• October 7, 2013

    When Comets Crack

    by Bonnie Meinke


    As Comet ISON nears the Sun, the big question is whether it will remain intact or shatter into pieces. Since comets are mostly made up of ice, it’s reasonable to suspect that the heat of the Sun causes them to disintegrate. But that’s not the whole story. To discover why comets fall apart, we need to look at the force currently keeping your feet firmly on the ground — gravity.

    Gravity is the weakest of the four physical forces. We can really only see its effects when some massive body is involved, so the best examples of gravity are usually astronomy-related. Despite its relative weakness, gravity is responsible for some pretty profound things: our spherical Earth, its ever-changing oceans, and for keeping all the planets in the solar system orbiting around the Sun.

    Earth’s tides are one way we all commonly experience gravity. The shoreline at the beach comes in for high tide and goes out at low tide because of the Sun-Earth-Moon system. Our Moon is big and close enough that it gravitationally attracts the oceans, causing a bulge on opposite sides of the Earth. The solid Earth spins underneath this bulge every day, causing the calm, (in most places) diurnal cycle of the tides. Tides are a gentle reminder of how gravity of one planetary body can influence another without physically “touching” it.


    This graphic shows how tides are oriented due to the Moon’s gravitational influence on Earth’s oceans. Note: Not to scale. Credit: Wikimedia Commons User Jhbdel


    Don’t let the gentle lapping of the waves fool you, though — tidal forces can be quite violent. A tidal bulge is really just the flexing of a body due to the differential force on the near side versus the far side. Imagine a more extreme flex, and the Earth could go from a basketball to a football and eventually to an arrow. Ocean tides work because the part of the Earth closer to the Moon feels slightly more gravitational pull from the Moon, causing liquid water (an easily malleable fluid) to move and reshape to the tidal bulge. But what might happen if the oceans were more rigid? What if the Moon were bigger or closer? Tides can eventually rip a body (planet, moon, comet) into pieces!  We can see this violent yet beautiful end for comets  when they swing by the giant planets or the Sun.


    The tidal forces of the Sun (yellow) are stretching the satellite (blue/purple) into a football shape. The smaller the distance to that Sun, d, the larger the stretch, Δd. Credit: Wikimedia Commons User CWitte, drawn by Theresa Knott


    The breakup of Comet Shoemaker-Levy 9 is an example from recent history. This comet was pulled into orbit around Jupiter due to the planet’s large mass — and thus its gravity.  As the new orbit brought the comet closer to the surface of Jupiter, Jupiter’s gravity tidally ripped the comet into many smaller pieces. The result was a spectacular light show! Each glowing sub-nucleus, 21 in total, smashed into the planet separately and caused scars that lasted for months. Gravity caused all this turmoil without any direct contact! Tidal ripping is one of the most spectacular ways things break up and involves no collisions, no pyrotechnics, no Hollywood budget! Just gravity. Michael Bay would be jealous.


    Hubble image of the pieces of Comet Shoemaker-Levy 9, which was ripped apart by Jupiter’s gravity in 1994.


    Comet Shoemaker-Levy 9 met its torn-to-shreds fate only after being captured in Jupiter’s gravity and crossing the “Roche Limit.” The Roche Limit is the location where tidal forces from a central mass, such as the Sun or a planet, balance the self-gravity of the satellite. This is the point where stretching becomes breaking. The Roche Limit is not strictly one value; rather it is dependent on the internal density of the satellite. A denser, more compact object may survive closer to the central massive body than a “fluffy” aggregate can. Think of how much easier it is to pull apart cotton candy in your hands than it is a jaw breaker. Internal strength of the satellite comes into play as well. Satellites held together by more than just self-gravity, like spacecraft, can orbit well inside the classical Roche Limit. Now compare that cotton candy to a Nerf ball. 


    Illustration of satellite (blue/purple body) being stretched out and broken up by tidal forces as it nears and crosses the Sun’s Roche Limit (white line).  Credit: Theresa Knott


    Comet orbits are very long and elliptical, so they can bring a comet very close to the Sun. Comet ISON came from the Oort Cloud, thousands of times farther from the Sun than Earth; however, it will pass within 1 million miles (1.6 million km) of the Sun’s searing surface, well within the Roche Limit of 1,475,000 miles (2,374,000 km) from Sun’s center. So will Comet ISON break up as it passes around the Sun? That depends on its internal density and strength. If the comet is a “rubble pile” like many asteroids — just a collection of rocks held loosely together by mutual gravity, the comet will likely be torn to shreds. If the comet has some internal strength, which is likely due to its frozen nature, then it could survive in one piece. 


    Image of asteroid 25143 Itokawa taken by the Hayabusa spacecraft in 2005.  Its “rubble pile” structure is evident.  Credit: ISAS/JAXA


    The wild card here is activity! As Comet ISON nears the Sun, its ices will begin to warm up and sublimate. When it’s grazing the Sun’s surface? That thing will be fizzing like crazy! Increased activity might also mean jets, which can cause rotation. If the comet is spinning faster, centrifugal force might put a physical strain on the comet that makes it even more difficult to stay in one piece and not spin off smaller chunks. So it is possible that even if it is internally strong, it will be compromised because it will be quickly losing its icy rigidity.

    Perhaps now you can understand why we stand to learn so much from Comet ISON as it approaches the Sun and are keenly interested in its activity levels. If it breaks up, we can infer its structure and composition, which in turn tells us about its formation. As Comet ISON nears the Sun this fall, I’m rooting for a break up. How about you?