What is Comet: Definition and 153 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. Nik_2213

    Writing: Input Wanted Coagulating iceteroidal 'fines'

    As I understand it, a warmed comet nucleus or 'iceteroid' will out-gas as it crosses relevant 'ice lines', shed silicate dust ranging from cm-scale down to 'fines' unto electron microscopic. As 'fines' are generally near-fractal, porous and 'fluffy', it is not useful as-is. Worse, too much has...
  2. pinball1970

    Stargazing 'Devil' comet visible tonight 21.04.24

    From the article: "Viewers in the mid- to southern latitudes of North America can look toward the Taurus constellation immediately after sunset to spot the comet. It will be just below the constellation, with the prominent 'V' shape of Taurus pointing down towards it. The comet will appear just...
  3. S

    B The green comet is just a weak smudge now

    I think I has observed it a few weeks ago (maybe it was something else), and it was long & bright, but now it's barely visible . (I'm using a 12X, 60mm binocular.)
  4. H

    I Comet Sublimation: Why Does Halley's Comet Still Exist?

    Having watched a dropped cube of ice sublime to nothing at the bottom of the freezer of my refrigerator in my kitchen over the course of a week, I would like to know why comets don't do this. Presumably the tail is the product of sublimation, but how long can that go on? How can Halley's Comet...
  5. S

    I Gas Density of Lunar Crater Rims: Comet vs. Asteroid Impacts

    I mean - when they form. What are typical gas densities at lunar crater rim when the crater is made by a comet? An asteroid? Both a comet and an asteroid, hitting Moon at several km/s, initially heat up to several thousand K, and even asteroid produces some vapour... at hypocentre. However, by...
  6. T

    Should we hit Mars with The Behemoth Comet?

    I want to know why no one thinks we should hit Mars with The behemoth comet, C/2014 UN271 (Bernardinelli-Bernstein) after watching this fun video It is for the very highly educated, I know, that's what I am. Basically,, how much of a propellant push could nuking the behemoth comet result...
  7. 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...
  8. fresh_42

    I Comet Bernardinelli-Bernstein enters inner solar system

    I start this thread about a comet that probably (hopefully) will be of common interest in the coming months (or years). And, no, Earth orbit will not be affected. https://arxiv.org/pdf/2109.09852.pdf
  9. matteo446

    Time that a comet spends inside Earth's orbit

    I tried in the first place to use the effective potential of a parabolic orbit which is 0 to get the angular momentum L. Evaluating the function U(r) at r = rP i get U(rP) = L^2/(2m(rP)^2) - GmM/rP = 0. Here I get L = m√(2GMrP). Now the relationship between angular momentum L and areal...
  10. K

    B "Snow" on comet 67P/Churyumov-Gerasimenko is actually stars

    You may have seen: 10 Space Pictures That Look So Good You Won’t Believe They’re Real Starts With A Bang Ethan Siegel 9.) Snowy weather on comets. The ESA’s Rosetta mission witnessed cometary “snow” firsthand. However, landru79 (twitter) has rearranged the frames to show that most of the...
  11. Kaguro

    Question regarding time period and distances of a comet

    We can find that (1+e)/(1-e) = 8 => 1+e = 8 - 8e => 9e=7 => e=7/9. I'm not sure if I need this. We can also find the time period of Earth ## T=\frac{2 \pi r}{v} = 3.14* 10^7 s## I think I need more information from somewhere else. What am I missing?
  12. A

    Did I Calculate the Speed of a Comet Correctly?

    Hi all, I just want someone to double-check my calculation. Using some typical values of a comet, if the comet has a speed of 300 km/sec when it is 100 million kilometers from the sun, how fast is the comet moving when it hits the radius of Jupiter from the sun (5 Au)? I think I did everything...
  13. Buzz Bloom

    I How does the perihelion passage affect the orbital period of comet Neowise?

    The orbit diagram https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)#/media/File:Comet_2020_F3-skyview.png' in the Wikipedia article https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE) shows some very odd spirals. My first guess is the picture shows what Neowise does in one Earth year (mostly...
  14. LCSphysicist

    What is the Period of a Near Rectilinear Orbit with Zero Minor Axis?

    I made and understood the letters a b c, but i don't understand the letter d. I thought it would be the double of the answer C [], but seems that's not right, since the book says the answer is two times the answer of B [ two times this[ ]
  15. 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...
  16. T

    What element produces a vapor in comet tails?

    I would like to find out what element produces a vapor or a stream in a heavy charged ionic vacuum would lithium produce such a thing? Comet tails.. just wondering
  17. G

    B Can We Use Nuclear Weapons to Stop a Comet Impact?

    Sooner-or-later, at random, the world will be faced with the threat of a large Comet impact. Our only recourse will be to Nuke it. In order to know how large the thermonuclear device needs to be to either deflect or destroy it there are two questions: - What is the expansion velocity of...
  18. jim mcnamara

    Stargazing Comet 46P/Wirtanen Visible to Naked Eye Dec 16

    On Dec 16 it will be the closest to Earth in its orbit, should be visible to the naked eye. https://ksby.com/news/2018/12/09/brightest-comet-of-2018-to-pass-the-closest-to-earth-in-december
  19. Alexanddros81

    A comet at its closest approach to the Sun

    Homework Statement 14.102 A comet at its closest approach is 125 x 106 km from the center of the sun (between the orbits of the Earth and Venus), where its speed is 45.5 km/s. (a) Show that the comet has an elliptical orbit around the sun. (b) Determine the period of the orbit in years (1 year...
  20. mfb

    NASA NASA finalists for New Frontiers: Rosetta's comet or Titan

    NASA selected two finalists for the next round of the New Frontiers program, missions with a cost below $850 million. Both get funding to further refine the proposals, in early 2019 one of them will be selected. CAESAR wants to go back to 67P/Churyumov-Gerasimenko, previously visited by the...
  21. C

    B First Interstellar Asteroid Found

    The first asteroid ever seen from another solar system is whizzing through our own, and astronomers are racing to observe the visitor before it slips away. Links: Nature http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/
  22. O

    Effective cross-section of catching a comet with the Sun

    Hello. 1. Homework Statement I have to find formula for effective cross-section (σ) of catching a comet with the Sun. I know mass of the Sun (M), radious of the Sun (R) and initial velocity (v) of the comet (at a big distance from the Sun). Homework Equations σ=πb^2, where b is on the...
  23. O

    Effective cross-section of catching a comet with the Sun

    Hello. 1. Homework Statement I have to find formula for effective cross-section (σ) of catching a comet with the Sun. I know mass of the Sun (M), radious of the Sun (R) and initial velocity (v) of the comet (at a big distance from the Sun). Homework Equations σ=πb^2, where b is on the...
  24. Vanessa Avila

    Finding the speed of the comet

    Homework Statement Comets travel around the sun in elliptical orbits with large eccentricities. If a comet has speed 2.2×10^4 m/s when at a distance of 2.6×10^11 m from the center of the sun, what is its speed when at a distance of 4.2×10^10 m Homework Equations L = rp = r(mv) The Attempt at...
  25. Const@ntine

    "Vomit Comet" SpaceCraft (0)-V & H at Max Height

    Homework Statement Physics for Scientists and Engineers, 8th Edition, Ex. 47 M4 A spacecraft flies from 24.000 ft to 31.000 ft, in which it enters a parabolic trajectory with a velocity of 143 m/s at an angle of 45, above the horizontal. It exits with a velocity of 143 m/s at an angle of 45...
  26. bri7

    Max Aphelion Distance of Kuiper Belt Comet w/216 yr Orbital Period

    Homework Statement Find maximum possible aphelion distance for a Kuiper Belt comet with an orbital period of 216 years? What is the eccentricity of this orbit? What prevents a periodic comet from following this precise orbit? Homework Equations perihelion = a(1-e) aphelion = a(1+e) 2a =...
  27. H

    B Aerogel comet particle capture experiment?

    some years ago it was big news when some aerogel bulk launched at the tail of a comet and bits of the comet dust got stuck in the gel. this was going to be recovered somehow and analysed. I do not know any more about that particular experiment but it died out of the mainstream news. anyone...
  28. Hamish Taylor

    Velocity of Comet at a given distance

    Homework Statement Halley’s Comet travels in a highly eccentric (non-circular) orbit. At its closest approach, it is about 9×10^7 km away from the sun and travels with a speed of 54.6 km/s. What is the comet’s speed when it crosses the orbit of Neptune, about 5 billion kilometres from the sun...
  29. J

    I The Largest Air Burst on Earth

    On June 30, 1908, the remote Siberian region of Tunguska made headline news when an explosion knocked down 80 million trees in an area of only 770 square miles. The mystery was that there was no crater, which led to the assumption that the space bomb responsible for the catastrophe detonated in...
  30. A

    Electrical power system of a spacecraft mission to a comet

    Homework Statement Calculate the solar array power capacity required at the start of the mission Mission duration is 10 years Distance from the Sun at end of mission is 4.8x[10][/8] km The power requirement for the spacecraft at the end of life is 510 W Solar array degradation over mission...
  31. ccarit3007

    What is the work done on a comet orbiting a star?

    If a comet is orbiting a star is there any work done? I understand that work is force * displacement, but the force must be in the direction of motion. In the case of a comet and star the star exerts a gravitational force on the comet, but this force is not in the direction of the motion of the...
  32. L

    (Tricky) Comet Picking up Mass - Differential Equations

    Homework Statement A comet in deep space picks up mass as it travels through a large stationary dust cloud. It is subject to a gravitational force of magnitude Mf acting in the direction of its motion. When it entered the cloud, the comet had mass M and speed V. After a time t, it has traveled...
  33. M

    How big is the comet based on a 735x731px surface picture taken from 3km away?

    Homework Statement There is a picture, size 735x731px, that show the surface of comet that was taken from a probe distanced 3 kilometers from the comet. If the resolution of the camera that recorded the surface is 3.5 times lower then the resolution of Tycho Brache (that is 1 angular minute...
  34. F

    Unraveling the Mystery of the Comet Dust Layer

    I was just watching the time-lapse footage of the Philae landing and was wondering: How does a comet acquire and keep a dust layer on it's surface? Can anyone say? It just seems a bit odd to me.
  35. MelkoRR

    Calculation of a comet orbiting Sun

    Homework Statement a comet is orbiting around the Sun in a parabolic orbit . center of the sun to perigee of the comet's orbit distance is 58 km . find the time required for the comet to transit into the Earth orbit around the sun . Homework Equations The Attempt at a Solution no clue
  36. M

    Comet on a parabolic orbit around the Sun

    Homework Statement given: the speed at perihelion What is comets full energy? What is its distance from Sun at perihelion? How much time is the comet located inside Earth's orbit? Homework Equations parabolic orbit equation 3. The attempt at a solutin wrongly calculated l
  37. E

    Question on Comet 67P tail in Rosetta image

    I cam across this picture from the Rosetta mission the other day. You'll notice the sun is illuminating the right hand side of the comet. Additional the tail is coming off the right hand side. This goes against what I've been taught about comets in that the solar wind pushes the tail the...
  38. spark802

    Capturing Orion and Comet Lovejoy in Arizona: A Journey Through Astrophotography

    Captured these two images from my trip to Arizona Feb 2015. I shot Orion in wide=field as a piggyback on my scope set up...Canon 550d,f/5.0,436 secs,ISO3200, kit lens at 36mm. For Lovejoy I shot through my 6 inch newt at ISO 800,for 300 secs guided.
  39. W

    Comet orbiting a Star - True and False

    Homework Statement Consider the path of a comet orbiting a star, the system of the comet plus the star. Which of the following statements are correct? Select all that are True. 1) As the comet slows down, the kinetic energy of the system decreases. 2) As the comet's kinetic energy increases...
  40. P

    Investigating Comet 67P's Elliptical Orbit

    Homework Statement Currently, I am trying to prove the "conservation of energy" concept within a comet's elliptical orbit by finding the mechanical energy of the aphelion and perihelion point to see if they're equal. However, they don't equal each other when I calculate both points. (aphelion...
  41. H

    Comet Speed Around Sun: Solving with Energy & Momentum

    Homework Statement Comets travel around the sun in elliptical orbits with large eccentricities. If a comet has speed 1.5×104m/s when at a distance of 2.1×1011m from the center of the sun, what is its speed when at a distance of 5.1×1010 Homework Equations Conservation of energy The Attempt at...
  42. davenn

    Where Can I Find Comet Lovejoy in the Sky Right Now?

    Comet Lovejoy is well placed in the sky for the next week or so... it has just gone past peak magnitude of M4.2 around 4 days ago ( 9th Jan). But it is still a bright and easy object particularly with binoculars Visually ( from a dark site) or in binoculars, its a bright fuzzy blob ... sort of...
  43. davenn

    Unexpectly captured comet Comet C/2014 Q2 Lovejoy

    well this was an unexpected bonus from my trip to the centre of Australia last week In the early hours ( ~ 3:30- 4:00 AM) of the last day at Uluru ( Ayres Rock) I decided to get up and do a few starfield photos from a dark sky site Tonite, going back over my images, I discovered that I...
  44. H

    Heavy water mismatch between comet 67P and Earth's oceans

    On the recent news announcement that finds a mismatch in the heavy-to-regular water ratio on 67P found by the Rosetta mission and Earth's oceans, I wonder if the passage of time has been considered? Could the several billion years that comets have been exposed to radiation have increased their...
  45. C

    Stopping or re-routing comet 67p

    Hello, I am new here and I would like to know if anyone on here can answer a question for me. How far could a saturn V rocket (http://en.wikipedia.org/wiki/Saturn_V) push comet 67p off course if it were 100 million miles away? Also, how long would it take this rocket to stop the comet if it...
  46. 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...
  47. 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...
  48. davenn

    Vale Bill Bradfield - The loss of a comet hunter

    Bill was a fellow Australian and prolific comet hunter and discoverer Quoted from the Ice In Space site He will be truly missed in the astronomical community :( Regards Dave
  49. C

    Time Comet Remains Within Earths Orbit

    Homework Statement A comet is going in a parabolic orbit lying in the plane of the earth’s orbit. Assuming that the earth’s orbit is a circle of radius a. The points where the comets orbits intersects the earth’s orbit are given by: cos θ = −1 + 2p/a where p is the perihelion distance...
  50. C

    A comet revolves around the sun

    Homework Statement A comet revolves around the sun in a closed elliptical trajectory. Ignore any force acting upon it besides gravity. Prove that the angle between the position vector (sun in the origin) and the velocity vector of the comet at its perihelion and aphelion is 90º. The...