Off on a Comet

By John McCormick

COMETS HAVE BEEN HARBINGERS of doom practically forever but, unlike asteroids or NEOs (Near Earth Objects), we actually know very, very little about them. There are theories that I will discuss, but we don’t even know where comets originate.

Some, perhaps many, of our questions about comets are about to be answered by the European Space Agency. It has recently invested one billion euros in sending a probe to follow comet 67P/Churyumov-Gerasimenko, which the satellite Rosetta caught up with on August 6.

The Rosetta probe is actually named for the Rosetta Stone. For those who skipped ancient history and modern archeology, the Rosetta Stone contained three translations of the same information—a decree by the pharaoh Ptolemy V created about 200 B.C.E. in Greek, Egyptian hieroglyphics, and demotic (another form of Egyptian using an alphabet). This stone with the three nearly identical wordings in different languages provided the first understanding of hieroglyphics, the Egyptian symbol language.

In the same way, Rosetta’s 100 kg Philae lander will try to decipher what this comet is made of and therefore, what made up the solar system 4.6 billion years ago. (Philae is an island in the Nile that held an obelisk with inscriptions which was the final information needed to decipher the hieroglyphic language on the Rosetta Stone.)

The term “comet” is rather loosely applied to wandering objects with “hair”—in fact, the word derives from a Greek phrase meaning long-haired star.

These days, the line between comets with tails (caused by evaporating gases due to heating by the sun) and asteroids that are almost entirely rock is blurred to the point of being nearly meaningless.

Following the analysis of Fred Whipple who was at Harvard for 70 years, for the past 50 years or so, comets were thought to be snowballs with some stones scattered through them, like a raisin-filled muffin. Now the latest theory is that some comets are mostly stone with bands of ice: icy dirtballs, as opposed to dirty snowballs.

In addition to writing about comets, Dr. Whipple saved many lives by inventing the machine that cut aluminum foil into the chaff used to blind radars. He also invented the Whipple Shield: a lightweight, stand-off layer of material that protects spacecraft from micrometeorites by causing many of them to shatter before coming in contact with the spacecraft’s main shields.

In addition to the gases that are frozen in the body of the comet, many are thought to contain hydrocarbons and even amino acids, leading to the theory that, instead of comets being harbingers of doom, they may be the source of life on planets, at a minimum giving life a kickstart when they hit a body with an atmosphere.

When a comet has had all the volatile compounds boiled off its surface, it no longer produces the characteristic tail and what is left is essentially an asteroid. There appear to be very few comets whose solid bodies are less than 100 meters in diameter. This observation has remained true even after we launched very powerful telescopes into orbit for a closer investigation; there just seems to be very few small comets.

The largest cometary nuclei are less than 40 miles across, although the coma, the glowing gaseous portion produced by solar heating, can be really gigantic on a solar scale.

Those of you with a knowledge of physics should be as surprised as astronomers were in the mid-1990s when it was discovered that comets were another x-ray source.

Normally we see x-rays coming from high energy events, not frozen blocks of air. It turns out that all those outgases that come from the coma and tail of a comet can be excited by impact with the same solar wind that causes the tail to always point away from the sun. These energetic particles ionize atoms in the coma gases and generate x-rays.

The solar wind, which consists of ionized particles emitted by the sun and is always moving directly radially from the sun, cause the gases evaporated from the comet to form a tail directly away from the sun.

Comets often have two tails, the second one coming from the dust particles released as the volatiles evaporate. This dust tail is more strongly affected by other forces, mainly solar radiation, and therefore forms a separate tail from that created by the ionized gases.

Closeup view of a comet

The Rosetta program was begun ten years ago. Because the comet is moving very fast, no booster would be able to keep up with it. Four swings past planets (one of Mars and three of Earth) for gravity boosts were required to get the probe moving fast enough. Currently the probe is transmitting images of the comet back to analysts who are working to select the best impact point for the lander.

Because the surface gravity of the comet is about 100,000 times less than Earth gravity, the lander won’t stick to the surface even though it will land at a walking pace. The designers solved this problem by adding harpoons that will anchor to the comet's surface.

The mission will end December 2015, if all goes as planned, as the comet moves away from the sun after its August 2015 closest approach to the sun.

While on the surface, the lander will analyze the comet using ten onboard instruments. It will drill down to about a foot under the surface for samples. In addition to microscopic examination of the comet, the lander will also use spectrometers, determine material density, texture, thermal properties and more.

Among the new technologies developed for Rosetta are low-intensity, low temperature solar panels, making it the first probe able to pass the asteroid belt running entirely on solar power. Other deep space vehicles always used thermal generators powered by radio isotopes. Rosetta is able to generate solar power even with only four percent of the solar power that we have in near Earth orbit. These solar panels are each 14 meters long with a surface area of 64 square meters.

The duration of the mission means that all of the technology on Rosetta is at least one decade old. There are built-in automated system controls that let the spacecraft take care of itself to a certain extent.

Ten years equals several generations in the computer field, so there was no possibility of preprogramming the onboard computer. In any case it would probably be impossible, even today, to plan for enough variables that the entire mission could be preprogrammed. But more automated routines could probably be stored onboard, reducing the need to deal with the long communication delays.

You might be wondering how large is this deep space vehicle. In this instance, the entire package measures 2.8 × 2.1 × 2 meters, and weighs one-and- one-half metric tons, about half of which was fuel load. The lander that will sit directly on the comet weighs a mere 100 kg.

This wasn’t the first mission to look at comets. In 1986, five probes were launched to perform fly-bys of Comet Halley, but they only flew past, very fast. The difference with Rosetta is that the spacecraft will be in close orbit around the comet [shown at right], and send down the lander for the first soft landing on a comet.

NASA's Dawn satellite made a closeup trip past Vesta and is headed for Ceres. In 2005, Japan's Hayabusa probe actually landed on Itokawa, but those were all asteroids.

Although this is only one billionth of the total number of comets thought to be associated with our solar system, the mission will provide astrophysicists with important knowledge that can be applied to all the others.

Comet Origins

There are two basic kinds of comets: short period and long period. Comets are believed to be the oldest unchanged objects in the solar system. Planets have extensive volcanic activity and experience intense comet and asteroid collisions.

A short period comet is one that has an orbital period on the order of 200 years or less. A long period comet will (obviously) have a period longer than about 200 years. Just how much longer I can’t say because any repeating comet would fit this category even if its orbit is in the tens of thousands of years. If the orbits are ellipses (closed orbits) then they will return, eventually. If not, they are just passing through and we will never see them again unless the Enterprise runs across them in deep space.

These two classifications apply only to comets that are in orbit around the sun. Objects (such as Arthur C. Clarke’s RAMA I, II, and III) that are in hyperbolic (non-closed conic sections) visit us once and then head back out into the great space between the stars.

The Edgeworth-Kuiper belt is a mass of relatively small objects (Pluto size or smaller) that form a toroid around the inner solar system at a distance of about the orbit of Neptune to double that distance from the sun. Haumea, Makemake, and Pluto are the three largest bodies known in the Kuiper belt, which is thought to be mostly composed of ice-type bodies such as comets.

The belt was an obvious candidate for the origin and home of short period comets, but work done about 25 years ago has led astrophysicists to believe most objects in this region are in dynamically stable orbits. This means they aren’t a major source of the multitude of comets we see each year.

The Oort Cloud is the other candidate for the home of the comets. The Oort Cloud is really just a theoretical region which extends from just outside what we consider the solar system to about half way to the next nearest star in Centauri. Although no physical observation of any segment of or individual object in the Cloud exists, many scientists are convinced it is out there. In other words, since there is no other known or theoretical candidate, this Cloud really “oort” to be the home of long-period comets.

Comet Social Aspects

Thanks in large part to our wonderful educational system here in the U.S., where school boards debate whether to teach evolution instead of begging scientists to lecture at the schools or require scientific and math literacy before granting a high school diploma, the social threat from comets unfortunately isn’t something confined to the Dark Ages. As late as the 1970s, The Children of God members were panicked by the predicted appearance of the Comet Kohoutek into giving all their money to the church’s founder and moving to communes.

Those of you were alive and semi-adults (over the age of about 13) during the period from 1968 through 1977 may recall that when astronomers spotted the comet it appeared to be making its first visit to the inner solar system—therefore they mistakenly tried to generate some enthusiasm for science by claiming the comet would be the astronomical sight of the century.

Although Kohoutek turned out to be just another smudgy blob in the sky, it did perform about average for a good naked eye-visible comet and instead gave a boost to religion.

All religions seem to have at their heart either a prohibition against sex, or an enthusiastic embracing of sex. Those that are triggered by some astronomical event either gives some individual desperate to control others a lever, or the event toggles some latent switch in an otherwise normal person, making them a bit crazy.

Robert Heinlein’s “Stranger in a Strange Land” described a religion based on real miracles and plenty of sex. Non-fiction religions do something similar. They often decree that any kind of sex not directed at causing pregnancy is immoral and a sin, while ignoring the fact that most of the planet’s ills are due to a growing population.

Shakers built a lot of rather primitive furniture; their religion insisted on abstinence, not just for priests and nuns but for everyone. This obviously didn’t work out too well because no sex tends to limit the number of new communicants who get indoctrinated.

If I’m not mistaken, the Heaven’s Gate comet cult required male castration if you were to be accepted as one of the chosen ones to sail off to heaven on the spaceship hiding in the comet’s tail.

The real Missionary Position?

The Children of God, having been created just at the peak of the sexual revolution (about the time Massachusetts finally made it legal to buy and sell condoms), embraced flower power and recruited the young and sexually frustrated by whoring out its more attractive members to lure new recruits.

As with most everything humans do, there is nothing new under the sun here. Historically, temple prostitutes were used both as a form of painless taxation and to keep male church members coming (so to speak).

A more recent religion based on comets is the Hale-Bopp/Heavens Gate incident that I mentioned in last month’s article on UFOs. It combined sex, comets, and space-going aliens.

Every bit as sensible as some of these newly spawned religions, in one episode of “The Simpsons,” Bart got back at Principal Skinner for making him help the teacher observe the stars when Bart discovers a comet headed right for Springfield.

Lots of rock groups either named, referenced, or apparently were inspired by Kohoutek in some of their music.

And, of course, “Lucifer’s Hammer,” by Jerry Pournelle and David Niven, refers to Kohoutek in multiple places.

Will the landing of Rosetta’s probe right on a comet for the first time ever lead to a new wave of science fiction, art, and religious fervor? Or, for the first time in history, will human beings who pay any attention actually take it in their stride? The actual landing on the comet is currently scheduled for November. We should find out next year when the data are made public. 

John McCormick is a trained physicist, science/technology journalist, and widely-published author with more than 17,000 bylines to his credit. He is a member of The National Press Club and the AAAS. His full bibliography can be accessed online.