Of course photons go $c$ in a vacuum, and seems there's data on electrons getting to  [$0.999999999976c$ and protons to $0.99999912c$.](http://boards.straightdope.com/sdmb/showpost.php?s=762868241d8a92b8aaf96f2b94fa1b29&p=2269018&postcount=4) 

For the twins paradox to be plausible, one of the twins must reach ["99.995% the speed of light" (Lorentz factor of γ = 100 )](https://en.wikipedia.org/wiki/Twin_paradox) without being atomized. However, I can't find any examples of molecules remaining stable as they are accelerated anywhere near $c$. Given that most accelerators use magnets and/or very high static voltage (e.g. linear accelerators), both of which operate 'best' on charged things, my refined question might read: 

> 
> What is the highest fraction of $c$ a [polyatomic ion](https://en.wikipedia.org/wiki/Polyatomic_ion) has been accelerated to without it breaking up? And, if that's not more than say $.90c$, is ion break-up indeed the limitation? 
> 
> 
> 

equally useful: Any idea on what some of the fastest *observed* polyatomic ions have been clocked at? ( I'm picturing a measurement on something accelerated by a charged solar wind, or maybe gravity accelerated in a mostly linear manner by an interstellar process. )

**Update** (2015-04-19):

So, I don't really have a good answer, but the best so far is a pathetic 0.0043$c$ ...(?!?) 

These the the best 3 datapoints I've found, all from earth reference frame: 

> 
> A comet observed at 0.0018 c ( from <https://astronomy.stackexchange.com/questions/1011/is-c-2012-s1-aka-ison-the-fastest-comet-on-record-to-date> )
> 
> 
> Elements in the constellation Leo at 0.0013 c ( using the upper bound of 380 km /sec from: <http://image.gsfc.nasa.gov/poetry/ask/a10552.html>)
> 
> 
> The ion CF3 a 0.0043c (from the paper behind the paywall at: <http://www.sciencedirect.com/science/article/pii/S0168583X97004138> - using that 4 MeV applied as per nucleon )
> 
> 
> 

Regarding the link on lead from Anna v - by the time the lead ions get anywhere near high fractions of c, indeed all the electrons are stripped away (<http://press.web.cern.ch/press-releases/2010/11/cern-completes-transition-lead-ion-running-lhc> ). Unfortunately, a lead ion (even with its electrons) isn't a polyatomic entity/ion, and it's empirical, observational support for the stability of multi-atomic entities at high fractions of c, relative to earth-reference-frame, that I'm interested to see the data on

Anyway, the best of the above is coming in at .0043c. Since I, somehow, can't imagine that's right, if anyone can do better, or could validate that that is even approximately correct, I'd check it as the answer!