What is Comets: Definition and 37 Discussions

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS). 27 Manx comets were found from 2013 to 2017.As of April 2021 there are 4595 known comets, a number that is steadily increasing as more are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

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  1. D

    Can comets create a microclimate crater on Mars 30km deep with 0.7bars?

    TL;DR Summary: Can comets either one or many create simultaneously overlapping impact craters for higher air pressures to breed algae and plant life as a micro climate? Is there a depth limit to crater creation on a planet? Let’s say Mars for a depth of 30km can it be done? Assuming the...
  2. Rendering

    Impact events: when were they first seen as potentially catastrophic?

    (This is a history of science question, so please let me know if it's not appropriate to this forum.) When did someone first realize that major Earth impact events (asteroids, etc.) could potentially be catastrophic? To be clear, I don't mean in the purely theoretical sense (the likely given...
  3. V

    Orbital Periods of Planets vs Comets

    Could someone please tell me if I'm on the right track with understanding this. The periods of the comet and Uranus is different because of Kepler's second law? Is it because planets orbit the sun in a circular path whereas for comets it's a highly elliptical orbit? Thank you.
  4. A

    I What causes Kuiper/Ort cloud objects to become comets?

    I was reading about comets and know they come from the Oort or Kuiper Cloud. However, being composed of materials, comets that continually orbit the sun shed the substance that makes them up and, from what I've read, will only thus last for thousands, maybe tens of thousands of years. I know...
  5. xpell

    B How hard is it to detect extinct comets?

    Hi! I have read that cometary nuclei have a very low albedo, even lower than coal or asphalt. Because of this, they absorb lots of light and heat rather than reflecting them. So I was wondering how hard is it to detect extinct comets. Could the solar system be full of dead nuclei buzzing around...
  6. R

    Why are orbits of Comets unstable?

    Homework Statement The orbits of the planets remains extremely stable over long times however this not always true for comets. Can you explain why not? What hazards might they encounter during their travels? Homework Equations None The Attempt at a Solution I think that due to the comet's not...
  7. V

    B Comets, Water & the Sun: Investigating Earth's Origins

    A big part of water on Earth come from comets at the very early ages of Earth so I suppose that the comets has same age as the Earth. The Earth and other objects come from a waste after creation of the Sun. The Sun come from cloud of hydrogen and helium mostly. It is star of 3rd-generation so I...
  8. wolram

    Was water brought to Earth on comets?

    This article has a different spin on how water came to Earth. https://www.newscientist.com/article/2118993-Earth's-water-must-have-arrived-here-earlier-than-we-thought/ The arrival of water on our planet wasn’t a last-minute job. Water came to Earth on icy comets after most of the planet and...
  9. Astronuc

    Stargazing Comets 252P/LINEAR and P/2016 BA14

    Be on the lookout for Comets 252P/LINEAR and P/2016 BA14 https://in-the-sky.org/news.php?id=20160315_18_100 http://www.skyandtelescope.com/observing/p2016-ba14-closest-comet-in-almost-250-years03162016/ https://in-the-sky.org/solarsystem.php?obj=78283 252P/LINEAR is about perihelion.
  10. D

    Could Solar Wind and CMEs be Causing Magnetic Storms on Comet Catalina?

    Just having a think about comets this morning . Im guessing the tails associated with comets are fragments of rock and ice from the surface of the comets but what is causing them to come off the surface if the comet is traveling through no medium to cause a force to move the bits that a make...
  11. T

    Comets' High Deuterium-to-Hydrogen Ratios Explained

    Comets have high Deuterium to Hydrogen ratios. They are enriched with extra neutrons. Comets periodically plunge close to the sun. And, the sun generates flares, which generate neutrons. Separately, I wonder if those neutrons come from fusion of solar corona gas, trapped on magnetic field...
  12. anorlunda

    Rocky Bits in Comets: Formation from Dust?

    Wikipedia says "Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[" I don't see where the rock comes from. In my mind, rock can be formed from dust only by application to lots of heat and...
  13. bitznbitez

    Are There Documented Polarities in Comets' Magnetic Fields?

    Comets have what is called an "induced magnetosphere". Has anyone been able to document polarity in any of the observed comets magnetic fields yet ? If they have I'm curious how it relates to the orbital plane of the comet. I haven't been able to locate that info if it exists so just...
  14. L

    Solar radiation force on comets.

    I see this comet being chased by ESA, they said it is presently about 500 million km from the sun. That puts the radiation received by that comet at about 10% (roughly) as what we receive on Earth, well at least on top of the atmosphere, at 1355 watts per meter ^2. Looking at the size of...
  15. E

    Exploring Comets: Is it Technologically Feasible?

    The popular press has been alive lately with stories of asteroid interception and even asteroid recovery. Has there been any thoughts of a spacecraft closely approaching a comet? How big are they? How fast do they travel? Is it remotely possible using existing technology? Could we land on...
  16. J

    How does Jupiter protect Earth from comets and such?

    How does Jupiter "protect" Earth from comets and such? I've commonly heard that Jupiter protects Earth by flinging comets out of an orbit that would otherwise leave them heading for Earth. I understand that Jupiter can throw comets and other objects out of their current orbit when they pass by...
  17. Y

    Question Regarding the Orbit of Comets

    Recently, I attended a lecture at my university entitled "The Life and Death of a Star-Grazing Comet" presented by John Raymond, an astrophysicist at Harvard Smithsonian. In this lecture, he talked about Comet Lovejoy and how it allowed us to help study the corona of the sun. As the comet was so...
  18. Vorde

    Comets in the Northern Hemisphere vs the Southern?

    Hey all, I haven't been alive for that long, so I guess my data probably isn't as good as it should be, but in my experience there have been far more noteworthy comets appearing only to the southern hemisphere, and not to the northern one. In fact, I haven't ever seen a comet (since I was a...
  19. O

    Apparent size of comets in images from the SOHO satellite

    I've gotten into an amusing discussion with another individual on the topic of the apparent size of comets as seen in SOHO satellite images of Sungrazing or Sun-impacting comets. I won't go into the crackpot conspiracy theories I've heard, but will instead ask: why do these tiny comets appear so...
  20. Straw_Cat

    The Aftermath of a Large Comet Impact on Mars

    Suppose a large, 'dirty snowball' comet, 25-50km in diameter or more, were to hit Mars at some speed, say 75,000 km/h. I am quite certain it would vapourize on impact. And the impact will create super-heated high-velocity shock waves which will travel around Mars at least once, probably...
  21. Astronuc

    Comet Death Dive Into Sun Seen in Detail for 1st Time

    Comet's Death Dive Into Sun Seen in Detail for 1st Time http://www.space.com/14288-sungrazing-comet-death-dive-sun-observed.html by Charles Q. Choi, SPACE.com Contributor Date: 19 January 2012 I was surprised to learn that sun-diving comets are common. I guess in the past, sun-diving comets...
  22. C

    Why do comets tails come out of the front and curve around to the back?

    I've been researching this for a while now, and none of the theory's can predict it. I've studied comets for a while now, and the tails come out of the front and then has a weird curve which goes to the back. If you guys have any random ideas or thoughts of why this is happening, that would be...
  23. S

    Why is Ice More Prevalent in Comets than in Early Planetary States?

    I have read a few times that comets are significant transports of ice (perhaps in having supplied water to fill Earth's oceans). That implies that water is more abundant in the few comets (in accumulated mass) that have impacted Earth than that which would have existed within the initial planet...
  24. D

    Orbital energy required to change a comets orbit

    Homework Statement Use the information in Section A.3 and the data in Table A.19 to calculate the heliocentric radial distance (in AU) of Comet Halley at 12h UT on July 16, 1994. - Done, Section A.3 just talked about finding dates using Epochs and such. Table 19 had comet details which will...
  25. C

    Supernova's Gamma Rays and Comets

    Hi guys, me again, with two questions. Just saw a documentation about supernova, when supernova happens, a huge amount of gamma ray is emitted since it is vacuum in space, why wouldn't the gamma ray hit us? Even if its far and takes a few thousand years for it to reach us, it would eventually...
  26. R

    Size Limit of Comets: Understanding Kepler's Laws

    Hi all Just a quick question What would be the maximum size to which a comet could actually go to to be able to maintain an elliptical orbit so as to follow kepler's laws To reword it what would be the least amount of eccentricity that a comet could have and what would this be dependent on?
  27. K

    Would you expect comets to follow the ecliptic? Explain.

    Would you expect comets to follow the ecliptic? Explain.
  28. Z

    Shouldn’t ice on comets burn up quickly?

    I see that the current explanation for a comets tail is that as a comet approaches the sun, it starts to heat up. The ice transforms directly from a solid to a vapour, releasing the dust particles embedded inside. Sunlight and the stream of charged particles flowing from the sun – the solar wind...
  29. B

    Harnessing Comets for Planetary Colonization

    This might be a good idea for a thesis. Could we alter a comet's course and make it break free of orbit and head towards another sun and populate other planets? Some of them are made up of life sustaining elements right? Could we use nuclear power to melt it to create a biosphere?
  30. M

    The Elliptical Orbits of Comets: Exploring Causes and Consequences

    If the planets are forced into circular orbits by inertia and gravity, why are the orbits of comets oval shaped? What keeps them from entering the sun.
  31. P

    Birght comets like Comet McNaught

    Some comets like Comet McNaught glow very brightly in the sky. However, they are not a sun, obviously. They are just large chunks of rock? It can't just be from the sun's reflection? Or is it as the bright ones have an ice coating around them. I know they have large kinetic energy but not enough...
  32. D

    Alternative Solutions to Deflecting Asteroids or Comets From Earth

    As most now accept an astroid or comet ended the reign of the dinosaurs. Hollywood's idea of dealing with the threat is to fly crews to the object and blow it up. NASA's solution involves mass drivers, impact airbags of gases and dust, lasers, solar saiks and etc. **Throwing out Hollywood's...
  33. Ivan Seeking

    Can Watching Comets on THC Reveal Hidden Connections in History?

    A thread for strange connections or coincidences. I was watching Comets on THC, and it came to my attention that the Heaven's Gate cult - all of whom committed suicide - believed that the Earth was going to be recycled by the mother ship. Is is just me or does this smack of "intergalactic...
  34. J

    Small Comets, Dr Louis Rich, & Water In Space

    Lately, I have become quite interested in how the astronomy field has begun to determine that water can be found almost anywhere. And since it is the second most common molecule in the galaxy, at least here, I became intrigued with how it gets around so easily. This brings me to Dr Louis...
  35. V

    Comets and orbital periods and such

    The orbital period of Comet Halley is calculated to be 75.5 years. Many other comets have orbits that extend much farther from the sun, and hence have much longer periods. At aphelion, a typical long-period comet is about 8*10^12 km from the sun; at perihelion, it passes inside the Earth's...
  36. O

    How do Oort Cloud objects become short-period comets?

    Something I've thought about before but never asked anybody...The existence of short-period comets is a problem if one believes the solar system is 4 billion years old, because obviously after that length of time all of the short-period comets would have disintegrated. So, we need a way of...
  37. Greg Bernhardt

    Some comets are only visible with the help of a telescope

    While some comets are only visible with the help of a telescope, this comet, seen in 1965, was visible to the naked eye even during the day! What comet was it?