How (Some) Comets Survive Solar Death Dive

Some comets get vaporized when they dive through the sun's "comet kill zone," yet others survive -- why?

Consider a tale of two comets. Last July, NASA's Solar Dynamics Observatory (SDO) satellite was on hand to witness what happened when the comet SOHO (a.k.a. C/2011 N3) passed just 100,000 kilometers above the Sun's photosphere. Poor SOHO was utterly destroyed by the encounter.

Five months later, in December, comet Lovejoy (a.k.a. C/2011 W3) also passed through the Sun's "comet kill zone," roughly 140,000 kilometers above the photosphere - but Lovejoy survived, although it did lose a lot of mass in the process.

Why did Lovejoy survive, and SOHO perish? Astronomers have puzzled over this phenomenon since the 1980s, and now a team of astronomers think they have the answer to why some comets make it through this solar trial by fire, and others don't. They presented their findings March 30 at the National Astronomy Meeting in Manchester.

Team co-leader John Brown, Astronomer Royal for Scotland, explains that the secret lies in the mass and orbital paths of comets.

Most of the dusty iceballs we call comets hail from when our solar system first formed, and usually pass their time orbiting around the Oort Cloud. But sometimes those orbits are disrupted, and the comets find themselves redirected toward the sun.

Once a comet enters our inner solar system, the ice melts and forms long tails that are swept behind them by the solar wind. How much material a comet sheds depends on its size. Comet Hale Bopp, for instance, was huge, with a mass around 10 million million tons, so it survived its passage through the "kill zone" just fine.

Smaller comets, which are more common, have masses around 1,000 tons. These are the bodies that vaporize, usually from the combined factors of sunlight and friction from atmospheric gas.

Comets also shed their mass in different ways, according to Brown et al.'s analysis. "Sunplungers" have orbital paths that reach into the lower atmosphere, about 7,000 kilometers beneath the photosphere.

In this case, sunlight is not the culprit; mass is stripped away by the drag of solar gas that surrounds the comet. This usually happens very rapidly, and such comets are usually destroyed as they collide with the denser layers of the lower solar atmosphere. That kind of explosion should be detectable since they would be similar to solar flares.

Then there are "sunskimmers" like SOHO and Lovejoy. These are the comets that lose their mass through the more conventional combination of sunlight and atmospheric drag, and emit weaker extreme UV radiation that nonetheless should still be detectable. Such comets usually perish in a kind of "slow fizzle"; how long that fizzle lasts depends on their mass.

One reason Comet SOHO was destroyed, and Lovejoy wasn't? The latter had more mass. Lovejoy was able to outlast the slow fizzle.

That was Brown et al.'s hypothesis, anyway, and it fits well with the data SDO collected last year year in the passage of comets SOHO and Lovejoy (and the sad passing of the former). The team is looking forward to the next "sunplunger" to see if that, also, fits their model's prediction Images: Top: Comet Lovejoy dashes from behind the sun after a death-defying dive (SDO/NASA). Middle: NASA astronaut Dan Burbank photographs Comet Lovejoy from the space station on Dec. 22, 2011 (Dan Burbank/NASA)