Ive seen the picture of the kuiper belt asteroid.

 

They are not spherical .

 

Even some of them are very long shape and very strange shape.

 

Any reason?

 

Any theory?

 

But Asteroid main belt asteroids are spherical- not sphere.

That is the distinction between a minor planet and a small object. Minor planets are massive enough that they can self-gravitate and form a more-or-less spherical object. Small object are shaped like potatoes because they have enough mass to randomly pull objects toward them but do not have enough mass to make those random conglomerates of mass form a sphere.

Ive seen the picture of the kuiper belt asteroid.

 

They are not spherical .

 

Even some of them are very long shape and very strange shape.

 

Any reason?

 

Any theory?

 

But Asteroid main belt asteroids are spherical- not sphere.

 

Actually you are incorrect, main belt asteroids are seldom spherical and many Kuiper belt objects are. The shape of these bodies has do with size. The bigger an object is the more spherical is will become under it's own gravity. The largest main belt asteroid, Ceres, is thought to be more or less spherical the next one down in size Pallas and the next Vesta somewhat less so. the smaller they get the less regular their shape is.

 

A Kuiper belt object would need to be of similar size to be spherical.

Let's keep in mind that it is somewhat more complicated than the size of the body. It's mass, hence the density of its component units, is key. Also the mineralogy, structure, tehmperature and phase will determine its strength. Composition, especially for those asteroids which are primordial, will set possible temperature ranges based on the quantity of short lived elements present, which in turn will dictate melting and differentiation.

Spherical Condition

 

I have not found proper material yet.

 

How about this?

 

At least a object had next condition during the past some period.

 

1)High boiling point solid case

At least 2/3 or more part( modifications required)of the object from the center was liquified. Liquifying temperature of the object is a function of pressure(P) and temperature.

 

P=G (diameter /2) (density)

G;Gravity, diameter function

2)It had been radiated by high energy source like Sun.

3)It's vapor phase(light gas) was changed to solid or liquid.

 

Any other case? Or others?