Evidence that comets and asteroids are the same thing comes from the physical observations of their surfaces from its rocky terrain, boulders and especially regolith (material covering a surface).
Previous missions to fly near or land on asteroids (NEAR-Shoemaker mission, Hayabusa mission, asteroid Lutetia during the Rosetta mission), have shown their surface having a rocky surface with boulders and some parts of the asteroid covered in 'regolith'. The Rosetta Mission to comet 67P has shown similar terrain.
Science theory suggests that comets are dirty snowballs and asteroids are rocky objects.
Are they similar objects just on different orbits?
Regolith on asteroids and comets
Asteroids have regoliths developed by meteoroid impact. The final images taken by the NEAR Shoemaker spacecraft of the surface of Eros are the best images of the regolith of an asteroid. The recent Japanese Hayabusa mission also returned clear images of regolith on an asteroid so small it was thought that gravity was too low to develop and maintain a regolith. The asteroid 21 Lutetia has a layer of regolith near its north pole, which flows in landslides associated with variations in albedo.
The Electric Universe theory would suggest that the regolith does not come from meteor impacts. The 'dirt' comes from spark machining (electric discharge machining) or spluttering as the electrically charged asteroid or comet interacts with the suns electrical plasma environment.
The EU theory has constantly predicted that comets are rocky objects, not ice mountains, they are the same as asteroids.
(Asteroid Lutetia) The most recent large impact feature is a 21-km diameter crater cluster in the Baetica region, close to the north pole. Most of the ejecta from the initial impacts seems to have failed to reach escape velocity and fallen back to the surface. As a result, the surrounding areas are partially covered by smooth material.
Some craters have been partially or completely buried by huge landslides.
Rosetta reveals mysterious Lutetia
The Electric Universe theory would suggest that how do these asteroids manage to survive such immense impacts that create relatively massive craters and if such a large impact how did the ejected material not escape from the asteroids weak gravity?
If they are electrically machined then a lot of the machined rock material would remain be able to remain on the asteroid, hence why the craters are buried in regolith and there are landslides.
Asteroids and comets are the same thing, just with different orbits. They will have different composition but comets are not ice mountains.
(Asteroid) Itokawa looks as if it consists of two attached masses. The asteroid surface has two distinct surface types: a smooth zone with regolith (a layer of sand or gravel on the surface), and a bumpy zone, covered with a great number of rocks.
Close-Up on the Asteroid Itokawa