9.8 Earth hours. Venus has the longest solar day; it equals 243 Earth days. One solar day on Earth takes about 24 hours.

Uniqueness of EarthThe Earth formed in just the right place with just the right ingredients for life to flourish.

Earth has many properties that cannot be found anywhere else in our solar system. The unique properties of the Earth are the most important to Earth's lifeforms. A few of the properties are listed below.

Earth is the only planet in our solar system where water can be found in solid, liquid, and gas forms.

Earth is the only major planet in our solar system that has an atmosphere that is breathable for lifeforms like those found on Earth.

Earth is the only planet in our solar system that maintains a relatively small temperature range, which is moderate enough for many organisms to survive in.

Earth is an abundant source of substances that contain carbon, nitrogen, oxygen, sulfur, and phosphorous. These chemical nutrients provide building materials for Earth's organisms to use for growth, reproduction, and repair.

Earth's proximity to the Sun allows organisms on Earth to use incoming electromagnetic radiation from the Sun as a source of energy.

Earth's atmosphere and magnetic field provide some protection from harmful radiation and solar wind.

Earth's WaterOne of the most important reasons why the Earth is able to support life is because water can exist in all phases of matter (solid, liquid, and gas) on Earth. Thus, systems such as the water cycle could develop to recycle Earth's water. Also, water is necessary for plants to grow, and plants are necessary for animals to eat. So without water, food chains and food webs would not be possible; life would not be possible.

Earth's AtmosphereEarth's atmosphere is breathable to many of the organisms that live on Earth. Although Earth's atmosphere did not always contain significant amounts of oxygen, today it contains the gas in amounts that can support organisms that obtain energy from their food using the process of cellular respiration. This process requires oxygen and produces carbon dioxide and water. Cellular respiration is complementary to photosynthesis—the mechanism used by plants and some microorganisms to produce food. That is, the carbon dioxide and water produced by cellular respiration are starting materials for photosynthesis. Many other planets, such as Mars, do not have atmospheres that contain oxygen, so they cannot support life forms that use oxygen to get energy from food they eat.The atmosphere is important to Earth's organisms also because the air that makes up the atmosphere circulates, which helps to warm cooler areas and

cool warmer areas. As a result, many areas on Earth experience only small daily changes in temperature.Finally, the Earth's atmosphere helps sustain life on Earth because its component gases help trap heat from the Sun in a process called the greenhouse effect and helps keep the temperature on Earth in a range that is compatible with life. However, there are other factors that affect Earth's surface temperature.

Earth's TemperatureEarth is the third planet in our solar system and located 150 million kilometers away from the Sun. Although this seems like a large distance, it is apparently the perfect distance to maintain life. Mercury and Venus, which are located closer to the Sun, are too hot to sustain life. Gas giants Jupiter and Neptune, which are located farther from the Sun, are too cold to sustain life.

Earth's NutrientsEarth's lifeforms are carbon-based. That means that they are made up largely of chemical substances that contain the element carbon. Earth's atmosphere, lithosphere, and water are all rich in carbon-containing substances that organisms can take in and use to live and grow. Earth also contains other chemical substances, such as nitrogen, oxygen, sulfur, calcium, and phosphorous that living organisms need to breathe and make proteins, bones, and other building materials. If Earth was not rich in these nutrients, lifeforms that need these materials could not be abundant here.

Energy from the Sun

Our Sun is a star that is located at the center of our solar system. Life on Earth is dependent on the Sun. The Sun's light enables plants to make their own food. Animals eat plants and use them for food, and other animals eat

the animals that eat plants. In this way, energy from the Sun is the basis of almost all of the food chains and webs on Earth.The Sun's energy also drives the water cycle. When the Sun warms the Earth's surface or atmosphere, water gains that energy and is able to evaporate. This evaporated water then condenses and precipitates, which helps to recharge the freshwater on Earth's surface and under the ground. Living organisms rely on this supply of freshwater in order to survive.

Layers of Protection for Living OrganismsTwo of Earth's structures—the ozone layer and the magnetosphere—provide protection for Earth's lifeforms. The ozone layer is a portion of the upper atmosphere. It blocks much of the Sun's UV radiation and prevents it from reaching Earth's surface, where it can damage the cells of living organisms.The magnetosphere is a magnetic field that surrounds the Earth. It deflects much of the solar wind and keeps Earth's atmosphere from being scoured away.

Earth's SatellitesThe Earth is orbited by one moon, many artificial satellites, and debris.

The MoonEarth, like other planets, is large enough to hold smaller objects in orbit around itself through the force of gravity. Earth has only one large, natural object that revolves around it. This object is the Moon. The Earth is unique in that it is the only planet in the solar system that has only one moon.

Earth's moon is the only natural satellite orbiting the Earth.

Artificial SatellitesThe Earth has a large number of small man-made satellites that orbit it, which are used for various purposes, ranging from telecommunications to space observation. Humans' activities in space have also put many pieces of man-made debris (also known as "space junk") in orbit around the Earth. NASA tracks the location of every piece of space debris that is 10 cm or larger. The image below represents the locations of all known pieces of space debris in orbit around Earth. Many of these pieces of debris were created in 2009, when a private satellite collided with a Russian satellite that was no longer in use.

Image is courtesy of NASA Orbital Debris Program

The black spots in this image represent all known pieces of space debris in orbit around Earth.

Because NASA knows the location and orbital path of all space debris, they can prevent collisions between pieces of debris and the International Space Station by changing the orbit of the station slightly.

Studying the UniverseScientists use a number of instruments to study various properties of the universe.

Tools for Studying the UniverseScientists use a number of tools to study the universe. The main types of unmanned tools include:

spectroscopes telescopes satellites probes

SpectroscopesElectromagnetic radiation interacts with objects and materials in different ways. But each material tends to behave consistently in terms of which types of radiation it absorbs, reflects, or emits (gives off). When a material absorbs or emits a unique set of different colors of visible light, the characteristic set of color bands observed through a spectroscope is the material's spectrum.For example, the element helium was named after the Sun because its spectrum was first found coming from the Sun. Viewed through a spectroscope, the light coming from hot helium produces a spectrum like the one shown below.

Since scientists know the characteristic spectra of different materials, such as helium, present here on Earth, they can compare these spectra to others obtained by viewing distant objects in space through a spectroscope. Just by looking at the spectra of glowing objects such as galaxies, stars, and nebula, scientists can tell what they are made of.

TelescopesThere are many different types of telescopes used by scientists to study distant objects. Some telescopes are set up on the ground. Others are placed on satellites that transmit data back to Earth. These space-based telescopes can gather better data because there is no interference from Earth's atmosphere. Any telescope can help astronomers gather information about distant objects, including what they are made of, how fast they are moving towards or away from Earth, and even what temperature they are.As telescopes have improved, scientists have been able to gather more detailed information about other stars. For example, planetary systems like that in our solar system used to be considered rare. The first extraterrestrial planet was discovered in 1995. Since then, more and more extrasolar planets have been found. We now know that planets are much more common than was originally believed.Different kinds of telescopes are designed to observe different parts of the electromagnetic (EM) spectrum. Because different wavelengths of light are produced and blocked by different things, each telescope can reveal unique parts of the universe.

X-ray telescopes are used to examine the x-ray radiation that comes from different objects. They cannot travel far through the Earth's atmosphere. Astronomers have identified only some of the sources of x-ray radiation in the universe. Hot gases and the remains of supernovae are known to produce x-rays. X-rays are produced by many high-energy objects in space.

Image courtesy of NASA/ROSAT Project

The above image is a map of the entire sky made by x-ray telescopes. The black streaks are places where data is missing. Blue indicates the most energetic x-rays, and red indicates the least energetic x-rays.

Light telescopes are tools used to examine objects using light in or near the visible range. These telescopes allow people to see the objects as they would if they were much closer to the objects.

Image courtesy of Goddard Space Flight Center, NASA

The image above is of part of the night sky as seen with visible light. Specifically, it is the part of the night sky through which the Milky Way runs. The image should be packed with the bright stars that fill the center of our galaxy, but dust obscures the view, blocking out the visible light.

Radio telescopes are used to examine the radio waves that come from different objects. Radio waves are not easily scattered, so they can travel long distances. Radio waves are also the most likely way we would hear from extraterrestrial civilizations, assuming they are out there. Ground-based radio telescopes can collect data during the day or night and in any weather conditions.

It was a radio telescope that first found the cosmic microwave background. It is a low level of microwave radiation that comes from everywhere in the universe. It is some of the best evidence supporting the big bang theory, which states that all the matter and energy in the universe originated at a single point.

© Max Planck Institute for Radio Astronomy (Bonn, Germany)

The above image is a map of the entire sky taken with radio telescopes. Light in this part of the spectrum indicates the presence of strong magnetic fields,

much stronger than the one that causes the needle of a compass to point north here on Earth. The Milky Way Galaxy runs along the center of the image. The Sun is not shown here, but the Sun is the brightest radio source in the Earth's sky.

SatellitesA satellite is an unmanned spacecraft that is put into orbit around the Earth. Satellites have become very common over the past two decades, and there are currently about 5,000 satellites orbiting the Earth. Most of these 5,000 satellites run on solar power, but a few run on nuclear power. These satellites have a great impact not only on space exploration but also on technologies here on Earth. Some of the technologies that depend on satellites are cellular phones, global positioning systems, and weather tracking systems that can track weather on a global scale.There are also many satellites in orbit around the Earth that are meant to help study the universe. Some of these satellites are telescopes, the most famous of which is the Hubble Space Telescope.

ProbesA probe is an unmanned spacecraft carrying instruments intended for use in exploration of the physical properties of celestial bodies other than Earth. It is launched with enough energy to escape the gravitational field of Earth.Probes have traveled further than any other man-made object in history. As a matter of fact, multiple probes have left our solar system. Much of the information we have about the other planets and moons in our solar system actually came from probes. Every planet in our solar system has been closely observed by a space probe.Space probes have the ability to examine many properties of planets and moons. They can examine atmospheres, weather, magnetic fields, surface features, and many other features of planets and moons. Some probes have actually entered the atmosphere of other planets. We have sent probes into the atmospheres of Venus, Mars, and Jupiter.

NASA's Great ObservatoriesTo help get a more complete picture of our universe, NASA has developed a set of observatories that gather data using radiation in different regions of the electromagnetic spectrum.

NASA's four Great Observatories, shown below, were intended to produce images of, and collect data from, our universe using four different types of electromagnetic radiation.

Image courtesy of NASA

The Great Observatories in order of launch date are the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

Hubble Space TelescopeThe Hubble Space Telescope was the first of the Great Observatories to be launched into orbit. It began its service in 1990, but gained its full capability in 1993 after a servicing mission by astronauts aboard the Space Shuttle. Hubble uses light in the infrared, visible, and ultraviolet ranges of the electromagnetic spectrum to produce images of space objects and send them back to Earth. The Hubble Space Telescope is credited with helping scientists to discover the age of the universe.

The Hubble Space Telescope in orbit above the Earth

Compton Gamma Ray ObservatoryThe Compton Gamma Ray Observatory was deployed in 1991, and its service ended in 2004. It was the second of the Great Observatories. The Compton Gamma Ray Observatory is known for allowing scientists to see a large structure at the center of the galaxy that may be a remnant of the eruption of a black hole. This structure is 50,000 light years in size.

Chandra X-ray ObservatoryThe Chandra X-ray Observatory is the third Great Observatory. It was placed in Earth orbit by the Space Shuttle in 1999, and it is still in use

today. Chandra has the smoothest mirrors ever made. It uses radiation in the X-ray region of the electromagnetic spectrum to make images of high-energy objects and events. The data Chandra gathers helps scientists understand change in the universe. For example, it has produced pictures of what remains after a star explodes.

Spitzer Space TelescopeThe Spitzer Space Telescope is the last of the four Great Observatories. It was deployed into space in 2003. This telescope uses light in the infrared region of the electromagnetic spectrum to produce images of the universe. An infrared telescope is a particularly useful tool for seeing through clouds of dust and gas which would obstruct the view from telescopes, such as the Hubble, that use visible light.

Solar System ExplorationOnce rockets were invented that were powerful enough to escape Earth's gravity, humans started exploring space with a series of satellites, space probes, and manned missions.

Humanity's exploration of space began in 1957 when the U.S.S.R. launched Sputnik, the first artificial satellite, into orbit around the Earth. The U.S.S.R. once again broke ground when they sent the first man into space in 1961 aboard Vostok 1. The United States followed less than a month later, sending Alan Shepard, the first U.S. astronaut, into space. These early milestones began a long history of space exploration, in which the United States has played a lead role.

Apollo Missions to the Moon

The main purpose of the Apollo missions was to put a man on the Moon and return him safely back to Earth. This goal was reached in 1969 on theApollo 11 mission when Neil Armstrong and Edwin "Buzz" Aldrin set foot on the Moon and then returned safely to Earth. There were six more Apollo missions meant to go to the Moon after Apollo 11, and all butApollo 13 set people on the Moon. In all, fourteen Apollo missions were launched between 1967 and 1972. 

During the Apollo missions, astronauts walked on the Moon, performed many experiments, and collected many samples of the Moon's materials, which were returned to Earth for study by scientists. Through these missions, scientists learned that the Moon is lifeless, dry, and geologically inactive today. They also learned many important things about what the Moon is made of and the history of the solar system.

Mariner 10 Mission to MercuryThe Mariner 10 spacecraft was launched in November 1973. Its main purpose was to study the planet Mercury. Mariner 10 flew close by Venus in February 1974. The spacecraft used the gravity field of Venus to accelerate itself toward Mercury. This "gravity assist" saved valuable fuel and became an important part of many future space missions.

Mariner 10 was the first spacecraft to use the gravity of one planet to reach another.

Mariner 10 first flew by Mercury in March 1974, and it flew by the planet two more times over the next year. The spacecraft took many photographs of Mercury, revealing the heavily cratered nature of the planet's ancient surface. Scientists also discovered that Mercury has a weak magnetic field and lacks a permanent, significant atmosphere.

Magellan Mission to VenusLaunched in 1989, the Magellan spacecraft orbited the planet Venus from 1990 to 1994. Its primary purpose was to map the planet's surface using radar, which unlike visible light can easily penetrate Venus' thick, cloudy atmosphere.

This global topographic map of Venus was created from radar data collected by the Magellan spacecraft. The lowest

elevations are shown in blue, while the highest elevations are shown in reddish pink.

The spacecraft also took electrical measurements and measured the planet's surface topography and gravity. From this mission, scientists learned that Venus' surface is mainly volcanic in nature and relatively young compared with most other terrestrial planets' surfaces. Venus has also been shaped by tectonic activity, but it does not have a complex plate tectonics system like Earth.

Missions to MarsOne of the main reasons humans are so fascinated with Mars is that it is likely the best possible site for future human colonies beyond Earth. Humans could better cope with Mars' temperature ranges than those on other extraterrestrial bodies. Also, evidence suggests that large volumes of ice are present beneath the surface, which humans could use as a water source. An artificial environment would have to be created, though, because humans cannot breathe in Mars' thin carbon dioxide atmosphere. In the meantime, humans have been sending spacecraft and robots to Mars for decades. Some important missions to Mars are described below.Viking

The Viking mission, which lasted from 1975 to 1982, had twin orbiters and landers that studied Mars in great detail. Thousands of images were taken from orbit and from the ground at two landing sites. The landers also

performed experiments, including soil analysis for the presence of life (none was found).

The image on the left, taken by the Viking 2 lander, shows a thin, temporary coating of water ice on the surface of

Mars. The image on the right, taken by the Viking 1 orbiter, shows a rampart crater (or "splosh" crater). When an

asteroid or comet struck the surface to make this crater, mixtures of rock, dust, and melted ice flowed away from the

crater. Evidence like this for water on Mars drove scientists to send more spacecraft to Mars in later years.

Mars Global Surveyor

Mars Global Surveyor (MGS) was on orbiter mission lasting from 1996 to 2006.MGS had a special camera (Mars Orbiter Camera, or MOC) that captured images in detail never before seen for another planet at that time. These images were as detailed as many images taken of Earth. The spacecraft had several other instruments, including a laser instrument (Mars Orbiter Laser Altimeter, or MOLA) that measured the planet's topography. The global topography map below was created from MOLA data.

In this global topography map of Mars, low elevations are shown in blue, and high elevations are shown in red and

white. The left image shows Mars' western hemisphere, and the right image shows the eastern hemisphere.

Mars Exploration Rover

The Mars Exploration Rover mission (2003 to the present) has twin rovers (Spirit and Opportunity) that have studied two different areas of Mars. These rovers have traveled farther on an extraterrestrial body than any other robotic rover so far. They have captured many images showing layered rock, atmospheric events, and ice in the soil. They have also studied the composition of many surface materials.

This image was captured by the Mars Exploration Rover Spirit. The red arrow points to a dust devil, which is a type of

sandstorm somewhat like a tornado.

Mars Reconnaissance Orbiter

Launched in 2005, the Mars Reconnaissance Orbiter has collected a wide variety of data from Mars. Included in the spacecraft's instruments are cameras to study the planet's surface and weather systems, a spectrometer to study surface materials in visible and infrared wavelengths, a radiometer that studies atmosphere conditions, and a radar instrument that detects underground ice. In addition to providing a wealth of scientific data, the Mars Reconnaissance Orbiter has also provided support for other missions and in choosing landing sites for future missions.

This image was taken by the HiRISE camera on board the Mars Reconnaissance Orbiter. The image shows an area

explored by the Mars Exploration Rover Opportunity.

Missions to the Outer PlanetsVoyager 1 and 2

In the summer of 1977, the twin spacecraft Voyager 1 and Voyager 2 were launched to explore Jupiter, Saturn, and some of their moons. Voyager 1 arrived at Jupiter in March 1979; Voyager 2 reached Jupiter in July of the same year. Together, the two spacecraft captured more than 33,000 images and took several measurements of Jupiter and its largest moons. Jupiter's moon Io was discovered to be volcanically active. Prior to this, the Earth was the only body in the solar system known to have current volcanic activity. Both Voyager 1 and Voyager 2 used a gravity assist from Jupiter to then move on to Saturn.Voyager 1 arrived at Saturn in November 1980; Voyager 2 arrived in August 1981. The spacecraft discovered that Saturn's atmosphere, like Jupiter's, is star-like in composition (mostly hydrogen and helium). Many images of Saturn, its rings, and its moons, including Titan, were captured, and other measurements such as winds speeds and temperatures were taken. Voyager 1 left Saturn on a path to leave the solar system. As part of an extended mission, Voyager 2continued on to Uranus with a gravity assist from Saturn.Voyager 2 arrived at Uranus in January 1986. The spacecraft imaged the planet, its moons, and its rings. Magnetic field readings and other measurements were also taken. A gravity assist by Uranus enabled Voyager 2 to continue its extended mission and reach Neptune in August 1989, about twelve years after its launch from Earth. The spacecraft collected similar data as for the other gas-giant systems, including images of the moon Triton. Voyager 2 then left the Neptune system on a path that will take it, like Voyager 1, beyond the solar system.As of May 14, 2010, Voyager 1 was nearly 16.9 billion km from Earth, traveling at a velocity of about 99,320 km/hr relative to Earth. Voyager 2 was about 13.7 billion km from Earth, traveling at a relative velocity of about 85,640 km/hr. It is estimated that both spacecraft will reach the edge of the solar system, known as the heliopause, by the year 2015. At that point, they will enter interstellar space.

This image is an artist's conception of Voyager 1's and Voyager 2's positions as they approach the edge of the solar

system. Voyager 1 is destined to be the first human-made object to leave the solar system.

Galileo

The Galileo spacecraft, which was launched in 1989, studied Jupiter and its four largest moons in great detail from 1995 to 2003. The spacecraft included an orbiter and a probe, which was sent into the atmosphere of Jupiter. The probe revealed that Jupiter's atmosphere has less water than previously thought, wind speeds of 450 mph, stronger but less-concentrated lightning activity than Earth, and a composition very similar to that of the Sun.The Galileo orbiter measured Jupiter's magnetic field, discovered that the planet has rings, took compositional readings, and captured thousands of images of the the planet and its moons. Scientists learned that the moon Io is the most volcanically active body in the solar system, and they found more evidence that a liquid ocean likely exists beneath the icy surface of the moon Europa. The presence of this ocean, which could possibly support life forms, led scientists to intentionally crash Galileo into Jupiter's atmosphere at the end of the mission to avoid a future collision with Europa.

This collage of images shows the main bodies studied by the Galileo spacecraft. Part of Jupiter is shown at left. From top to bottom are Io, Europa, Ganymede, and Callisto. The sizes of these bodies are shown to scale, but their true positions and distances between them are not represented.

Cassini-Huygens

Launched in 1997, the Cassini-Huygens spacecraft arrived at the Saturn system in 2004. The spacecraft originally consisted of two parts—the Cassini orbiter and the Huygens probe. The Huygens probe parachuted through the atmosphere of the moon Titan and landed on its surface in 2005. The probe showed that Titan's atmosphere is mostly nitrogen, methane, and argon, and it photographed Titan's surface while descending and from the ground. Titan appears to have an icy surface, with rivers and lakes of liquid methane and ethane. Also, like many other moons of the outer planets, Titan may have a liquid ocean beneath the surface. In Titan's case, the ocean is probably made of water and ammonia.The Cassini orbiter has made many observations of Saturn, its moons, and its rings. Images have revealed many new discoveries, such as a possible ring system around the moon Rhea and water jets rich in organic molecules shooting from the icy moon Enceladus. These jets suggest that warm, liquid water might be present beneath the icy surface. As with Jupiter's moon Europa, such an environment could be one that supports life.

Benefits of Space TravelIn preparing for the challenges of space exploration, people have developed tools and products that have become very important to enriching human lives.

The technological advances that have been made for space travel and exploration have had a major impact on society. These technologies have led to advances in many different fields. Some of the fields that have been impacted by the space program include communications, computing, weather forecasting, medicine, transportation, and global positioning.

Communications

The technology developed for putting satellites in space has had a marked effect on society. Many private companies now have satellites in orbit around the Earth. These companies are able to sell products and services that make use of data or capabilities provided by the satellites.

Cellular phones depend on satellites, as do many satellite TV and radio signals. Satellites have made global communication a reality.

Personal Computing

NASA can be credited with prompting the development of the personal computer architecture. NASA needed a small computer to be used onboard the Apollo Command Module. This contributed to the development and availability of the hand-held calculator.

Protective Clothing

The material making up modern body armor was developed by the space industry. This material is stronger than steel and much lighter. This makes it ideal for clothing capable of stopping bullets. The space industry is also responsible for developing the material now used in most firefighter uniforms. The material is extremely resistant to heat and fire.

Weather Forecasting

Satellite technology and the ability to launch satellites into space are products of the space industry. Weather satellites have become a very common tool for weather forecasters. It is now possible to track weather on a global scale and to better predict storm patterns. Lives can be saved because people in the path of a dangerous weather event, such as the hurricane shown, can be given advanced warning.

Medical Technology

Many spin-offs of space exploration and research have improved medical technology. Pacemakers, x-ray machines, and portable medical equipment all have their roots in space exploration and research.

Food Preparation

NASA helped facilitate the development of countertop microwave ovens. Before NASA asked contractors to develop small, light microwave units for reheating food on spacecraft, microwaves were very large, heavy, and expensive.

Global Positioning

Citizens, military, and other governmental agencies often use global positioning systems. This is yet another technology that depends on satellites. This technology can locate exactly where something or someone is on Earth as long as they carry a tracking device. Many cars now have global positioning systems in them that link with satellites so that the people driving them always know how to get where they are going.

Motion of the Earth, Moon & Sun

The Earth, Sun, and Moon are constantly rotating. The Earth and Moon are also revolving.

RotationWhen the Earth, Sun, and Moon spin on their own axes, they are performing a motion called rotation. It takes one day, or 24 hours, for the Earth to make one complete rotation on its axis.

Rotation of the Earth on its axis

The rotation of the Earth is responsible for the change between night and day. When one part of the Earth is rotated toward the Sun, it is daytime there. When the same part of Earth is rotated away from the Sun, it is nighttime there.The Sun and Moon appear to rise in the East and set in the West each day. At midday, the Sun appears to be almost directly overhead. But this apparent motion of the Sun and Moon is a result of the rotation of the Earth on its axis. Similarly, the stars appear to move across the sky each night. This is also because the Earth is rotating.The video below shows how the Earth's rotation results in the change from day to night and back again.The Sun also rotates on its axis. The Moon rotates on its axis as well. But we only ever see one side of the Moon, because it rotates at the same speed at which it revolves around the Earth.

RevolutionWhen the Earth revolves, it moves in an orbit around the Sun. The orbit is elliptical, which means that it is similar to an oval in shape.

Revolution of the Earth-Moon system around the Sun

The revolution of the Earth around the Sun and the tilt of Earth's axis are responsible for the changing seasons. It takes the Earth one year, or 365 1/4 days, to make one complete revolution around the Sun. Because the Earth's axis is tilted, different parts of the Earth will be tilted toward or away from the Sun at different times of the year. It is winter where the Earth tilts away from the Sun. The days are short, and the Sun is low in the sky, even at noon. It is summer where the Earth tilts toward the Sun. The days are long, and the Sun is high in the sky at noon.In December, it is summer in the South Hemisphere and winter in the North Hemisphere.

In June, it is summer in the North Hemisphere and winter in the South Hemisphere.

The white lines in the images above show the paths taken by different parts of the Earth as it revolves on its axis. Notice how the Arctic circle stays in the sun almost all day in June and in the dark almost all day in December. Days are shortest in the winter and longest in the summer. There are two days each year when the day and the night are the same length. These are the spring and fall equinoxes.The Moon orbits, or revolves around, the Earth, and it also revolves around the Sun as part of the Earth-Moon system. It takes the Moon about one month to revolve once around the Earth.

Phases of the MoonThe changes in how much of the Moon's lit portion we can see from Earth are called the phases of the Moon.

Half of the Moon is always lit by the Sun. But because the Moon is revolving around the Earth, the amount of that lit portion we can see from Earth constantly changes.

Phases or Orbital PositionThe Sun shines on exactly one-half of the Moon, except during a lunar eclipse. From Earth, we can generally see only the part of the Moon that is lit by the Sun. As the Moon orbits the Earth (once about every 28 days), the amount of the lighted half of the Moon that we can see from Earth changes. These changes give us the phases of the Moon. The Moon makes one complete cycle every 29½ days.

Orbital Positions of the Moon and Corresponding Phases

As the Moon orbits the Earth, different parts of the Moon are lit by the Sun, and different amounts of the lit surface are visible from the Earth. These differences in the Moon's appearance are what we see as the Moon's phases. This diagram shows how each phase corresponds to the Moon's orbital position and to the part of the Moon being lit by the Sun.

Definitions New Moon—The phase that results when the Moon is on the same side of

the Earth as the Sun. During new moon, the entire lighted surface of the Moon is facing away from the Earth. Therefore, the Moon is invisible from the Earth.

Crescent Moon—A crescent moon looks less than half full, but not completely dark.

Quarter Moon—A quarter moon is when half of the side of the Moon facing Earth is lighted and the other half is dark. There are two quarter moon phases in a cycle.

Gibbous Moon—A gibbous moon looks more than half full, but not completely full.

Full Moon—A full moon is the phase that results when the Moon is on the opposite side of the Earth as the Sun. During the full moon, the entire lighted surface of the Moon is facing the Earth. Therefore, the Moon appears brightest on Earth during this phase.

Waxing—From the new moon to the full moon, the moon is waxing—more of its lit surface is visible from Earth each night than the night before.

Waning - From the full moon to the new moon, the moon is waning—less of its lit surface is visible from Earth each night than the night before.

The Sun, Moon, and TidesThe tides of the ocean result from the gravitational pull on the Earth by the Sun and the Moon.

Gravity and TidesTides are a rhythmic rising and falling of sea levels due to gravitational forces from the Moon and Sun. Though the Sun is far more massive, the Moon has a greater effect on the Earth's tides because it is so much closer to Earth. However, the gravity between the Earth and the Sun influences the tides as well.

Daily PatternsThere are both daily and monthly patterns to the tides. The daily patterns are due to the rotation of the Earth about its axis. As the planet rotates about its axis, the Moon's gravity exerts the greatest pull on the side of Earth that faces the Moon.When that area is an ocean, the Moon's gravity pulls the water "upward." In fact, the water forms a "dome" above the Earth's surface as it is pulled toward the Moon. When a body of water is in a "dome" phase, this is called high tide.

The blue line shows the location on Earth represented by the tides shown at the bottom of the image.

High tides happen twice a day, about 12 hours apart because they happen on both sides of the globe. About 6 hours after a high tide, there is a low tide. At low tide, ocean water pulls back much farther from the beach and shore.

Monthly PatternsThe monthly tidal patterns are due to the Moon's orbit around Earth. Spring tides generally occur twice a month—during new and full moons, when the Earth, Sun, and Moon line up in a row. In this arrangement, the gravities of the Sun and Moon work together and have the strongest pull on Earth. This produces the largest difference between high and low tide.

The yellow region shows the effect of the Sun's gravity on the tides, and the pink region shows the effect of the Moon's

gravity on the tides.

Neap tides occur during quarter moons, when the Sun and Moon form a right angle. Due to their arrangement, the gravitational forces of the Moon and Sun partially cancel each other out. So, unlike spring tides, there is only a moderate difference between the sea levels at high and low tides.

The yellow region shows the effect of the Sun's gravity on the tides, and the pink region shows the effect of the Moon's

gravity on the tides.

The tides that occur during all of the other possible arrangements of the Earth, Sun, and Moon are called intermediate tides.

EclipsesThere are two types of eclipses involving the Sun and Moon: solar eclipses and lunar eclipses.

A lunar eclipse occurs at the full moon, when the Moon moves through the Earth's shadow. A solar eclipse occurs at the new moon, when the Moon moves directly between the Earth and the Sun.

Solar EclipsesDuring a solar eclipse, light from the Sun is blocked out of view for a certain part of the Earth by the Moon. During a solar eclipse, the Earth, Moon, and Sun are aligned as shown below.

Lunar EclipsesDuring a lunar eclipse, light from the Sun is kept from reaching the Moon because it is blocked by the Earth. During a lunar eclipse, the Moon, Earth, and Sun are aligned as shown below.