Comets have been observed for over 2,000 years. During that time, several different systems have been used to assign names to each comet, and as a result many comets have more than one name.
The simplest system names comets after the year in which they were observed (e.g. the Great Comet of 1680). Later a convention arose of using the names of people associated with the discovery (e.g. Comet Hale–Bopp) or the first detailed study (e.g. Halley's Comet) of each comet. During the twentieth century, improvements in technology and dedicated searches led to a massive increase in the number of comet discoveries, which led to the creation of a numeric designation scheme. The original scheme assigned codes in the order that comets passed perihelion (e.g. Comet 1970 II). This scheme operated until 1994, when continued increases in the numbers of comets found each year resulted in the creation of a new scheme. This system, which is still in operation, assigns a code based on the type of orbit and the date of discovery (e.g. C/2012 S1).
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Jupiter's Moons: Crash Course Astronomy #17
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Asteroids: Crash Course Astronomy #20
Transcription
This episode of Crash Course is brought to you by Squarespace. As we saw in the last episode, Jupiter is by far the largest and most massive planet in the solar system. That means it has a very strong gravitational field, which also means it can hold on to a lot of moons. A lot. Right now, as we record this episode, there are 67 that have been confirmed. And how many it really has depends on how small an object you're willing to call a "moon." In 1610, Galileo pointed his telescope at Jupiter, and witnessed a revolution. Oh, hey, literally! He saw three little stars lined up on either side of Jupiter, stars he could not see with his naked eye. And they moved! A week later he saw a fourth one, and he knew he was seeing objects revolving, orbiting around Jupiter. It was proof that not everything in the solar system revolved around the Earth. That was a pretty big deal. Those four moons are now called the Galilean moons in his honor. Not bad for a week’s work. All four are really big, too. If Jupiter weren’t there, drowning them out with its glare, they’d be visible to the naked eye. In that case we might even call them planets, too. The biggest of Jupiter’s moons is Ganymede. At 5270 km across, it’s the biggest moon of any planet. It’s even bigger than the planet Mercury—in fact, in size it’s halfway between Mercury and Mars! Size isn’t the only planet-like characteristic of Ganymede, either. It’s mostly rock and ice, but it probably has a liquid iron core. It even has a magnetic field, likely generated by that liquid core. The surface is similar to our own Moon in that there’s very old, cratered terrain as well as smoother, younger areas. Ganymede is also criss-crossed with large grooves. It’s not clear what the origin of those grooves is, but it may be related to stress and strain on the surface caused by the tides from the other large moons as they orbit Jupiter and pass each other. Ganymede has a surprise well below its surface, too: Oceans of water! Measurements of Ganymede’s magnetic field, made during multiple passes by the Galileo spacecraft in the 1990s, combined with Hubble observations of the moon, indicate Ganymede has quite a bit of salty liquid water, deep beneath its surface! As we’ll see in a sec, it’s not alone in that regard. The next biggest moon is Callisto, at 4800 km in diameter. In many ways it’s similar to its big brother Ganymede, mostly rock and ice. It probably has a rocky core, then a layer of mixed rock and ice above that. The surface is mostly ice, but mixed with darker material as well. It has a magnetic field, too, but it probably doesn’t have a metallic core. The surface is heavily cratered, and there’s no indication of any volcanoes or tectonic activity. That means the surface is very old, maybe as old as Callisto itself. It even has an atmosphere, but it’s a tad thin: roughly one one-hundred-billionth the pressure of Earth’s air at the surface! Callisto orbits Jupiter farthest out of the four biggies, almost 2 million km away. That’s too far to gravitationally interact with the other three; when I talk about the moons affecting each other, it’s really the other three interacting. Next up is Io. It’s only a little bit bigger than our own Moon, and orbits Jupiter so tightly it only takes about a day and a half to go around the planet. When the Voyager 1 space probe passed Io in 1979 it revealed a surface that was really weird. It was yellow and orange and red and black, and didn’t seem to have any obvious impact craters. An engineer, Linda Morabito, noticed that in one image there appeared to be what looked like another moon behind Io, partially eclipsed by it. But that was no moon: It was a volcano on Io erupting, its plume shooting up from the surface and opening up into a wide arc. Io is the most volcanic object in the entire solar system, with over 400 active volcanoes. Quite a few of them are erupting at any given time, and images taken even a few months apart show changes in the surface due to ejected material. A lot of the erupted material is rich in sulfur, which is why the surface has all those odd colors on it. The energy for all this activity comes from the other moons: As they pass Io in their orbits they flex it via tides, heating its interior through friction. A lot of that sulfur ends up as a very thin atmosphere around Io, and some of those sulfur atoms are then picked up by Jupiter’s powerful magnetic field as it sweeps past Io and accelerates them to very high speeds. This has created a tremendous donut-shaped radiation belt around Jupiter, like Earth’s Van Allen belts, but far more powerful. The radiation there is so intense it would kill an unprotected human in minutes. Of course, if you’re floating in space near Jupiter unprotected, you might have some more immediate concerns. Oh, one more thing: Both Ganymede and Io are magnetically connected to Jupiter. Charged particles flow from those moons along the lines of magnetism to Jupiter, which then slams them down at Jupiter’s poles, just like the Earth does with the particles from the solar wind. On Earth this creates the aurorae, the northern and southern lights, and it does at Jupiter, too. You can even see the ultraviolet glow where each of the moons connects to Jupiter; their magnetic footprints in the planet’s atmosphere! And now we come to Europa, the smallest but perhaps most exciting of all the Galilean moons. Slightly smaller than our moon, it was known for decades to be very reflective, meaning its surface was probably loaded with water ice. But even so, the Voyager observations were shocking. They showed a surface completely lacking in craters, meaning something had resurfaced the moon like Io or Venus; but Europa has no volcanoes. Even more intriguing, the surface was covered in long cracks, dark streaks all over the moon, as well as complex ridges. These and other features appear to be due to material from the interior of Europa welling up and forming the new surface, kind of like the way lava does on Earth. But in this case, the material is water. It’s now thought that Europa has an entire ocean of water, sealed up under a solid crust of ice several kilometers thick. Water welling up and moving under the crust causes it to shift, creating all the various surface features. The amount of water that may be locked up on Europa is staggering; easily more than all the water in all the oceans on Earth! Like Ganymede and Io, the interior of Europa is kept warm by tidal flexing from the other moons, keeping the ice melted. Now get this: A lot of Europa’s material is silicate rock, like on Earth and other terrestrial planets, located in a layer under the ocean. If this interacts with the ocean in the same way Earth’s oceans interact with the sea floor, this could make the subsurface Europan water salty. In fact, those dark cracks on the surface have been found to be rich in salt and organic materials - in other words, carbon-based compounds! This is pretty exciting. We think Earth’s life originated in salty ocean water. If there are carbon-based molecules actually in Europa’s water, it’s not too crazy to wonder if the same spark that occurred here also happened there. We think Europa has everything it needs to spawn life. We just don’t have any direct evidence of it yet. Some people have proposed sending a space probe to Europa specifically to look for life. It would land near a crack in the ice, where the crust is thinner, and somehow penetrate it (perhaps melting its way down). Chemical sampling could then look for signs of biological activity. That’s amazing to me: The idea of life in Europa, even if it’s just microbial life, is taken very seriously by astrobiologists, scientists who study the possibility of life in space. It used to be science fiction. Now it’s a topic of scholarly research. Astronomers have a concept called the habitable zone: The distance a planet can be from its parent star where the temperature on the planet’s surface can support liquid water. It’s a fuzzy concept; Venus and Mars are both technically in the Sun’s habitable zone, but Venus is too hot and Mars too cold for liquid water. Atmospheres make a big difference. But it’s still a useful concept as rule of thumb for potential habitability. But Europa changed that. Jupiter is way, way outside the Sun’s habitable zone, yet there’s Europa, all wet. It’s a great example that we need to let our ideas breathe a bit sometimes, let them relax and flow outside the boundaries we set for them. When we look for signs of life on planets orbiting other stars, I bet we’ll have to keep our minds open to types of life we’ve never considered before. Those are just the four big moons of Jupiter, each thousands of kilometers across. They probably formed along with Jupiter, coalescing from the eddies and whorls around the protoJupiter as it formed billions of years ago. But the planet has dozens of other moons, too. About the only thing they all have in common is that they’re tidally locked to Jupiter; they all rotate once for every time they go around the planet. Jupiter’s tides are hundreds of times stronger than Earth’s, so no surprise there. The next biggest moon after The Big Four is way smaller; named Amalthea, it’s an irregular lump about 250 km across its longest dimension. It was discovered in 1892, and it’s red—probably polluted by sulfur from Io. It orbits just over 100,000 km from Jupiter’s cloudtops; if you stood on Amalthea’s surface, Jupiter would fill half the sky. The moons get smaller and more irregularly shaped from there, with Himalia and Thebe and Elara and Pasiphae, down to Hegemone, Kale, and Kallichore, which are no bigger than hills. Many of the irregular, distant moons of Jupiter orbit the planet backwards relative to the others, in what are called retrograde orbits. They may be captured asteroids from the nearby asteroid belt. Many of the moons have orbital characteristics that are very similar, too, which may indicate they were once a single object that broke up. Several such families of moons orbit Jupiter. The smallest moons we’ve seen are roughly a kilometer across. If they were sitting on Earth they might be hard to pedal up on a bicycle, but orbiting Jupiter they hardly rate as more than debris. There are probably thousands of moons the size of houses circling the planet, and who knows, maybe millions the size of tennis balls. Should we even call those moons? Maybe. But I don’t really worry about that kind of thing. The important thing to remember is that these are worlds, big and small, each fascinating, rich, and diverse. And there’s still a lot more left to explore about them. Today you learned that Jupiter has lots of moons, and four big ones. They’re mostly rock and ice, though Ganymede, the biggest, may have an iron core. Io is riddled with volcanoes, and Europa has an undersurface ocean that is the object of intense study for scientists looking for life in space. Io, Europa, and Ganymede are close enough to interact gravitationally, providing a source of heat for their interiors. There are lots and lots of littler moons, but at the moment we really don’t know much about them. Someday. Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head over to their channel and find even more awesome videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, and the graphics team is Thought Café.
Named by year
Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year when they appeared e.g. the "Comet of 1702".
Particularly bright comets which came to public attention (i.e. beyond the astronomy community) would be described as the great comet of that year, such as the "Great Comet of 1680" and "Great Comet of 1882". If more than one great comet appeared in a single year, the month would be used for disambiguation e.g. the "Great January comet of 1910". Occasionally other additional adjectives might be used.
Named after people
Possibly the earliest comet to be named after a person was Caesar's Comet in 44 BC, which was so named because it was observed shortly after the assassination of Julius Caesar and was interpreted as a sign of his deification. Later eponymous comets were named after the astronomer(s) who conducted detailed investigations on them, or later those who discovered the comet.
Investigators
After Edmond Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Halley's Comet.[1] Similarly, the second and third known periodic comets, Encke's Comet[2] and Biela's Comet,[3] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.
Discoverers
The first comet to be named after the person who discovered it, rather than the one who calculated its orbit, was Comet Faye – discovered by Hervé Faye in 1843. However, this convention did not become widespread until the early 20th century. It remains common today.
A comet can be named after up to three discoverers, either working together as a team or making independent discoveries (without knowledge of the other investigator's work). The names are hyphenated together, using en dashes where possible. For example, Comet Swift–Tuttle was found first by Lewis Swift and then by Horace Parnell Tuttle a few days later; the discoveries were made independently and so both are honoured in the name. When the discoverer has a hyphenated surname (e.g. Stephen Singer-Brewster), the hyphen is replaced by a space (105P/Singer Brewster) to avoid confusion.
From the late 20th century onwards, many comets have been discovered by large teams of astronomers, so may be named for the collaboration or instrument they used. For example, 160P/LINEAR was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) team. Comet IRAS–Araki–Alcock was discovered independently by a team using the Infrared Astronomy Satellite (IRAS) and the amateur astronomers Genichi Araki and George Alcock.
In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names (but only for periodic comets); thus Comets Shoemaker–Levy 1 to 9 (discovered by Carolyn Shoemaker, Eugene Shoemaker & David Levy). Today, the large numbers of comets discovered by some instruments makes this system impractical, and no attempt is made to ensure that each comet is given a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Systematic designations
Original system
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet 1969i (Bennett) was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet 1969i became Comet 1970 II (it was the second comet to pass perihelion in 1970)[4]
Current system
Increasing numbers of comet discoveries made this procedure awkward, as did the delay between discovery and perihelion passage before the permanent name could be assigned. As a result, in 1994 the International Astronomical Union approved a new naming system. Comets are now provisionally designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids). For example, the fourth comet discovered in the second half of February 2006 was designated 2006 D4. Prefixes are then added to indicate the nature of the comet:
- P/ indicates a periodic comet, defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage.[5]
- C/ indicates a non-periodic comet i.e. any comet that is not periodic according to the preceding definition.
- X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets).
- D/ indicates a periodic comet that has disappeared, broken up, or been lost.[5] Examples include Lexell's Comet (D/1770 L1) and Comet Shoemaker–Levy 9 (D/1993 F2)
- A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet. An unused option for many years, this classification was first applied in 2017 for 'Oumuamua (A/2017 U1) and subsequently to all asteroids on comet-like orbits.
- I/ indicates an interstellar object,[6] added to the system in 2017 to allow the reclassification of 'Oumuamua (1I/2017 U1). As of 2019[update], the only other object with this classification is Comet Borisov (2I/2019 Q4).
For example, Comet Hale–Bopp's designation is C/1995 O1. After their second observed perihelion passage, designations of periodic comets are given an additional prefix number, indicating the order of their discovery.[7] Halley's Comet, the first comet identified as periodic, has the systematic designation 1P/1682 Q1.
Separately to the systematic numbered designation, comets are routinely assigned a standard name by the IAU, which is almost always the name or names of their discoverers.[8] When a comet has only received a provisional designation, the "name" of the comet is typically only included parenthetically after this designation, if at all. However, when a periodic comet receives a number and a permanent designation, the comet is usually notated by using its given name after its number and prefix.[9] For instance, the unnumbered periodic comet P/2011 NO1 (Elenin) and the non-periodic comet C/2007 E2 (Lovejoy) are notated with their provisional systematic designation followed by their name in parentheses; however, the numbered periodic comet 67P/Churyumov–Gerasimenko is given a permanent designation of its numbered prefix ("67P/") followed by its name ("Churyumov–Gerasimenko").
Interstellar objects are also numbered in order of discovery and can receive names, as well as a systematic designation. The first example was 1I/ʻOumuamua, which has the formal designation 1I/2017 U1 (ʻOumuamua).
Relationship with asteroid designations
Sometimes it is unclear whether a newly discovered object is a comet or an asteroid (which would receive a minor planet designation). Any object that was initially misclassified as an asteroid but quickly corrected to a comet incorporates the minor planet designation into the cometary one. This can lead to some odd names such as for 227P/Catalina–LINEAR, whose alternative name is 227P/2004 EW38 (Catalina-LINEAR), derived from the original provisional minor planet designation 2004 EW38.
In other cases, a known asteroid can begin to exhibit cometary characteristics (such as developing a coma) and thus be classified as both an asteroid and a comet. These receive designations under both systems. There are only eight such bodies that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 4015 Wilson–Harrington (107P/Wilson–Harrington), 7968 Elst–Pizarro (133P/Elst–Pizarro), 60558 Echeclus (174P/Echeclus), 118401 LINEAR (176P/LINEAR), (300163) 2006 VW139 (288P/2006 VW139), (323137) 2003 BM80 (282P/2003 BM80), and (457175) 2008 GO98 (362P/2008 GO98).
References
- ^ Ridpath, Ian (3 July 2008). "Halley and his Comet". A brief history of Halley's Comet. Retrieved 14 August 2013.
- ^ Kronk, Gary W. "2P/Encke". Gary W. Kronk's Cometography. Retrieved 14 August 2013.
- ^ Kronk, Gary W. "3D/Biela". Gary W. Kronk's Cometography. Retrieved 14 August 2013.
- ^ Arnett, William (Bill) David (14 January 2000). " 'Official' Astronomical Names". International Astronomical Union. Retrieved 2006-03-05.
- ^ a b "Cometary Designation System". Minor Planet Center. Retrieved 2011-07-03.
- ^ "MPEC 2017-V17 : NEW DESIGNATION SCHEME FOR INTERSTELLAR OBJECTS". Minor Planet Center. International Astronomical Union. 6 November 2017. Retrieved 6 November 2017.
- ^ "Cometary Designation System". Committee on Small Body Nomenclature. 1994. Retrieved 2010-08-24.
- ^ "Guidelines for Cometary Names". Solar System Support Pages at the University of Maryland - IAU Division III. IAU Division III Committee on Small Body Nomenclature. Retrieved 19 March 2019.
- ^ "Comet Names and Designations; Comet Naming and Nomenclature; Names of Comets". International Comet Quarterly. International Comet Quarterly. Retrieved 19 March 2019.
This page was last edited on 27 March 2024, at 06:27