Planet Impact
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Science Background
Words from the Scientist

How can this be used?

The science background is written by teachers for teachers and students. If you unfamiliar with the information contained in the lesson, this text will give you the background you need to feel confident in using the lesson. Students also can use these pages to do research on related topics or to read as a follow-up to the lesson. The questions marked with an asterisk, *, can be used as higher-level assessment questions.

Science Background:

1. What is gravity?
Gravity is the attraction of every body to every other body due to the masses of each body. The larger the mass, the greater the force. It also depends on the distances: the closer the bodies, the greater the force. Gravity is directed toward the center of a body, and the distance is measured from the center. Gravity keeps the moon going around the earth, the earth going around the sun, and the sun going around the center of the Milky Way.
Gravity is the weakest of the four fundamental forces of nature, yet it is the force that governs motion in the universe. Electromagnetic, weak nuclear, and strong nuclear are the other three forces.

2. What factors determine the force of gravity between two bodies?
The force of gravity (F) depends on the masses of the two bodies (m1, m2), and the distance between the bodies' centers (r). There is a direct proportion between mass and gravitational force: If you double the mass of one body, the gravitational force between them is also doubled. The gravitational force is inversely proportional to the square of the distance: If you double the distance between the two bodies, the force of gravity is reduced to one-fourth its original value. The equation relating these ideas is: F = G(m1m2)/r2 , where G is the universal gravitational constant equal to 6.67 x 10-11 Nm2/kg2 ( or m3/s2kg).

3. What keeps objects in orbit? (What keeps the moon from falling to the earth or the earth from falling into the sun?)
Objects remain in orbit around a massive body due to gravity and their sideways motion. Objects in orbit are moving sideways, approximately at right angles (90 degrees) to the force of gravity. An object would travel in a straight line with a constant speed if it were not for the gravitational attraction of a massive body. The attractive force changes the motion of the object from a straight line to a closed curve, as it begins orbiting the massive body. In effect, the object is "falling" around the massive body

4. What is an unbalanced or net force?
An unbalanced or net force causes changes in an object's speed and/or direction. Only one force acting on a body is unbalanced because there is no counter-force to cancel the force's effects. A person remains at rest when sitting in a chair because there are two balanced forces acting on that person. One of those forces is gravity, which pulls the person downward. The other force is the chair pushing upward on the person. The two forces are equal in magnitude (size) and opposite in direction. So, they balance each other, and the person doesn't change direction or speed. On the other hand, Earth's gravitational influence on the moon is unbalanced. The moon is constantly changing its direction of motion, so it is experiencing acceleration. Any time there is an unbalanced force, the object will undergo a change in direction or speed. So, a change in direction or speed means there is an unbalanced or net force at work.

5. * How does a large body, like a planet, capture a small body, like a comet?
According to Newton's first law of motion, an object in motion tends to remain in straight-line motion at a constant speed unless acted upon by an external, unbalanced or net force. When a comet or asteroid comes close to a body with a large gravitational force, a planet for example, the path of the comet or asteroid is altered due to the unbalanced force of gravity on the body. It moves toward the planet as described in Newton's second law: When an unbalanced force acts on a body, the body experiences acceleration in the direction of the force. A force, which tends to make a body move in a curved path, is called a centripetal force. Occasionally, the comet will be close enough to the planet to become trapped by the gravitational force and will begin orbiting the planet. The comet would like to continue traveling in a straight line, but the planet is pulling the smaller body towards its center, making it travel in a curved path around the planet.

6. Why does the angle of approach affect the ability of a planet to capture a comet?
The more direct the approach, the easier it is for the planet to capture a comet because the comet comes closer to the planet. As discussed above, the closer the objects come to each other, the stronger the force of gravity.

7. What role does speed play in the capture of a comet by a planet?
The faster an object is moving, the greater the kinetic energy. In order for an object to be trapped by the gravity of a planet, the object's kinetic energy (Ek = 1/2 mv2 where Ek = kinetic energy, m = mass of comet and v = speed) must be less than the gravitational potential energy (U = GMm/r where U = potential energy, G = gravitational constant, M = mass of planet, m = mass of comet and r = distance from the center of the planet to the center of the comet). The comet's total energy is equal to the kinetic plus potential energies. But the potential energy is negative, so a comet can only escape a planet's gravitational pull if the kinetic energy is larger than the gravitational potential energy.

8. Does a comet's mass play a role in its capture by a planet? Or, why doesn't a comet's mass affect the path it follows?
When a comet travels near a planet, there is a gravitational force between the comet and the planet (Fg= GMm/r2 where Fg = gravitational force and the others as defined above). This force provides a centripetal acceleration, which changes the comet's path so that it begins orbiting the planet. (Fc= mac where Fc = centripetal force and ac = centripetal acceleration). These two forces are the same. If we set them equal to each other, the mass of the comet factors out of the equation.

Fc = Fg
mac = GMm/r2
ac = GM/r2

The comet's centripetal acceleration is independent of its mass in the same way that the acceleration of a body near the earth's surface is independent of its mass. Two objects dropped simultaneously from the same height will hit the earth at the same time (provided air resistance is negligible).

9. How does the mass of a planet affect its ability to capture a comet?
As shown in the equation above, the amount of centripetal acceleration on a comet depends on the mass of the body causing the acceleration. The greater the acceleration, the more easily the comet's path is changed and the more likely it is to be captured. This means a massive planet can capture a comet more easily than could a less massive planet .

10. What causes a comet to break up, and what are tidal forces?
The force that makes a comet orbit a planet is also responsible for the breakup of a comet. And that force is gravity. Because the gravitational force increases as the distance between the bodies decreases, the force of gravity on the nearer side of a celestial body is stronger than the force of gravity on the far side, and a tidal force arises. These forces can exist between any two celestial objects in orbit about each other. Some celestial bodies are not perfectly rigid, so they become distorted when subjected to such tidal forces. It is as if they are being pushed from the top and bottom, and a bulge forms on either side of the body — one directed toward the central body and the other on the opposite side. But there isn't a force above and below the body. What is happening is that the part of the orbiting body closest to the central body moves toward that body by a larger amount than the middle of the orbiting body. This causes a bulge on the side toward the central body. To explain the bulge on the opposite side, apply the same logic: the middle of the orbiting body feels a greater pull than the far side, so it moves toward the central body more than the outer part. This leaves a bulge of material behind. If a celestial body is very rigid or is not held together well, instead of getting pulled out of shape the tidal forces can actually tear the body apart, as happened with comet Shoemaker-Levy 9.

11. What is the Roche Limit?
The Roche Limit is the distance at which the tidal forces of a planet (or other massive celestial body, such as a star) become greater than the internal cohesive forces of a comet (or other small object). As the comet approaches the Roche limit, the side closest to the planet experiences a stronger gravitational pull than does the far side. Thus the two sides of the comet tend to move apart because they are acted upon by different magnitude forces, and the comet breaks up. This mathematical limit is at different distances for different planets and depends on a planet's diameter.

12. How are centripetal forces related to centrifugal forces?
Centripetal forces are true forces, which cause a body to move in a curved path. The force of gravity on a satellite causes it to orbit a planet. The force is directed toward the center of the planet and causes the satellite to alter its path toward the planet. Otherwise, the satellite would travel in a straight line, tangent to the orbit. Centrifugal forces are pseudo-forces that arise when a body is undergoing a centripetal acceleration. An example of this is the amusement park ride known as the "Round-Up." You stand on the ride and it spins in a circle (and then tips upwards). You feel as if you are being pushed backwards, toward the outside of the ride. This force is a centrifugal force. In reality, the ride is exerting a force on you toward the inside of the circle. Your body would like to go in a straight line, tangent to the circle, and you feel an outward force because of the inward force of the ride that keeps you moving in a circle.

13. What is Shoemaker-Levy 9 (SL9)?
Shoemaker-Levy 9 is a comet discovered by David Levy, Eugene Shoemaker, and Carolyn Shoemaker on the night of March 24, 1993. Instead of seeing a single coma and tail, the threesome discovered a coma in the shape of an elongated bar and several tails extending beyond it. Later, more detailed photographs showed the bar to be many individual fragments of the original comet. From July 16, 1994 to July 22, 1994, these fragments of Shoemaker-Levy 9 crashed into Jupiter. This was the very first time that scientists knew ahead of time where to view the collision of two bodies in space. The impacts were observed by amateur and professional astronomers, along with other scientists. The impact was recorded by satellites and telescopes, both Earth-based and space-based.

14. What is the background of the discoverers of Shoemaker-Levy 9?
Eugene Shoemaker was a retired geologist whose interest in comets and meteorites led him to search the world for craters that recorded their impacts. Carolyn Shoemaker, the wife of Eugene, is a planetary astronomer who collaborated with her husband throughout his career. David Levy, an amateur astronomer, has worked closely with Eugene and Carolyn Shoemaker for years. He has discovered 21 comets; eight of them with his own home telescope.

15. How big was Shoemaker-Levy 9 before and after the breakup?
According observations by the Hubble Space Telescope, the comet was at most 5 km in diameter before the breakup. As it approached Jupiter, the comet broke apart into at least 21 pieces, but the sizes are uncertain. The diameters of the brightest pieces appear to have been 2 to 3 km.

16. When did the comet break up and how long did it take it to collide with Jupiter?
When Shoemaker-Levy 9 apparently broke up on July 7, 1992, its distance from the center of Jupiter was about 91,000 km, or about 1.3 Jupiter radii. The fragments collided with Jupiter over a seven-day period starting with fragment "A" on July 16, 1994 and ending with fragment "W" on July 22. The period between the breakup and the collision was a little more than two years. During that time, the fragments of Shoemaker-Levy 9 moved farther and farther away from each other. See the images of comet SL9 taken six months apart in the Grab Bag section of this document.

17. If Shoemaker-Levy 9 wasn't discovered until 1993, how do we know it broke up in 1992?
Based on the path the comet followed after discovery, the day when the elliptical orbit of the comet brought it closest to Jupiter was calculated to be July 7, 1992. Tidal forces were strongest when the comet was closest to Jupiter, which is when scientists believe the comet broke up.

18. Why did Shoemaker-Levy 9 crash on Jupiter?
Comets usually orbit the Sun but Shoemaker-Levy 9 was captured by Jupiter's gravity and appears to have orbited the planet for about two decades before the breakup. After Shoemaker-Levy 9 broke into fragments, it was in an orbit around Jupiter that had a period of two years. The energy lost in the breakup of the comet lowered the point of closest approach ("perijove") of the subsequent orbit to within one Jupiter radius of that planet's center.

19. How often does a comet/asteroid collide with Earth?
According to David Levy, a half-mile-wide object should hit the Earth on the average of once every 100,000 years. However, small objects the size of a grain of sand or a piece of gravel hit the Earth each minute. The frequency of a 100-m asteroid/comet hitting Earth is about once every 100 years. The chances could be higher or lower because these small objects are not easy to see with our telescopes, so their number is not well known.

20.* How are solar system objects affected by gravity-induced impacts?
The craters on the moon were caused by impacts with other objects. Craters on Earth are evidence that large objects have hit Earth. Many scientists believe that an asteroid or a comet was responsible for the extinction of the dinosaurs. The current theory of the formation of the moon is linked to a collision or close encounter with a very large body. The oceans are believed to have formed from the impacts of many water-rich planetesimals and cometesimals. An asteroid hit the sparsely populated region of Tunguska, Siberia on June 30, 1908, causing destruction of many trees and reindeer. Craters on most solar system bodies provide evidence of collisions with asteroids or comets. If the impacted body is small, it can be forced into a different orbit and find itself captured by a nearby larger body. Some astronomers believe that the moons of Mars are really asteroids that ventured too close to the planet and were trapped by its gravity.

21. How fast was Shoemaker-Levy 9 traveling and how much energy did it have when it hit Jupiter?
The fragments of Shoemaker-Levy 9 were traveling at an impact speed of 60 km/sec when they struck Jupiter with a kinetic energy equivalent to 600 times the world's estimated nuclear arsenal.

22. What did the impact sites tell us about Shoemaker-Levy 9?
Scientists are still not certain whether the Shoemaker-Levy 9 was a comet or an asteroid. At present, many scientists favor a cometary origin. But we may never know the answer because comets and asteroids have so much in common. Comets and asteroids are both small bodies. Both are primordial, having formed 4.6 billion years ago. And they can be found near Jupiter. Comets generally contain a large amount of water; asteroids do not. Some analyses tend to favor that Shoemaker-Levy 9 was a comet. One such analysis shows the nuclei had comae before the impacts. However, some data still leave doubt as to the origin of Shoemaker-Levy 9. One finding, for example, reveals an absence of a strong indication of water in the impact debris.

23. What did the impact sites tell us about Jupiter?
The impacts into the Jovian atmosphere have provided scientists with a natural tracer of Jovian winds. The high-speed easterly and westerly jets turned the dark spots into "curly-cue features." Hubble's ultraviolet observations showed the debris particles sinking into Jupiter's atmosphere. The observation provided a three-dimensional perspective of Jupiter's wind patterns. At lower altitudes the impact debris flowed east and west; in the higher stratosphere they moved primarily from the poles toward the equator because auroral heating drove them in that direction. Auroral heating is caused by the interaction of ions with Jupiter's strong magnetosphere, producing heat and light.

24. What lasting effects did Shoemaker-Levy 9 make on Jupiter?
The individual impact sites are fading, leaving behind a thin band of debris in the Jovian atmosphere. The material left from Shoemaker-Levy 9 will continue to sink into the depths of Jupiter's atmosphere, leaving no permanent change in Jupiter's appearance.

Words from the Scientist:

Patience is one of the marks of a good scientist — things rarely happen quickly, and you have to be lucky (and alert) to catch them when they do happen. The collision of comet Shoemaker-Levy 9 (SL-9) into Jupiter was one of those times when I was in the right place at the right time, and I was to get in on some real scientific action. Normally, science proceeds incrementally, with small gains accumulating slowly over time. Yet when we realized SL-9 was going to hit Jupiter, we had a chance to observe a process that used to be common in the solar system, but lately has become rare: the accretion of a planetary building block (a "planetesimal") into a planet. Planets were formed out of millions of these planetesimals, 4.5 billion years ago, but the formation process petered out pretty quickly. We were fortunate to be able to see an impact!

We were fortunate in many ways — the impact occurred when Jupiter was easily visible from Earth, and occurred right on the "terminator limb" — the edge of the planet (as seen from Earth) where dawn was occurring. While many bemoaned the fact that the actual impact was hidden by the edge of Jupiter, this arrangement gave us the best possible view of the plume that resulted — the really interesting (and unpredicted) effect. The plume images made by the Hubble Space Telescope are even more awe-inspiring when you understand the geometry and realize that some of the observations are glowing hot in Jupiter's shadow, not lit by the Sun.

Before the impacts, there was much speculation about what would happen and we tried to arrange our experiments to observe the predicted effect. But we were wrong on almost every count. Instead of small, white clouds (such as occur in the earth's atmosphere when a large meteor passes through), we saw large dark circles. Instead of rings of waves expanding from the impact sites like ripples on a pond, we saw nothing beyond the "splashdown marks" of plumes of material blown off the planet during the impact. We did see the slow change in shape and position of the dark material in Jupiter's stratosphere, as the winds in this otherwise transparent region moved the material around, and this went pretty much as expected. But otherwise we were looking at the unknown. This is where science gets really exciting — when we observe things we didn't expect to see.

The excitement was very high in the control room of the Hubble Space Telescope when the first images of the first impact came in. There is a video of the scientists there, gathered around the monitor, watching the data appear. They are sober, controlled, until the second image shows a bright blob right where the first image showed one — it's not an artifact, the collisions are causing a real effect! The sober faces break out in grins and shouts (all except Reta Beebe — she told me later she was thinking about all the work we would have to do to understand this stuff). Scientists are human beings, and the release of seeing something after months of planning and work was palpable.

The SL-9 impact was, to me, the best of science: we observed something, made explanations and predictions, and observed some more. When the further observations didn't match the predictions, we revised our explanations and made more predictions. The cycle goes on and on, providing successively more accurate views of reality.

One final point — one mustn't forget history. It turns out that the great astronomer Cassini, while observing Jupiter from the Observatoire de Paris in the early 17th century, recorded the occurrence and evolution of a black spot in the middle of Jupiter. Over the course of a week it got larger and dissipated. This singular observation was forgotten until after the SL-9 impact — apparently Cassini had seen the mark left by another body bombarding Jupiter. Never throw out an observation just because you can't explain it!

Alex Storrs
Towson University
8000 York Road
Towson, MD 21252


See the Grab Bag page for a complete list of Web sites, books, and other materials that can be used as references on gravitational forces, comets, and SL-9's impact on Jupiter.

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