Cassini's Big Finish Begins

Saturn eclipsing the sun, seen from behind by the Cassini orbiter. Earth can be seen as a small dot between the rings on the upper, left-hand side. Image Credit: NASA/JPL/Space Science Institute

(My apologies for the "big finish." But because the other phrase is so prevalent, I just had to find something to say other than "grand finale,” at least once.)

The NASA Cassini mission is winding down, quite literally. On April 26, the Cassini spacecraft became the first to dive between Saturn and its ring system. This begins the spacecraft’s “grand finale” in which it will make 22 amazing orbits and, on September 15, enter Saturn’s atmosphere and burn up. The spacecraft will be useful until the very last moment–it will be sending back data continuously, including measurements of the composition of Saturn’s atmosphere, rotation rate and interior structure.

Until that time, the instrument teams have several new observations to make. These include understanding a Saturn radiation belt, discovered inside the rings in 2004, and taking close-up pictures of the rings and other features.

Read the 2004 Article: New Radiation Belt

The spacecraft will also image Saturn’s cloud tops at close range, weigh its ring system (which will indicate just how old it is), sample the atmosphere of the planet and its rings, and measure Saturn’s internal structure.

Read the NASA PDF Resource on the Mission (published 1997)

Originally called Cassini-Huygens, the mission involves ESA, NASA and the Italian Space Agency. The idea of the mission began in 1982, when the European Science Foundation and the American National Academy of Sciences formed a working group to investigate future cooperative missions. Two European scientists suggested a paired Saturn Orbiter and Titan Probe as a possible joint mission. In 1983, NASA's Solar System Exploration Committee recommended the same Orbiter and Probe pair as a core NASA project. NASA and the European Space Agency (ESA) performed a joint study of the potential mission from 1984 to 1985. ESA continued with its own study in 1986, while American astronaut Sally Ride, in her 1987 report, also examined and approved of the Cassini mission.

In 1988, NASA Associate Administrator for Space Science and Applications Len Fisk wrote to his counterpart at ESA, Roger Bonnet, strongly suggesting that ESA choose the Cassini mission from the three candidates at hand and promising that NASA would commit to the mission when ESA did.

Read the Article: Launch of Cassini Spacecraft

On October 15, 1997, a Titan IVB/Centaur rocket launched from Cape Canaveral Air Force Station in Florida, sending the Cassini orbiter and its Huygens probe on a seven-year, 2.2-billion mile journey to the Saturn system.

Read About the Spacecraft

The mission arrived at Saturn on July 1, 2004. The mission was originally planned for four years. But Cassini-Huygens was so successful that the mission multiple times, eventually to 2017. It has flown past seven of the larger satellites, including giant Titan – which is larger than the planet Mercury. The orbiter passed Titan more than 70 times. Flying within 880 km of the moon, it studied Titan’s orange clouds and nitrogen-rich atmosphere. It also mapped its surface with an imaging radar.

On Christmas Day 2004, Huygens separated from Cassini. Three weeks later, it entered Titan’s thick atmosphere, becoming the first probe to land on the surface of a planetary satellite (other than Earth’s moon). Protected by a heat shield, the probe slowed from 18,000 to 1,400 km per hour in just three minutes. Soon after, a large parachute opened. At a height of about 160 km, the probe began to take pictures and study the atmosphere. For more than two hours, data from Huygens were received and stored on Cassini as it flew overhead.

Read More

To learn more about Cassini, Huygens, Saturn, Titan, the rest of the Saturn system, and the Grande Finale, check out these links.

Read About the Mission

See the Mission Timeline

Read the FAQ for Cassini – The Grand Finale

Read the 2017 Article: Bittersweet feeling as Cassini mission embarks on its ‘grand finale’ ahead of death plunge

Read the 2014 Article: Cassini 10 Years at Saturn Top 10 Discoveries

Read the 2017 April Article: Cassini Completes Final -- and Fateful -- Titan Flyby

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What is an Orbit?

or-bit (ˈȯr-bət) noun. 1. a. A path described by one body in its revolution about another (as by the earth about the sun or by an electron about an atomic nucleus); also, one complete revolution of a body describing such a path. b. A circular path. [Middle English, from Medieval Latin orbita, from Latin, rut, track, probably from orbis]

An orbit is a regular, repeating path that one heavenly body takes around another body. A body which orbits another body is a satellite of the other body. So, the moon is a satellite of Earth, while at the same time, Earth is a satellite of the sun, along with the other planets, comets, asteroids, and many other bodies in the solar system. And remember that the sun orbits the center of our Milky Way galaxy. So, the sun is a satellite of the Milky Way.

A satellite can be classified as natural or artificial. The moon, Earth and other heavenly bodies are natural satellites. All orbiting objects made by humans are artificial satellites. The first artificial satellite was Sputnik 1, launched in 1957. A more recent example is the International Space Station, which was launched in pieces and assembled in orbit.

Most orbiting bodies move along or close to an imaginary flat surface. This imaginary surface is called the ecliptic plane.

What Shape Is an Orbit?

Orbits come in different shapes. All orbits are elliptical, which means they are the shape of an ellipse, similar to an oval. For the planets, the orbits are almost circular. The orbits of comets have a different shape. They are highly eccentric or "squashed." They look more like thin ellipses than circles.

Satellites that orbit Earth, including the moon, do not always stay the same distance from Earth. Sometimes they are closer, and at other times they are farther away. The closest point a satellite comes to Earth is called its perigee. The farthest point is the apogee. For planets and other bodies that orbit the sun, the point in their orbit closest to the sun is perihelion. The farthest point is called aphelion. Earth reaches its aphelion during summer in the Northern Hemisphere. The time it takes a satellite to make one full orbit is called its period. For example, Earth has an orbital period of one year. The inclination of an orbit is the angle the orbital plane when compared with Earth's equator.

This diagram of an Earth orbit demonstrates an elliptical orbit and shows apogee, perigee, aphelion, and perihelon. Image Credit: NOAA

How Do Objects Stay in Orbit?

An object in motion will stay in motion unless something pushes or pulls on it. This statement is called Newton's first law of motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward Earth. A constant tug-of-war takes place between the satellite's tendency to move in a straight line, or momentum, and the tug of gravity pulling the satellite back.

An object's momentum and the force of gravity have to be balanced for an orbit to happen. If the forward momentum of one object is too great, it will speed past and not enter into orbit. If momentum is too small, the object will be pulled down and crash. When these forces are balanced, the object is always falling toward the planet, but because it's moving sideways fast enough, it never hits the planet. Orbital velocity is the speed needed to stay in orbit. At an altitude of 150 miles (242 kilometers) above Earth, orbital velocity is about 17,000 miles per hour. Satellites that have higher orbits have slower orbital velocities.

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What is an Eclipse?

e-clipse (i-ˈklips) noun. 1. a. The total or partial obscuring of one celestial body by another. b. The passing into the shadow of a celestial body. Compare OCCULTATION, TRANSIT. [Middle English, from Anglo-French, from Latin eclipsis, from Greek ekleipsis, from ekleipein to omit, fail, suffer eclipse, from ex- + leipein to leave]

An eclipse happens when one heavenly body, such as a moon or planet, moves into the shadow of another heavenly body. There are two types of eclipses as seen from Earth: an eclipse of the moon and an eclipse of the sun.

An eclipse of the moon...

The moon moves in an orbit around Earth, and at the same time, Earth orbits the sun. Sometimes Earth moves between the sun and the moon. When this happens, Earth blocks the sunlight that normally is reflected by the moon. Instead of light hitting the moon’s surface, Earth's shadow falls on it. This is an eclipse of the moon—a lunar eclipse. A lunar eclipse can happen only when the moon is full.

A lunar eclipse can be seen from Earth at night. There are two types of lunar eclipses: total lunar eclipses and partial lunar eclipses.

A total lunar eclipse happens when the moon and the sun are on exact opposite sides of Earth. Although the moon is in Earth's shadow, some sunlight reaches the moon. The sunlight passes through Earth's atmosphere, which causes Earth’s atmosphere to filter out most of the blue light. This makes the moon appear red to people watching from Earth.

A partial lunar eclipse happens when only a part of the moon enters Earth's shadow. In a partial eclipse, Earth's shadow appears very dark on the side of the moon facing Earth. What people see from Earth during a partial lunar eclipse depends on how the sun, Earth and moon are lined up.

A lunar eclipse usually lasts for a few hours. At least two partial lunar eclipses happen every year, but total lunar eclipses are rare. It is safe to look at a lunar eclipse.

A lunar eclipse. Image credit: NASA

An eclipse of the sun...

Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

There are three types of solar eclipses. The first is a total solar eclipse. A total solar eclipse is only visible from a small area on Earth. The people who see the total eclipse are in the center of the moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.

The second type of solar eclipse is a partial solar eclipse. This happens when the sun, moon and Earth are not exactly lined up. The sun appears to have a dark shadow on only a small part of its surface.

The third type is an annular (pronounced “ANN-you-ler”) solar eclipse. An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, it seems smaller. It does not block the entire view of the sun. The moon in front of the sun looks like a dark disk on top of a larger sun-colored disk. This creates what looks like a ring around the moon.

A solar eclipse. Image credit: NASA

During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the umbra (pronounced “UM-bruh”). This shadow gets smaller as it reaches Earth. It is the dark center of the moon’s shadow. The second shadow is called the penumbra (pronounced “pe-NUM-bruh”). The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.

The umbra and penumbra. Image credit: NASA

Solar eclipses happen approximately once every 18 months. Unlike lunar eclipses, solar eclipses only last for a few minutes.

You should never look directly at the sun. It can permanently damage your eyes. You must use proper safety equipment to look at any type of solar eclipse. More on this later.

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