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This source may be nebulous (no pun intended!) for some. It writes to say

Starting at around 95,000mph at the start of this month, Comet ISON will eventually reach an astonishing 845,000mph on November 28th when it catapults round the Sun at perihelion

I'm no astronomer; the only comets I even know the names of are

  • Shoemaker Levy
  • Swift Tuttle
  • Wild
  • Halley Bopp

To the best of my knowledge (feel free to poke me in the rib if i'm wrong!) none of the above list achieved a 3-digit velocity at perihelion. In contrast, going by the figure quoted above, S1 is expected to achieve a mind-boggling 375 km/s!

Is C/2012 S1 the fastest comet on record to-date?

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What would be the perihelion distance at 845,000 miles per hour? Theoretically no mass can attain that speed and remain an integral mass. Its internal gravitational impetus would be near zero force and it could not emerge from perihelion as an intact mass.( Shoemaker-Levy?) Destroyed as an intact body by its speed in orbiting Jupiter at its next to last orbit. –  user5453 Feb 2 at 15:16
    
Shoemaker-Levy was destroyed by Jupiter's tidal forces when it exceeded it's Roche limit. –  Joan.bdm Feb 3 at 8:04

2 Answers 2

ISON is a sungrazer. But there were legendary comets like the Kreutz Family of Sungrazing comets. The Great comet of 1843 is a member of the kreutz family and it passed even more close to the sun at perihelion. It was a time before SOHO was deployed and there are not accurate observations of the speed of the Great Comet.

Since, it was travelling closer to the sun, considering the angular momentum created by the gravity of the sun, The great comet might have been a bit faster than the ISON.

You cannot just term, speed of the comet. Comets travel with higher speeds due to the sun's gravitational pull and can be accelerated to extreme speeds when they catapult around the sun. However, when the comet is moving away from the sun, it'll be travelling slower because of the gravitational pull of the sun.

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A comet that travels to the Sun from the Oort cloud will be travelling roughly at its escape velocity at perihelion.

The fastest comets will be those that travel from the Oort cloud and plunge directly into the Sun.

If you want the comet to emerge from around the Sun, then its perihelion distance must be no lower than $\sim 7 \times 10^{8}$ m. At that point it will have approximately the escape velocity at that radius $v_{esc} = (2GM_{\odot}/r)^{1/2} = 617$ km/s or 1.39 million miles per hour.

Therefore I expect that lots of comets have attained something like or exceeding this speed before. Comet Lovejoy for example achieved a perihelion distance of $8.36\times10^{8}$ m and was travelling at 536 km/s (1.2 million miles per hour, and in fairly good agreement with the formula I quoted above - the aphelion was not infinity, but $\sim 157$ au, which largely accounts for the difference).

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