The Question of the Week activity has ended. The ten questions and answers for the Tracking a Solar Storm fall challenge are archived below.
The Sun is approximately 4.57 billion years old. It started out as a protostar and, over time, has matured into a main sequence star. The Sun, being about halfway through its main-sequence stage, will stay in its current state for another 4 to 5 billion years, all the while increasing in temperature, brightness, and size. Then, as the Sun runs out of fuel, it will expand into a red giant before turning into a white dwarf and slowly cooling over billions of years into a black dwarf.
To learn more, read this article on physical cosmology by Meredith Bower.
The Sun is approximately 4.5 billion years old. Currently the Sun's life cycle is in the main sequence stage of the stars life. The Sun is still halfway through the main sequence star stage; it should take approximately another 4.5 billion years to complete this stage. When a medium star like the Sun uses up some of its gases, it expands to become a red giant star. At this time, it will expand to cover the orbits of Mercury, Venus, and possibly Earth. It will remain that way for about a billion years. Then, the Sun will lose its outer shell, and the core will shrink to become a white dwarf star, a hot, small star. The white dwarf star will eventually cool and stop shining to become a black dwarf, which does not emit light anymore.
-Mr. Egan's class, grade 6
In short, the Sun shines because it is hot. Our star is made of very hot, highly dense gas primarily comprised of hydrogen and helium. Deep in the Sun's core, where temperatures reach 15,000,000 K (27,000,000 F), four hydrogen atoms fuse together to form one helium atom, a process called nuclear fusion. The mass of the four hydrogen atoms is greater than the mass of the helium atom that they form in the fusion process. This difference in mass is converted into energy, which the Sun radiates equally in all directions. The majority of this solar energy comes in the form of the visible light and infrared (heat) regions of the electromagnetic spectrum.
The sun shines because of the heat and light emitted during the nuclear fusion reactions that take place inside its core. There the temperature and the pressure are very high, so the atoms of hydrogen combine to create helium. The mass of a helium atom is lower than the mass of a hydrogen atom, so the difference of mass is converted into energy, according to Einstein's relation between mass and energy (E=mc2). Fusion reactions are accompanied by high emissions of energy, as there are millions of tones of hydrogen that are transformed into helium every second. This energy causes the sun gas to glow, giving off different kinds of light, like infrared light, ultraviolet light, and visible light.
-Ms. Prajea's class, grade 6 and grades 9-12
The sun shines due to its extreme temperature. It is very hot and contains much energy. The sun stays hot because it is powered by the proccess called nuclear fusion located within its core. When hydrogen and helium are fused together, the mass is changed into energy. Stars are made of clouds of dust and gas that pulls inward because of gravity. As the mixture compresses, it gets hotter and hotter until a special reaction occurs. The gas particles join and expand. The process creates energy deep in the center of the star. It is this energy that gives the star heat and light.
-Ms. Florio's class, grade 7
Stars are classified by their temperature and the spectrum of light that they emit, or in simple terms, their color. Using the Morgan-Keenan system, classification is done using a scale of letters: O, B, A, F, G, K, M, L, T, and Y, with O representing the hottest stars and Y representing the coolest. The hotter stars toward the O end of the scale are more blue and white in color, whereas cooler stars toward the Y end of the scale are more red and brown. Our star, the Sun, is relatively cool with a temperature between 5,200 - 6,000 degrees Kelvin (8,900 - 10,340 degrees Fahrenheit), thus earning a classification of G and a color description of yellow. Stars can be further classified according to their luminosity, or brightness, which is done using a system of Roman numerals I through VII. Because our sun is in its main sequence stage, meaning that it is still converting hydrogen to helium through the process of nuclear fusion, it is placed in class V as a dwarf or main-sequence star. These two classification systems, temperature/color and luminosity, are often represented in a scatter plot of stars called a Hertzsprung-Russell diagram. On the diagram below you can see the Sun plotted as a class G magnitude V main-sequence yellow dwarf star.
Although our star is classified as a yellow dwarf, its actual color is white. When viewing the Sun through Earth's atmosphere, the shorter wavelengths of light (violet, blue, green) are scattered, leaving us to see only the longer yellow, orange, and red wavelengths. However, if you were to travel into space above Earth's atmosphere, then you would see all the visible wavelengths of the Sun's light that, when mixed together, are seen as white light. Below is a picture representing the actual color of our Sun.
A G-type star is a star classified with G in the Morgan-Keenan system( meaning it has an average surface temeperature of 5300K-6000K, and mass and size are quite close to those of the sun). Main-sequence is the stage in a star's life while it transforms hydrogen into helium in its core by nuclear fusion, producing and emitting energy. The star doesn't grow or become smaller, so it is in an equilibrium stage. The term "dwarf star" refers to a medium star as regarding its mass, size and luminosity. A yellow dwarf is a main sequence star whose colour is convetionally stated as yellow, colour which in reality is yellowish-white. The Sun appeares to be yellow, sometimes orange, or red due to Earth's atmosphere (photons of the lower end of the spectrum- yellow, orange and red are less easily scattered than the ones of the higher spectrum, with short wavelenghts- blue, indigo and violet). The Sun is actually white because the light it emits is a mixture of all the colours of the rainbow. The human eye can't detect this colour even in the absence of the atmosphere, so from space we see it white as well.
-Ms. Prajea's class, grades 9-12
Stars are classified depending on temperature, luminosity, and spectrum. Spectral class G means that our Sun has a temperature of 5,000 to 6,000 Kelvins and emits radiation whose electromagnetic spectrum is yellow. It also means that our Sun is in yellow dwarf stage. The main sequence is a group of stars centered around the diagonal of Hertzsprung-Russell diagram. These stars are in constant ratio of temperature to luminosity, and these are also the ones in which fusion of hydrogen turns into helium. The dwarfs are main sequence stars, which belong to the luminosity class V. Depending on the spectral type, there are different colors of dwarfs. Our sun is a GV which means that, it is a yellow dwarf.
-Ms. Kafel's class, grades 9-12
The Sun has a north and south pole and rotates on its axis just as Earth does. However, because the Sun is a fluid gaseous body, it does not rotate uniformly as does a solid body like our own rocky planet. By observing the movement of sunspots, scientists have learned that different latitudes of the Sun rotate at different speeds. The Sun rotates faster at the equator and slower at the poles. It takes the Sun only 25 days to rotate at the equator, yet it takes progressively longer, up to 35 days, to rotate at the higher polar latitudes. This is known as differential rotation.
Because the Sun has a differential rotation, scientists need a system to compare locations on the Sun over a period of time, especially for the purpose of tracking sunspots. Astronomer Richard Carrington first noted that sunspots rotate every 27.28 days. This rotation period is known as the Carrington rotation and roughly corresponds to rotation at a latitude of 26 degrees, which is the typical latitude of sunspot activity. Beginning on November 9, 1853, the Carrington rotation system numbers each rotation of the Sun with a unique number called the Carrington Rotation Number. As of November 25, 2013, the current Carrington Rotation Number is 2144.
The Sun has a north and south pole, just as Earth does, and rotates on its axis. However, unlike Earth, the entire Sun doesn't rotate at the same rate because the Sun is not solid, but is instead a giant ball of gas and plasma, so different regions of the Sun don't stick together and thus rotate at different rates. This phenomenon, called differential rotation, makes the Sun's rotation period pretty hard to establish. The Sun rotates every 25 days at the equator, and at the poles the Sun rotates every 36 days. This is known as differential rotation. The Carrington rotation system allows us to determine an average period for the Sun's rotation. It refers to comparing the positions of certain regions of the Sun over time. By following sunspots, visibly dark spots on the surface of the Sun caused by magnetic activity, Richard Carrington calculated the rotation period of the Sun relative to the stars as 25.38 days. This means about 27.27 days as seen from Earth, since our planet orbits the Sun.
-Ms. Stoica's class, grade 6
A variable star is a star that changes brightness as seen from our perspective on Earth. A magnetic star is a star that generates a magnetic field through the differential rotation and convective motions of its electrically charged plasma. Our Sun does both, hence the description magnetic, variable star. First of all, the energy output of our Sun varies by about 0.1 percent over the duration of its solar cycle, resulting in subtle variations in its apparent magnitude (brightness). Secondly, the churning motion of the Sun's convection zone generates magnetic fields that float to the surface (photosphere) and appear as darker (cooler) sunspots, serpentine filaments, and lovely coronal loops. As we learned from question four, the Sun rotates faster at the poles and slower at the equator. Over time, this differential rotation twists the magnetic field lines into knots eventually causing them to break, resulting in violent events such as flares and coronal mass ejections. To learn more about this topic and to see some excellent images of the Sun, visit the American Museum of Natural History website on SunScapes: Our Magnetic Star.
Coronal holes are cooler, low density regions of the Sun's outer atmosphere (corona), where the Sun's magnetic field opens up and allows solar wind to escape. Visibly darker than their surroundings, these features can last for a few solar rotations until the magnetic field lines shift once again. Unlike flares and CMEs that occur more frequently during heightened solar activity, coronal holes are more commonly seen when solar activity quietens. To learn more, read this brief article by Karen C. Fox of NASA Goddard Space Flight Center, SOHO Views Large Coronal Hole Near the Sun's North Pole.
The Sun is a magnetic star, which means that it has a magnetic field. It is caused by the movement of the plasma in the interior part of the Sun, which is the result of convection. The magnetic field affects the plasma, increasing the pressure without gain in density. That magnetized region expands and reaches the photosphere, consequently creating sunspots and coronal loops. The variable star is a star that is characterized by changes in magnetic fields over the course of a solar cycle during which the magnetic poles interchange their positions.
-Ms. Kafel's class, grades 9-12
Coronal holes are areas of the Sun's corona where the magnetic field presents open magnetic field lines (lines of magnetic field emerging from one region that do not return to another region, but extend into space) and solar material. In fact, highly ionized hot coronal plasma is drawn out. Coronal holes are areas where the Sun is darker and colder and the density of plasma is lower than average because some of it flows out.
-Ms. Prajea's class, grade 6
The solar cycle, also referred to as the sunspot cycle, is an 11-year cycle on average. The end of one cycle and the beginning of the subsequent cycle is marked by the reversal of the Sun's magnetic field. Sunspots are characterized by north and south magnetic poles. In one cycle these poles will have a north-south orientation, but in the following cycle they will have a south-north orientation. This reversal happens at the end of the cycle when solar activity is low and very few, if any, sunspots are formed. This quiet period marking the end of one cycle and the beginning of a new cycle is referred to as solar minimum. In contrast, solar maximum occurs in the middle of the cycle when solar activity is at its peak, meaning that numerous sunspots are present on the surface of the Sun and prominences, flares, and CMEs are much more common. The Sun is currently in the period of solar maximum for solar cycle 24. Solar cycle 24 began in January 2008 and is indicating its peak in 2013. Read these three articles to learn more about solar cycle 24: Backward Sunspot, Solar Cycle 24 Begins, and Twin Peaks. Watch this NASA video to see an animation of the Sun reversing its magnetic poles.
Occasionally the solar cycle deviates from its 11-year norm. From 1645 to 1715, a period of about 70 years, the Sun showed very little activity and sunspots were exceptionally rare. This period became known as the Maunder Minimum, named after astronomer Edward Maunder who studied sunspot activity. Interestingly, the Maunder Minimum coincided with the coldest part of the Little Ice Age, a time when Europe and North America experienced extremely cold winters. NASA's Solar Radiation and Climate Experiment has made a possible correlation between low sunspot activity and colder winters on Earth. [Information adapted from Maunder Minimum and Little Ice Age articles in Wikipedia.]
The solar cycle lasts 11 years on average, but there exist shorter solar cycles, about 9 years, and longer ones, about 14 years. There are sunspots visible on the Sun almost all the time during the solar maximum. Some are very large and last several weeks. The recording of solar sunspot activity began in 1755. The 24th solar cycle is the current solar cycle and began on January 4, 2008, but there was minimal activity until early 2010. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14. The Sun went through a period of inactivity in the late 17th century. This was called the Maunder Minimum. Very few sunspots were seen on the Sun from about 1645 to 1715. This period of solar inactivity also corresponds to a climatic period called the Little Ice Age when rivers that are normally ice-free froze and snow fields remained year-round at lower altitudes.
-Ms. Prajea's class, grades 9-12
The solar cycle is a periodic change in the Sun's sunspot activity. The cause of these sunspots is that the magnetic poles of the Sun are shifting. The solar cycles have two parts: the solar maximum, and the solar minimum. A solar maximum is when sunspot activity is at its peak and the Sun's magnetic poles are reversing. The solar minimum however is the opposite when there is little or no sunspot activity. Solar cycles have an average duration of about 11 years. Our sun in its current cycle is about half way done. Currently, it is in solar maximum. However in 2013, after several months of surprising calm in solar activity during solar maximum, it was clear that the Sun's activity had flat-lined. This was very surprising since 2011 was a very active year in terms of solar activity. This unexpected calm has lead scientists to hypothesize that 2011 was merely part one of the solar maximum peak, and the second of this double-peaked solar maximum will be in early 2014.
-Mr. Egan's class, grade 7
A sunquake is a ripple of seismic energy formed by a pressure wave (shock wave) spreading across the surface of the Sun. Solar flares and, more recently, coronal mass ejections, have been found to be sources of the explosions that cause these pressure waves and subsequent quakes. A unique characteristic of solar seismic waves is acceleration. The solar waves accelerate before disappearing. In contrast, on Earth, water ripples from a splash in a pond travel outward at a constant velocity. For more information on sunquakes, read these articles:
A flare is an intense brightening that occurs in the solar chromosphere. Most flares occur around active regions associated with sunspot groups. However, occasionally a flare is observed well away from an active region or sunspot group. These flares are invariably associated with the sudden collapse of a large solar filament (or prominence) and are termed Hyder flares, named for Charles Hyder who published studies of such events in 1967. As these giant plumes of relatively cool, dense gas are buffeted by the winds and currents in the Sun's atmosphere or as new magnetic field lines poke through the Sun's surface beneath them, they can become unstable and suddenly collapse, crashing into the surface of the Sun and sparking a Hyder flare. Large Hyder flares may take 30 to 60 minutes to rise to peak intensity, and then they may last for several hours.
Fun Fact: Filaments and prominences are the same thing, but we use one term versus the other depending on our viewing perspective. A top view will show this feature as a dark ribbon hovering over the brighter surface of the Sun. We call this a filament. A side view, however, will show this feature as a bright loop dancing in the Sun's lower atmosphere contrasted by the black background of space. We call this a prominence. Filaments and prominences are composed of mostly charged hydrogen gas held aloft by the Sun's looping magnetic field, often for weeks at a time.
A sunquake is caused by the large amount of energy coming from a solar flare. Sometimes, this energy is unleashed in a short-lived seismic wave that will penetrate thousands of kilometers in the inner parts of the Sun. After this the wave gradually retreats back to the surface, which causes an outgoing ripple known as a sunquake. There is known to be more solar flares around active sunspot regions. However, some flares happen in areas with no sunspot activity. These are called Hyder flares.
-Ms. Florio's class, grade 7
In some flares, some of the resulted energy is released in the form of a seismic wave that penetrates several thousand kilometers into the solar interior. Over the succeeding hour most of this wave is refracted back to the surface, where its arrival is manifested by an outgoing ripple, called a sunquake. A Hyder flare is a flare that occurs distanced from a group of sunspots. It is described as a sudden disappereance of a dark filament. An important characteristic of Hyder flares is that they rise to their maximum brightness slower than common flares.
-Ms. Prajea's class, grades 9-12 and grade 6
As for the Earth, quakes may also appear on the Sun. They are called sunquakes. They are the result of a sudden release of stored energy, and consequently it leads to the creation of seismic waves that cause shaking of the Sun. Waves rise from the interior and reach the surface of the Sun, so we see a complicated pattern that depends on the waves' frequency. Some frequencies penetrate the Sun deeper than others, and conditions in the Sun's interior can either weaken or strengthen the waves. By observing them we can determine the temperature, density, pressure, and motion of internal layers. Sunquakes are associated with coronal mass ejections and are a direct consequence of solar flares. Hyder flare is the type of solar flare that occurs away from the active region of the Sun or group of sunspots. It is associated with a sudden disappearance of plasma filament on the surface of the Sun.
-Ms. Kafel's class, grades 9-12
Nitrogen and oxygen make up about 99 percent of Earth's atmosphere. When electrically charged particles from the Sun collide with these two atmospheric gases, the gases become energized causing their atoms to move around rapidly. Then, as the oxygen atoms and nitrogen atoms begin to settle down they let go of this energy, releasing it in the form of light that we see as the aurora. The color of the aurora varies from green to red to blue depending on which gas is being energized. Oxygen emits red and green light whereas nitrogen emits blue light.
So what determines whether oxygen creates a green or red aurora? Well, the transition of oxygen atoms is faster for green light than it is for red light. Therefore, green auroras are seen at lower altitudes where the atmosphere is thicker and there are more particles bumping into one another more frequently. At higher altitudes where the atmosphere is thinner and less dense, there are fewer particles bouncing into one another at a less frequent rate, giving the oxygen atoms more time to release energy in the form of red light.
The aurora is caused by charged particles in the magnetosphere and solar wind that collide with the atmospheric atoms and ions. The collisions cause the electrons of the atmospheric atoms to become excited. As the electrons return to their original energy levels, these atoms emit visible light of distinct wavelengths, to create the colors of the display we see. Each atmospheric gas produces a unique color.
-Ms. Stoica's class, grade 6
The aurora also known as the northern lights can be different colors. The aurora is caused by collisions between fast-moving particles from space and the oxygen and nitrogen gas in our atmosphere. The different colors of these beautiful northern lights depends on which gas is being excited by electrons and how much energy is being exchanged. Oxygen emits either a greenish-yellowish light or a red light. The most common color an aurora can be is greenish-yellow. Nitrogen gas gives off blue light. The oxygen and nitrogen molecules also emit ultra-violent light. The ultra-violent light however can only be detected by special cameras on satellites.
-Ms. Florio's class, grade 7
When electrical charged particles, which travel as solar wind, succeed in passing through Earth’s magnetosphere, they reach Earth’s upper atmosphere and strike the gas molecules. They transfer their energy to these gases and excite them to emit light. So when we see auroras, we see in fact the photons of light emitted by the excited gas molecules, which tend to return to their normal stages. Aurora can be produced in day time or night time, but we can see it only when it is dark because otherwise it is exceeded by the sun light. Auroras are seen at regions around Earth’s magnetic poles, especially around the North Pole. The different colors come from different atoms or ions: green and red from atomic oxygen, nitrogen ions and molecules make some pinkish-reds and blue-violet; purple is the appearance of combined colors from nitrogen ions and helium; neon produces the very rare orange. Colors in the aurora also vary with altitude because the molecules of gas lose their energy while getting down to the lower layers of the atmosphere. The ionosphere is home to most aurorae borealis, with 100-300 km being typical. This is where oxygen usually results in green, with red at the top. However, some particularly energetic particles penetrate much deeper into the atmosphere, down to perhaps 80 km. Purple resulted from nitrogen often comes from here.
-Ms. Prajea's class, grades 9-12 and grade 6
Aurora is a phenomenon consisting in shining the upper part of the atmosphere of the earth. They appear on both Poles. Auroras occur as a result of solar wind entering the atmosphere and exciting gases. That causes the emission of light photons of a certain wave length which practically determines a particular color. The color depends on the altitude where reactions take place as well as the variety of the elements involved. The green color is a direct effect of collisions of electrons with atoms of oxygen at an altitude of 110 km. At the height of 200-400 km collisions with oxygen atoms are the source of radiation of the red color. The colors of purple and blue color emerge as a consequence of collisions with atoms of nitrogen.
-Ms. Kafel's class, grades 9-12
Our planet is surrounded by layers of energetic charged particles that are captured from the solar wind and stored in what is called the Van Allen radiation belts. These radiation belts are held in place by Earth's magnetic field and are located in the inner region of Earth's magnetosphere. There is an inner belt, an outer belt, and a transient third radiation belt that was discovered recently by NASA's Van Allen Probes mission. The Van Allen belts are not symmetrically placed around our planet and range in altitudes between 125 and 800 miles. The belts dip closest to Earth's surface in a region off the coast of Brazil in South America termed the South Atlantic Anomaly. Here, in this low lying region, the International Space Station and other satellites in low Earth orbit pass through the radiation belts where for several minutes they are bombarded by the high concentration of charged particles. This can negatively affect data collection and the operation of on-board electronic systems as well as prematurely age computer and other spacecraft components. Astronauts, too, can be affected by the South Atlantic Anomaly and have reported seeing peculiar 'shooting stars' in their visual field while passing through this region.
The South Atlantic Anomaly is where the Van Allen belt is lowest and causes radiation to be the highest with satellites.
-Ms. Eden's class, grades 3-5
The South Atlantic Anomaly is an area where Earth's Van Allen belt gets close to the Earth. This relates to spacecraft and satellites by increasing the levels of radiation that they detect.
-Ms. Talbert's class, grade 8
The South Atlantic Anomaly is a zone where the inner radiation belt is the closest to the Earth's surface, reaching an altitude of 200 km. Here, the Earth's magnetic field is lower than its normal level. Because of the rarefied magnetic field, the charged particles and the radiations can easily reach lower altitudes. Even though the radiation does not directly affect the Earth, it affects the satellites and the spacecraft. It interferes and causes several damages to the equipment and also affects the health of the astronauts. It harms the retina of the astronauts, and it also disables communication for a short period of time. When the spaceships pass through that zone they need more shielding, and also the telescopes cannot provide images and for their safety they enter a stand-by mode. The International Space Station requires extra shielding to deal with this problem. The Hubble Telescope does not take observations while passing through the South Atlantic Anomaly. Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' seen in the visual field of astronauts. The South Atlantic Anomaly is believed to be the reason for the early failures of the Globalstar network's satellites.
-Ms. Stoica's class, grades 9-12
The IRIS spacecraft's orbit and altitude determine how long its instruments can view the Sun. The spacecraft orbits between 390 - 420 miles (620 - 670 km) above Earth's surface in what is called a sun-synchronous polar orbit. This means the spacecraft travels around Earth from north to south, passing nearly directly over the poles, in such a way that it crosses the equator at the same local time each day.
Earth's axis is tilted 23-degrees, which leads to yearly changes in sun angles that coincide with the seasons. In turn, IRIS's orbit is also inclined (about 98 degrees relative to Earth's orbit around the Sun). Thus, IRIS's orbital path rides the terminator – the ever-moving line that separates the illuminated "day" side of Earth from the dark "night" side of Earth. This path allows the spacecraft's instruments to continuously view the Sun at what we consider sunrise (dawn) or sunset (dusk) 24-hours a day for eight months during the year.
However, as Earth travels along the plane of the ecliptic making its annual journey around the Sun, it will routinely eclipse the Sun, temporarily blocking the Sun from IRIS's view. As the spacecraft passes through Earth's shadow, it is unable to make solar observations. This happens during each of the spacecraft's orbits between early November and early February. These eclipse periods can last up to 24 minutes each. Read this article to learn about how the Solar Dynamics Observatory contends with this same issue twice a year: NASA's SDO Observes Earth, Lunar Transits in Same Day
If IRIS were to have been launched into a slightly higher altitude orbit, it would not experience any Earth shadowing or eclipse time at all. However, these higher orbits were already too full or reserved for other spacecraft.
IRIS travels in a polar, sun-synchronous orbit, inclined 97.89 degrees to the
equator, but not at very high distances above Earth's surface ( 620-670 km) in order to avoid as much as possible Van Allen belts where the radiation is extremely high. Its orbital period is about 97 minutes. The altitude and inclination are combined in such a way that the satellite passes over any given point of the Earth's surface at the same local solar time. It travels around Earth, nearly directly over the poles, and it crosses the equator at the same local time each day. The angle between the orbital plane and the axis Earth-Sun remains almost the same. But the sun-synchronous orbits precess around the Earth's axis, and this axis is not in the plane of the ecliptic, so they don't hold a truly constant angle to the Sun. There might be periods of time, around the summer and winter solstices, when one of Earth's poles is in permanent shadow, when IRIS passes through Earth's shadow (when it passes that pole). For maximizing the time of being in permanent sun, IRIS satellite was launched on 27 June, 2013, immediately after the summer solstice.
-Ms. Prajea's class, grades 6 and 9-12
A Sun-synchronous orbit is a geocentric orbit that combines the altitude and the inclination in such a way that an object on that orbit ascends or descends over any Earth latitude at the same local mean solar time. Polar orbit satellite is the orbit that moves north-south direction in a plane perpendicular to the equatorial plane and passing through the poles. So a sun-synchronous polar orbit has an inclination close to 90 degrees and lasts 96-100 minutes. The illumination's angle surface will be nearly the same every time. Its trajectory also enables IRIS to intercept the sun's rays at roughly the same angle at all times. Theoretically, sun-synchronicity would make the sun visible the whole year round, but considering that the Earth is not perfectly spherical, it is not 100% efficient.
When IRIS will be located over the North Pole on the winter solstice, hence during polar winter, on the 22nd of December, the sun will not be visible. The same thing will repeat over the South Pole, during the northern hemisphere's summer solstice, on the 22nd of June. Also, occasionally, the moon will obstruct the spacecraft's view of the sun and since IRIS's trajectory is close to the Earth, the moon will appear large enough to block the sun as it would during an eclipse. In conclusion, the IRIS spacecraft's sun-synchronous trajectory enables it to have a clear view of the sun at most times, but certain exceptions will prevent it from being visible for small intervals.
-Ms. Stoica's class, grade 6
NASA Official: Darlene Gross
Last updated: January 30, 2014