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101 votes
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Could the dinosaurs have seen the asteroid that killed them?

The answer is yes; for a few nights prior to the impact (assuming they had eyes with a similar sensitivity to our own and could look up!). It could be a bit longer than this if the body was larger ...
ProfRob's user avatar
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58 votes

How can comets have tails if there's no air resistance in space?

There are two forces that can cause the formation of a tail: the solar wind and radiation pressure. The first misconception in your question is "the dust [travels] slower than the nucleus". The tail ...
James K's user avatar
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45 votes
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How do we know that a comet passed by earth 50,000 years ago?

50000 years is the comet's estimated orbital period. That does not necessarily mean that the comet was naked-eye visible from Earth 50000 years ago. That also does not necessarily mean that the comet ...
David Hammen's user avatar
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36 votes

How can comets have tails if there's no air resistance in space?

First, there is not just one tail, it is several, but when traveling far from a star, they are "aligned". When it gets closer the different materials behave differently, both depending on the ...
Viktor Mellgren's user avatar
32 votes
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Is it possible that the Rosetta orbiter moved the comet when it crashed?

Yes, it did. But not by much. The comet has a mass of about $10^{13}$ kg. Rosetta had a mass (after fuel had been used up) of about 1300kg. The "impact" was at 0.9 m/s. This means that ...
James K's user avatar
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31 votes
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Where is the Perseids meteor shower coming from?

The comet sheds material each time it get close to the sun. The sun heats this dirty snow ball, the ice evaporates and tears dust and smaller rocks with it. These dust particles then follow a similar ...
SpaceCore's user avatar
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31 votes

How is Halley's comet still in one piece?

The basic answer to your question though is that Halley's comet is unlikely to be very old (in the sense that it has not spent a large amount of time orbiting the Sun as closely as it does now). Most ...
ProfRob's user avatar
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21 votes

What is this celestial phenomenon in the video?

That is a re-entry of an artifical satellite, or rocket body. (As Darth Pseudonym mentions, its actually the upper stage from the Soyuz MS-25 launched two days earlier). It's certainly not a comet. ...
James K's user avatar
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17 votes
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How close to escape velocity are most Oort-cloud comets?

It's a consequence of the reversibility of orbits that anything that free-falls from a near-interstellar distance arrives at nearly escape velocity, or faster. Take the Vis-Viva equation, the formula ...
notovny's user avatar
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15 votes

Can a comet orbit a planet?

Its very unlikely for a comet to become a satellite of an inner solar system planet. Much less likely than it is for an asteroid. Most asteroids are on fairly circular orbits, and so the relative ...
James K's user avatar
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15 votes
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Why would the Perseids meteor rate fall off after maximum faster than the increase before maximum?

A very interesting question. Neither Bruce McClure at EarthSky nor I knew the answer, despite the fact that the observed gradual rise in numbers for the Perseids - and their quick drop-off - is ...
Deborah Byrd's user avatar
14 votes
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Do comets contain any significant amounts of iridium?

I also googled "iridium content of comets", and the first result was https://news.dartmouth.edu/news/2013/04/dartmouth-researchers-say-comet-killed-dinosaurs Now there is currently no consensus on ...
James K's user avatar
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14 votes
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Size and mass of comet ATLAS (C/2019 Y4)

The confusion comes from the difference between the nucleus and the coma. The nucleus is a small icy body, only a few km across. The coma is the cloud of gas and dust released from the nucleus as it ...
James K's user avatar
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14 votes
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Does Halley's Comet travel past the outer bounds of the Oort Cloud?

No. Halley's Comet has an aphelion of 35 AU, which is far less than the believed boundaries of the Oort cloud. Any object on an elliptical orbit with a perihelion of 0.6 AU and an aphelion of 2,000 ...
notovny's user avatar
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13 votes
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when are you able to see Halley's comet, and for how long?

Halley's comet orbits the sun and its orbit lasts 75 years. The orbit is a long elliptical orbit. For most of those 75 years it is a cold black dot, and frozen solid in the outer solar system. But a ...
James K's user avatar
  • 126k
13 votes

Could the dinosaurs have seen the asteroid that killed them?

A carbonaceous condrite has the same reflectivity as the moon at around 7-13%. If there was ice, if the tail was 10 times smaller than Hale-Bopp, it would have auspiciously covered half of the sky. ...
bandybabboon's user avatar
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13 votes

Comet Neowise c2020 f3 will return in ~6800 years, how can we be sure Earth will be in the right place to see it then?

The time of the orbit is not well known but the inclination can't change as much (small changes in velocity due to outgassing can significantly change the orbital period, but can't change the ...
James K's user avatar
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13 votes
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Do some comets spin? If so, how fast?

Yes, comets spin although measuring it can be tricky due to the coma and outgassing from the nucleus. It's easiest to measure the rotation period when the comet is inactive near aphelion although this ...
astrosnapper's user avatar
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12 votes
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Blue color of ion (plasma) comet tails

Close, but not quite right - the blue light is indeed emission from CO$^+$, but it's from the CO$^+$ ions themselves, with no need for recombination to CO; that (ionized) molecule has a strong set of ...
Eric Jensen's user avatar
  • 4,894
12 votes

How do we know that a comet passed by earth 50,000 years ago?

If I say "Orbital mechanics is easy" some people who know a lot about this will get very angry. But for the purposes of this question: orbital mechanics are easy. we know where the comet is ...
Borgh's user avatar
  • 229
11 votes
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Why does Kepler's "Big Picture" of comet 67P look so strange? And what is the significance?

One thing to keep in mind is that the Kepler instrument is not a telescope like Hubble. It is a photometer and though it uses CCDs to look at the sky, it doesn't return a picture in the usual sense. ...
Dave's user avatar
  • 801
11 votes
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Do astronomers generally agree that the distinction between comets and asteroids is not so clear?

If we're going to get technical, Asteroids are not really an official name anymore. In 2006, when the IAU redefined what a planet was (and thus demoted Pluto), they also decided to more formally ...
zephyr's user avatar
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11 votes

Where is the Perseids meteor shower coming from?

The comet releases lots of dust particles. The sunlight pushes these particles into orbits not quite the same as the comet Some particles will be pushed into faster orbits, some will be pushed into ...
James K's user avatar
  • 126k
10 votes

Can a comet orbit a planet?

Given that moons commonly orbit planets, why do we never encounter a comet orbiting a planet? By definition, it would no longer be a comet, but rather a moon (or more properly a satellite). Comets ...
zephyr's user avatar
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10 votes
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Does Jupiter help to protect the Earth against comets/asteroids impacts?

Wordy answer, mostly light on Math: The key word here (and the article uses this word) is "long period comets". First there's Jupiter impacts, but that's a relatively low percentage, cause even ...
userLTK's user avatar
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10 votes
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How do comets' nuclei stick together?

Van-der-Waals forces and gravitation are the main answer. The main property keeping together bodies smaller than a few $100 {\rm km}$ are their constituents surface forces between the grains (van-der-...
planetmaker's user avatar
10 votes
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How do we know that ice from the S1094b event was from Mars surface and not from the impactor?

From Richardson et. al, all the small, high-speed debris gets ejected from an impact first, and ends up furthest from the impact point. This would include any intermixing of the impactor with Martian ...
Connor Garcia's user avatar
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9 votes
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Comet Neowise c2020 f3 will return in ~6800 years, how can we be sure Earth will be in the right place to see it then?

It is important to appreciate the scale of the comet's orbit compared to the Earth's orbit. The Earth goes round the Sun in a nearly circular path. The average distance is one Astronomical Unit (AU). ...
Oscar Bravo's user avatar
9 votes

What is this celestial phenomenon in the video?

Objects that shed a lot of bits like that and move relatively slowly are usually human-made space junk from low-earth orbit that has slowed down enough to reenter and burn up. It's definitely not a ...
Darth Pseudonym's user avatar
8 votes
Accepted

How does this comet have alcohol in it?

Getting atoms together into molecules requires two basic things - getting the reactants together and them providing the means to overcome the "activation energy" that is due to the mutual repulsion of ...
ProfRob's user avatar
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