Robotic Missions in Sun-Earth Space - The Sun-earth Connection

One of NASA's primary research goals is to learn about the Sun-Earth connection. The Sun bathes the Earth with life-sustaining warmth. It also emits radiation, charged particles, and other substances that can disrupt electrical systems and electronic equipment vital to modern societies. Since the beginning of the space age scientists have put satellites in Earth orbit to monitor the Sun and its effects on Earth. During the 1990s NASA began sending spacecraft even farther out into space to investigate solar phenomena that directly impact our planet.

Lagrange points are places found in the space around a large body (like the Earth) orbiting another large body (like the Sun). At a Lagrange point, the gravity of the Earth and the gravity of the Sun are balanced, so that a satellite can remain at one of these points and not be pulled away by the gravity of either of the larger bodies. The concept was discovered by the Italian-born French mathematician Joseph-Louis Lagrange (1736–1813).

There are five Lagrange points in the Sun-Earth system. Because the Earth constantly orbits the Sun, the Lagrange points constantly move also. However, they remain a fixed distance from the Earth and the Sun. The L1 point is roughly one million miles from the Earth, between the Earth and the Sun. The L2 point is approximately the same distance in the opposite direction, outside the Earth's orbit. Point L3 is located on the opposite side of the Sun from the Earth at a distance equal to that of the distance from the center of the Sun to the center of the Earth. The L1, L2, and L3 points lie on a centerline that bisects (cuts through the middle of) the Earth and the Sun. The L4 and L5 points are located at the end points of two equilateral (equal-sided) triangles whose shared base is the segment of centerline that falls between the center of the Earth and the center of the Sun.

Any object at the L3 point is always hidden from Earth view by the Sun. Point L3 has been an intriguing location to science fiction writers who fantasize that an unseen mirror-image of Earth could be there. The idea was popularized in the 1951 movie The Man from Planet X.

Solar Wind

Solar wind is a constant flow of charged particles from the Sun into space. It moves away from the Sun at a speed of 200 to 400 miles per second and flows across the solar system reaching just past Pluto. The region of outer space exposed to the solar wind is called the heliosphere. Helios was the mythical Greek god of the Sun. His Roman counterpart was named Sol.

Astronomers see visible proof of the solar wind when they look at comets. Comet tails are blown outward by the solar wind and always point away from the sun. The solar wind also contributes to the magnificent auroras seen on Earth. An aurora is a magnetic phenomenon that causes colorful streaks of bright light to appear in the upper atmosphere near Earth's two polar regions. Aurora is a Latin word meaning dawn. It was also the name of the mythical Roman goddess of dawn.

The aurora centered on Earth's northern hemisphere is called aurora borealis or the northern lights. The southern lights are known as aurora australis. Usually auroras are only visible to people in the northern and southern regions of the Earth. For example, people in Canada, Alaska, and northern states are far more likely to see auroras than people in the southern United States.

The solar wind constantly pushes and shapes Earth's magnetosphere. This is the region of space around the Earth dominated by the planet's magnetic field. (See Figure 6.9.) The magnetosphere helps protect Earth from dangerous electromagnetic radiation moving through space. The outer boundary of Earth's magnetosphere is called the bow shock.

The speed, composition, density, and magnetic field strength of the solar wind are not constant but vary depending on conditions on the Sun. A "gust" in the solar wind can energize Earth's magnetosphere, producing beautiful auroras, but wreaking havoc on sensitive electronics.

Sunspots

Sunspots look like dark blemishes on the surface of the Sun. They are actually areas of plasma that are slightly cooler than their surroundings due to magnetic activity. Sunspots can be enormous in size, even bigger than planet Earth. Sunspots can appear, change size, and disappear. Each typically lasts from a few hours to a few months. Most sunspots are visible only through telescopes.

Sunspots were first identified in western literature by the Greek philosopher and scientist Theophrastus (c. 372–287 BC). However, some historians believe that Chinese astronomers knew about sunspots centuries before this. After the invention of the telescope in the 1600s many scientists reported seeing dark spots when they looked toward the Sun. Galileo argued that these spots were clouds moving across the Sun's surface. However, most astronomers of the time believed they were seeing a small unnamed planet moving near the Sun.

During the late 1800s European astronomers searched for this mysterious planet thought to orbit between Mercury and the Sun. It was called Vulcan. In 1826 an amateur German astronomer named Heinrich Schwabe (1789–1875) was looking for the planet Vulcan. He began keeping detailed records of the number of sunspots he observed every day. After nearly two decades of doing this he noticed a cyclical pattern to the data. By the early 1900s scientists knew that the number of sunspots peaks approximately every eleven years and then drops off dramatically. (See Figure 6.10.) This is called the solar cycle. The period of peak sunspot activity is called the solar maximum.

In 1952 the International Council of Scientific Unions proposed that scientists around the world cooperate in conducting extensive earth science investigations between July 1957 and December 1958. This was the next expected solar maximum. Scientists hoped to collect large amounts of data on the relationship between sunspots and solar activity and the resulting effects on Earth's magnetic fields. This research remained a priority throughout the space age. The most recent solar maximum occurred in 2000–01. The next peak is expected in 2011–12.

Solar Flares

Solar flares are sudden energy releases that burst out from the Sun near sunspots. They are most common during the solar maximums. Solar flares emit electromagnetic radiation, and can include energy particles and bulk plasma. They can last from minutes to hours.

Solar flares that erupt in Earth's direction can shower the planet with energetic particles within thirty minutes. FIGURE 6.9 The Earth and the magnetosphere Adapted from Roger P. Briggs and Robert J. Carlisle, "Figure 4-12. A 'Side View' of the Earth and Magnetosphere Showing Some of the Important Regions," in Solar Physics and Terrestrial Effects: A Curriculum Guide for Teachers, Grades 7-12, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Space Environment Center, June 1996, http://www.sel.noaa.gov/Curric_7-12/Chapter_4.pdf (accessed December 28, 2005)Solar flares are associated with auroras and magnetic storms on Earth and can cause radio interference if large amounts of x-ray radiation are released.

Scientists classify solar flares by the intensity of their x-ray radiation and its effects on Earth. These classifications are:

• C Class—Small flares with few consequences to Earth systems.
• M Class—Medium-sized flares resulting in brief radio blackouts in Earth's polar regions. May cause some minor radiation storms.
• X Class—Large flares that can cause worldwide radio blackouts and long-lasting radiation storms on Earth.

Auroras typically appear on Earth about two days after a solar flare occurs. On July 14, 2001, a powerful solar flare occurred that resulted in the northern lights appearing as far south as Florida. In November 2003 the largest solar flare ever recorded burst out from the Sun. Luckily it was pointed away from Earth and did not cause any major problems.

Solar Prominences

Solar prominences are giant clouds of dense plasma suspended in the Sun's corona (outermost atmosphere). They are usually calm, but occasionally erupt and snake out from the sun along magnetic field lines. Prominences can break away from the Sun and hurtle through space carrying large amounts of solar material.

FIGURE 6.10 The sunspot cycle, 1750–September 2005 Adapted from "Monthly Averages (Updated Monthly) of the Sunspot Numbers," in The Sunspot Cycle, National Aeronautics and Space Administration, Marshall Space Flight Center, October 25, 2005, http://science.nasa.gov/ssl/pad/solaaar/greenwch/spot_num.txt (accessed December 28, 2005)

Coronal Mass Ejections

Coronal mass ejections (CMEs) occur when billions of tons of particles are slung away from the sun by broken magnetic field lines. CMEs move away from the Sun into space at a tremendous speed (millions of miles per hour). These ejections happen frequently and are most common during the solar maximum, when they can occur several times a day. CMEs are known informally as solar storms.

When a CME blasts toward Earth the planet's magnetosphere is hit within two to four days by a cloud of electrically charged particles. This cloud distorts the entire magnetosphere, as shown in Figure 6.11. The resulting disturbance is called a geomagnetic storm. These storms can harm satellites and disrupt telecommunications and electric power generation around the world.

Space Weather

The term space weather was created by scientists to refer to the overall effects of solar activities on the space around Earth (or geospace). Space weather encompasses all the solar phenomena (solar wind, sunspots, flares, prominences, and coronal mass ejections) and the resulting conditions in Earth's magnetosphere and upper atmosphere.

The National Oceanic and Atmospheric Administration (NOAA) monitors space weather and issues warnings about geomagnetic storms and solar radiation storms (increases in the number of energy particles in geospace). The NOAA's Space Environment Center (SEC) is located in Boulder, Colorado. It serves as the national and worldwide warning center for space weather disturbances. The Space Weather Operations branch is jointly operated by NOAA and the U.S. Air Force.

The SEC uses a scale system to characterize space weather storms according to severity, effects, and frequency. Geomagnetic storms are ranked from level G1 (least severe) to G5 (most severe). Solar radiation storms are similarly ranked from level S1 to level S5. Minor storms occur far more frequently than major storms. The SEC also ranks radio blackouts from level R1 to level R5. These blackouts are caused by x-ray emissions from the Sun that disturb Earth's ionosphere.

FIGURE 6.11 Geomagnetic storm effects Norm Cohen and Kenneth Davies, "Figure 5. An Ejection from the Sun Travels to Earth and Distorts Earth's Magnetic Field, Resulting in Geomagnetic Activity," in Space Environment Topics: Radio Wave Propagation, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Space Environment Laboratory, 1994, http://www.sec.noaa.gov/info/Radio.pdf (accessed January 31, 2006)

Monitoring of space weather is important. A major solar event like a coronal mass ejection can cause a tremendous inflow of electrical energy into Earth's atmosphere. This is disruptive to electrical power grids and telecommunication systems on the ground and dangerous for Earth-orbiting satellites. NASA also worries about possible harmful exposure of astronauts to excessive radiation levels during solar storms.

Ongoing Solar Exploration Missions

NASA and other space agencies operate a number of missions dedicated to studying the Sun-Earth connection. Table 6.5 describes major missions ongoing as of February 2006. Most of these spacecraft are stationed in Earth orbit and study the effects of solar activity on the magnetosphere or upper atmosphere. Some of the spacecraft are in orbits far from Earth or on other paths designed for optimal study of the Sun-Earth connection. NASA spacecraft support several science programs, including Explorer, Discovery, and ISTP.

INTERNATIONAL SOLAR-TERRESTRIAL PHYSICS (ISTP) PROGRAM

During the 1990s NASA teamed with the European and Japanese space agencies in a project known as the International Solar-Terrestrial Physics (ISTP) Science Initiative. The purpose of the ISTP project is to combine international resources to conduct long-term investigations of the Sun-Earth space environment. The program includes both ground-based studies and space missions. The space activities fall into two categories: terrestrial (Earth-directed) and solar (Sun-directed).

Terrestrial space missions rely on satellites in Earth orbit that gather data about the sun's effects on the planet. This effort is supported by two other U.S. federal agencies—the National Oceanic and Atmospheric Administration (NOAA) and the Department of Energy's Los Alamos National Laboratory (LANL).

As of 2006 ISTP's solar space missions are ongoing. They are conducted by spacecraft called Cluster, Geotail, Polar, SOHO, and Wind. All but SOHO and Wind are kept in Earth orbit. SOHO and Wind are positioned a million miles from Earth to get a better look at the Sun.

SOHO and ACE (an Explorer mission) are in small orbits around the L1 Lagrange point. ACE collects samples of the solar wind and other particles emitted from the Sun. The spacecraft is equipped with a sophisticated communications system that allows it to transmit data very quickly. Many solar storms reach the L1 point an hour or more before hitting Earth. ACE is able to give scientists advance warning of impending storms that could disrupt Earth's magnetic field. The spacecraft has its own propulsion system and enough propellant to last until approximately 2019.

SOHO is the largest and most sophisticated solar observatory ever developed. It includes twelve instruments that collect data about the Sun's inner and outer workings and the solar wind. SOHO observes the Sun in visible light and four different wavelengths of extreme-ultraviolet radiation. This is ultraviolet radiation that lies TABLE 6.5 Ongoing solar missions Created by Kim Weldon for Thomson Gale, 2004closest to x-rays in the electromagnetic spectrum. SOHO sensors can detect waves of pressure under the Sun's surface, a process called helioseismology.

TABLE 6.5
Ongoing solar missions
Name	Primary mission sponsors	NASA science program	Launch date	Location in space	Mission
aEuropean Space Agency
bInternational Solar Terrestrial Physics
cJapan Aerospace Exploration Agency
SOURCE: Created by Kim Weldon for Thomson Gale, 2004
ACE (Advanced Composition Explorer)	NASA, California Institute of Technology	Explorer	8/25/1997	L1 orbit	Samples the solar wind, particles emitted during solar flares, and high-energy galactic particles. Monitors space weather and provides warning of impending geomagnetic storms.
Cluster	ESAa	ISTPb	7/16/00 and 8/9/00	Earth orbit	Includes 2 pair of two satellites that study the solar wind and its interaction with Earth's magnetospheric plasma.
FAST (Fast Auroral Snapshot Explorer)	NASA, University of California at Berkeley	Explorer	8/21/1996	Earth orbit	Studies Earth's aurora. Also supports ISTPb.
Geotail	NASA, JAXAc	ISTPb	7/24/1992	Earth orbit	Studying the dynamics of the Earth's magnetotail.
IMAGE (Imager for Magnetopause-to-Aurora Global Exploration)	NASA, Southwest Research Institute	Explorer	3/25/2000	Earth orbit	Satellite imaging Earth's magnetosphere. Also supports ISTPb.
RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)	NASA, University of California at Berkeley	Explorer	2/5/2002	Earth orbit	Explores the basic physics of particle acceleration and explosive energy release in solar flares. Named after the late NASA physicist Reuven Ramaty.
SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer)	NASA, University of Maryland, German Universities	Explorer	7/3/1992	Earth orbit	Investigating the composition of local interstellar matter and solar material and the transport of magnetospheric charged particles into Earth's atmosphere. Also supports ISTPb.
SOHO (Solar and Heliospheric Observatory)	NASA, ESAa	ISTPb	12/2/1995	L1 orbit	Solar observatory studying the Sun's internal structure, heating of its extensive outer atmosphere, and the origin of the solar wind.
Stardust	NASA	Explorer	2/7/1999	Comet encounter	Sampling ship sent to intercept comet Wild 2 as it orbits the sun and collect particle samples. Expected to return to Earth in January 2006.
TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics)	NASA, John Hopkins University Applied Physics Laboratory	Solar Terrestrial Probes	12/7/2001	Earth orbit	Studying solar influences on Earth's mesosphere and lower thermosphere/ionosphere.
TRACE (Transition Region and Coronal Explorer)	NASA, Lockheed Martin Solar & Astrophysics Laboratory	Explorer	4/1/1998	Earth orbit	Studies the connection between the Sun's magnetic fields and heating of its corona.
Ulysses	NASA, ESAa	Sun-Earth Connection	10/6/1990	Solar orbit	An observatory in polar solar orbit that studies the solar wind throughout the heliosphere.
Wind	NASA	ISTPb	11/1/1994	Traveling a path with multiple loops around L1, Earth, and L2	Investigating solar phenomena and their effects on Earth's magnetosphere. Provides almost continuous monitoring of the solar wind.

SOHO has captured hundreds of images of solar events, such as solar flares and coronal mass ejections, and has discovered more than 400 comets. The spacecraft is operated by NASA and the ESA. Dozens of universities and research institutions worldwide participate in SOHO as part of the ISTP initiative. SOHO is expected to remain in orbit at least until 2013.

Wind is an observational spacecraft that carries an array of scientific instruments. Following its launch in 1994 Wind was designed to follow a complicated path that winds back and forth between the L1 and L2 points. The path is a figure-eight shape with numerous loops around Earth in between. Wind collects data on plasma, energetic particles, and Earth's magnetic field. The spacecraft provides nearly continuous monitoring of the solar wind approaching Earth.

Ulysses is a multi-functional spacecraft on a far-flying mission. It travels around the Sun on an orbital path that reaches out near Jupiter. As shown in Figure 6.12 the Earth and Jupiter orbit the sun in the same plane. This is called the ecliptic plane and appears as a circle within the diagram. All of the planets except Pluto orbit the Sun in the ecliptic plane. Ulysses follows a different path that takes it high above and far below the ecliptic plane as the spacecraft circles the Sun.

This provides Ulysses with exposure to vast regions of the heliosphere. The spacecraft investigates the magnetic field and solar wind at various heliospheric altitudes. It can also detect bursts of radio and plasma waves, solar x-rays, and solar and galactic cosmic rays. During its Jupiter flybys the spacecraft studies the magnetic fields surrounding FIGURE 6.12 Ulysses mission trajectory "Trajectory," in Ulysses Mission Overview, National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, November 8, 1999, http://ulysses.jpl.nasa.gov/ (accessed January 31, 2006)that planet. Ulysses is also unique because it flies over the polar regions of the Sun. This provides the observatory with a high-latitude view of the Sun's outer atmosphere.

Ulysses is a joint mission operated by NASA and the ESA. The spacecraft is named after a hero of Greek legend who went on a long sea journey and had many adventures along the way. The story is contained in a book called Odyssey, written by Homer, an epic poet who lived in the ninth or eighth century BC.

Future Solar Missions

Between 2006 and 2010 NASA plans to launch a number of solar missions as part of its "Living with a Star" program to study the solar maximum of 2011–12.