There’s an awesome graphic making the Internet rounds right now – the city of Los Angeles and Comet Churyumov–Gerasimenko (67P), to scale.
That comet isn’t going to hit LA. It’s special because the Rosetta spacecraft just intercepted it this month, about midway between the orbits of Mars and Jupiter.
It looks big here, but Comet Churyumov–Gerasimenko, billions of years old, is really just a speck in the Solar System.
Imagine putting downtown Los Angeles physically out beyond Mars and the asteroid belt. Now, try not just to hit it with a rocket launched from South America, but also to have that rocket circle around downtown and then land in a park.
It took 10 years, but the European Space Agency’s Rosetta has just gotten there.
This semiautonomous little spacecraft is now trying to establish an orbit about a mile above the tumbling comet. Then, in November, it will put down a lander and will stay with the comet for the next year during Comet Churyumov–Gerasimenko’s close approach to the Sun.
If Rosetta can stay in position, it’s going to be a wild ride – one that will us a lot about how comets are put together and how they work.
Asteroids and comets
Like asteroids, comets are leftover debris from the formation of the Solar System.
Asteroids are generally dry and either stony or metallic.
Comets contain water and are loosely built out of ice, rocks, and dusty grains of silicates and minerals rich in carbon.
Scientists believe a comet nucleus is mostly water ice, with some frozen carbon monoxide, carbon dioxide, formaldehyde, methanol and other chemicals mixed in there as well. Dust makes up as much as a third of the nucleus.
That glow at the head of the tail isn’t the nucleus. It’s just the most actively degassing part of the coma (the atmosphere that forms around the comet).
Of note, asteroids generally don’t have atmospheres, although in January 2014 water vapor jets were detected on the dwarf planet Ceres, the largest object in the asteroid belt.
Astronomer David Levy says, in Impact Jupiter, that there is a spectrum between asteroids and comets.
Comets are found out beyond the orbit of Neptune, starting at the Kuiper Belt some 30 AU from the Sun (astronomical units – 1 AU is the distance from the Sun to Earth).
Kuiper Belt objects orbit the Sun in a plane, like the inner planets and asteroids. Most short-period comets, those whose orbits bring them sunwards more frequently than every 200 years, come from the Kuiper Belt. Halley’s Comet, with an orbital period of 76 years, is a short-period comet.
Beyond the Kuiper Belt, beyond even Voyager 1, is the spherical Oort cloud of small objects. This cloud starts at around 1,000 AU or more from the Sun. It may extend out as much as halfway to the nearest star, Proxima Centauri (4.24 lightyears away).
Long-period comets like Hale-Bopp (orbital period 2,400 years) start out in the Oort cloud.
Sometimes words from a knowledgeable source are more powerful than any image.
Robert Burnham, former editor-in-chief of Astronomy magazine, has given what I think is the best summary of how a comet rocks and rolls.
A comet approaching the Sun from deep space – beyond the orbit of Neptune – is an inert body wrapped in a layer of dust…As a comet approaches the Sun, the growing warmth triggers one kind of ice and then another to flash directly into vapor…Somewhere around the distance of Pluto’s orbit (about 40 astronomical units from the Sun), ices made of nitrogen and carbon monoxide pick up enough heat to change to gas…Then, near the distance of Neptune’s orbit – 30 AU – methane ice pops into vapor under the weak sunlight, barely 1,900th as strong as at Earth. As the comet comes inside the distance of Saturn, about 9 AU, ammonia ice bursts into gas, adding to the comet’s growing “atmosphere.” At a distance between Saturn and Jupiter (5.2 AU), frozen carbon dioxide (dry ice) gets going.
However, comets are not made of pure ice, so the activity shown by any given comet may not follow such neat and predictable rules…In any case, by the time a comet nucleus is twice as far from the Sun as Mars (or about 3 AU), a soft fog of gas envelops it. This resembles the haze of vapor swirling around a brick of dry ice at a picnic in July. (Exceptional comets, such as Hale-Bopp, are already quite active by this stage.) The point where the growing warmth causes water ice to start evaporating is sometimes called the “snow line.” As the ice evaporates, it creates a cloud of water vapor mixed with the other gases that were formerly locked up in the ice. The gases bursting free of the nucleus also carry off dust particles…
The result of a comet’s strengthening activity is the development of a coma, an atmosphere of gas and dusty particles that surrounds the nucleus…The coma grows in size as the comet approaches the Sun and can swell to a diameter of several hundred thousand miles. It grows spherically until the gas becomes so thin that the pressure of sunlight begins to distort its symmetry…
But the coma is not what makes comets so awe-inspiring – it’s their enormous tails…As Johann Kepler guessed, comet tails are born from the coma. They take two easily distinguished forms, one made of dust and the other of ionized gas…The dust tail often appears curved, as the nucleus and coma race away from the debris they have shrugged off and the discarded dust particles set out on their own orbital paths in the wake of the comet. Dust tails may stretch for 10 million miles…The ion tail always points directly away from the Sun like a windsock, even when the comet is moving outward from the Sun…
Ion tails differ from dust tails by their color, which is usually a distinct blue or blue-green, both to the eye and the camera…ion tails shine by fluorescence…Most of the light in a comet’s plasma tail comes from ionized carbon monoxide, nitrogen, and water – although carbon monoxide is the main ingredient in what we see.
Let’s just sit back and enjoy the show for a while.
Space missions to comets
Rosetta is one of 20 missions that have explored comets.
In 1986, the ESA’s Giotto mission, named after the 14th century artist who painted the Star of Bethlehem as a comet, gave us first view ever of a comet’s nucleus.
At a distance of less than 400 miles, Giotto got close enough to Halley’s Comet to photograph its surface to a resolution of 100 yards. This historic mission also confirmed that comets are indeed “dirty snowballs.”
Probably the most dramatic mission thus far has been NASA’s Deep Impact. Built in two sections (one of them awesomely named “the impactor”), it launched in January 2005 and headed for Comet Tempel I, arriving in July.
One half of Deep Impact did quiet science stuff like taking photographs and collecting data. The scientists then released the impactor section…which smashed into the comet.
Okay, “smash” is not in the scientist’s vocabulary. ESA scientists say, “The impact excavated debris from the comet, allowing the main spacecraft to analyse the composition of surface and interior materials of a comet.”
Analysis of the plume confirmed the long-held suspicion that comets are among the most primitive objects in the Solar System. In addition, it gave researchers insight into some of the events during the condensation of the solar nebula.
That sounds pretty esoteric, but it has very practical uses. It confirms some of the most important models of Earth’s internal structure and so has meaning for everybody from exploration geologists to emergency planners who are trying to make realistic seismic or volcanic hazard plans.
It is another two-part spacecraft. Rosetta is the name of the larger part, which has navigational computers as well as dust and gas analyzers, cameras and plasma detectors.
The other part is the lander – Philae – which will be packing a lot of instruments, including a mass spectrometer and seismograph, as well as cameras and various instruments to check out dust and gas on the nucleus of Comet Churyumov–Gerasimenko.
Rosetta is currently maneuvering around this irregularly-shaped comet nucleus to establish an orbit in September that will keep the main craft some 19 miles away from the surface.
Note that the comet in that video is tumbling as well as moving through space. What it’s not doing is degassing or shedding rocks and dust. At roughly 3 AU, the comet is still far enough from the Sun to remain fairly quiet, but close enough for the fireworks to soon begin.
If all goes well, humanity is going to try something stupid – no, something really brilliant. In November, the lander will try to set down on the surface of Comet Churyumov–Gerasimenko.
One of the most famous comets recently was Shoemaker-Levy 9. No one knows anything about its early history, but in the early 20th century, it was a 6-mile-wide object comet that was captured by Jupiter’s gravity and joined the small family of comets that orbit around that giant gas planet.
In July 1992, Comet Shoemaker-Levy 9 approached Jupiter again, this time close enough for gravitational tidal forces to tear it apart. Now in some 20 pieces, the comet carried on through its last orbit and struck Jupiter in December 1993.
Just imagine if it had hit the Earth…
We’ve already looked at impact catastrophes, specifically, the Chicxulub crater left by a space object – probably an asteroid – that at least contributed to the late Cretaceous mass extinction.
Could the Tunguska event in 1908 have been a comet air burst? No one knows for sure, but some researchers think it was.
Because comets are made of ice and dust – unlike stony or metallic asteroids – an incoming comet, if it doesn’t vaporize, might fragment on the way down, leaving a linear line of craters. Five such craters were tentatively identified in 1998.
There appears to be no scientific consensus on exactly when comets have struck Earth or which impact structures, if any, can be attributed to them.
However, there is evidence that such impacts have happened – water and the presence of life on Earth.
As NASA puts it,
It seems possible that the origin of life on the Earth’s surface could have been first prevented by an enormous flux of impacting comets and asteroids, then a much less intense rain of comets may have deposited the very materials that allowed life to form some 3.5 – 3.8 billion years ago.
For millennia, we have been amazed, frightened, and intrigued by the “hairy stars” that sometimes appear in the night sky. Now we are on the brink of landing on a comet. We have honed our ability to navigate the Solar System and may also gain a wealth of knowledge about comets, if the ambitious Rosetta comet mission succeeds.
…and now we’re trying to ride one around the Sun.
— NASA's Dawn Mission (@NASA_Dawn) August 24, 2014
That’s even more awesome than that graphic showing Comet Churyumov–Gerasimenko sitting on Los Angeles.
CALPURNIA: When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.
CAESAR: Cowards die many times before their deaths;
The valiant never taste of death but once.
Of all the wonders that I yet have heard.
It seems to me most strange that men should fear
— William Shakespeare, “Julius Caesar,” Act 2, Scene 2
— ESA Rosetta Mission (@ESA_Rosetta) September 3, 2014
Another awesome comet closeup:
— ESA Rosetta Mission (@ESA_Rosetta) September 8, 2014
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- Robert Burnham. Great Comets. Cambridge University Press. Cambridge. 2000
- History of comet missions. European Space Agency
- David H. Levy. The Sky in Early Modern English Literature. Springer. 2011
- David H. Levy. Impact Jupiter Plenum. New York. 1995