What caused Shoemaker-Levy 9 to split up? (Advanced)

My questions concerns gravitational pull. It is my understanding that the comet segments that recently struck Jupiter were the remnants of a structure (comet) that was pulled apart by the force of Jupiter's gravity during a previous passage. I can appreciate that Jupiter's even huge gravitational "pull" on the closer parts of the structure is stronger than on the farther parts but it would seem (???) that the difference would be quite small because the distance (even when squared) across the structure is small (if consistant with the size of most comets as I understand the subject) and therefore the cohesion between the parts would be more than sufficient to overcome any difference in gravitational pull regardless of the proximity during the passage and the comet or what ever would remain intact. As all particles accelerate under the sum of the forces. Is the weaker pull on the farther particles plus the cohesive force of the particles that make up the structure insufficient to overcome the stronger pull on the closer particles to render the comet into many big chunks?

And why some big chunks and some small ones? At every separation the diffential of gravitational forces would reduce as a function of the smaller distances across the components pieces. Guess it all depends upon the degree of cohesion but I would think (???) of a small rocky ice ball as having some appreciable cohesion in the cold of outer space. Or is it too cold for any degree of cohesion to exist and the comet's particles are only bound by very weak gravitation and molecular forces? There must be some cohesion as there was a formation in the first place and there were still chunks after the break up. What is wrong here? Could not a very high speed impact with a small mass produce the observed results of the break up? How does one explain that the chunks were strung out fairly far apart. Would differential gravitational forces produce that effect? Do not all size pieces behave the same having the same velocity (prior to the break up and all are acted upon by the same mass at essentially the same distances just after the break up even if the break up occurred over many passes. Did not all parts that struck have essentially the same path or did they have quite different paths and were just captured by Jupiter' huge gravitational pull as Jupiter swept through (literally) its orbit?

Are structures similarly wrenched apart as they approach the event horizon of a black hole? This must be the most extreme case of the effect of differential gravity across a structure.

What we're talking about here are tides, the same gravitational effect responsible for the rise and fall of the Earth's oceans. Your understanding is correct. One part of the comet is closer to Jupiter than the other, and since gravity depends on the distance squared, the closer part feels a stronger pull and the farther partfeels a weaker pull, resulting in the body feeling stretched.The strength of the stretching, the tidal force, depends on the distance to Jupiter cubed, the diameter of the comet, and the mass of the Jupiter.

Now, you are rightly puzzled about the cohesion of the comet. You or I would be ripped apart by the tidal force the Earth exerts on us, pulling harder on our feet and less hard on our heads, except that we are not held together by our own gravity --we have strength, cohesion. Similarly, if we pack together a snowball, we can see that it isn't pulled apart by tides, either. However, comets aren't packed together. They are quite tenuous in their construction. They form as ice and other substances slowly freeze onto them out of the vacuum of space on to bits of dust, and then they are subject to bombardent by other comets, as well as being continually pecked at by micrometeorites, the size of grains of sand. The comet is made of lacy pieces of ice of various sizes, held together only by the gravity of its own tiny mass.

That gravity holds it together pretty well, too -- unless it passes too close to a behemoth like Jupiter, where the tidal force stretching it apart will be greater than the gravity holding it together. Remember that Shoemaker-Levy 9 was the first comet observed with this pearl-string structure. Most comets don't pass so close to a planet without running into it!

Computer simulations show that if you pass an object made of small particles held together by gravity too close to a larger object, tidal forces will pull it out into a nice string of particles, just as observed. The pieces stay fairly close together as you expect -- the gravitational forces are fairly similar so they don't aquire too big a range of velocities. However, the gravitational forces on each particle are different -- remember, it was the tidal force, a difference between graviational forces, that pulled them apart in the first place! Over time, they would have been separated by greater and greater distances, until that fateful day when they crashed.

An impact with a small body with enough energy to break up the comet would have lead to a much greater dispersion of velocities. Instead of seeing a pearl-string, we would just see little tiny comet-lets on various orbits, and it would be nearly impossible for them to end up together in a string, even if they stayed in orbit around Jupiter. Gravity doesn't tend to herd things in orbit together. Instead differing orbits tend to get more different. (If you're wondering how, given this, the planets formed at all, you should remember that the early solar system was full of junk like gas, and dust, and things were constantly running into each other and experiencing friction.)

Your question about tidal forces around black holes is very perceptive! Yes, as an object falls into a black hole, it will be stretched by tides, a process known by the technical term "spaghettification." : ) These tidal forces would prevent any human craft using technology we can imagine from traveling into a black hole without being destroyed. (If we could ever discover and travel to a black hole, that is.) Similar strong tidal effects are also expected near neutron stars, which are very dense. (There's a wonderful short story called "Neutron Star" by Larry Niven, if you happend to be a fan of science fiction.)

This page was last updated on July 18, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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