Solar system objects with perihelia beyond the orbit of Jupiter ( q > 5 AU ) are too cold for water ice to generate an appreciable coma via sublimation .
Despite this , numerous high perihelion objects ( HPOs ) including many comets and recently escaped Kuiper belt objects ( “ Centaurs ” ) are observed to be active out at least to the orbit of Saturn ( q \sim 10 AU ) .
Peak equilibrium temperatures at 10 AU ( \sim 125 K ) , while far too low to sublimate water ice , are sufficient to sublimate super-volatiles such as CO and CO _ { 2 } ice .
Temperatures at 10 AU are also high enough to trigger the rapid crystallization of exposed amorphous ice , thus constituting another possible driver of distant activity .
While supervolatile ices can sublimate strongly ( as r _ { H } ^ { -2 } ) to at least Kuiper belt ( 30 AU ) distances , crystallization is an exponential function of temperature that can not be sustained much beyond \sim 10 AU .
The heliocentric dependence of the activity thus suggests an observational test .
If activity in high perihelion objects is triggered by crystallization , then no examples of activity should be found with perihelia q > > 10 AU .
If , on the other hand , activity is due to free sublimation of exposed supervolatile ices , or another cause , then distant activity might be detected .
We obtained sensitive , high resolution Hubble Space Telescope observations of HPOs to search for activity beyond the crystallization zone .
No examples of activity were detected in 53 objects with q > 15 AU , consistent with the crystallization trigger hypothesis .
However , sensitivity limits are such that we can not reject the alternative hypothesis that mass loss is driven by the sublimation of supervolatile ices .
We also searched for binary companions in our sample , finding none and setting an empirical 3 \sigma limit to the binary fraction of < 8 % .