Why is it difficult to observe the planet Mercury?

The earth takes $1$ year to make a complete revolution around the sun whereas Pluto takes nearly $248$ of our earth years to orbit the sun. What is the main reason for this?

What is the name of $400 \mathrm{km}$ thick bright layer around the sun?

Which of the following is not a natural satellite?

Mention any two achievements of Hubble Telescope.

What is an artificial satellite? Name any two types of artificial satellite and state their functions.

Which satellite is used for geographical survey?

Why do tail of comets formed, when it comes closer to sun?

Fill in the blank with the appropriate number:
The surface temperature of the sun is _____${}^0 \mathrm{C}$. ($2000$/$5500$/$3500$)