Hubble Space Telescope has completed its 25th year in space observing planets, stars, galaxies and to reveal other parts of our universe.

According to NASA, the telescope has made more than 1 million observations so far, which has been used by astronomers in more than 12,700 scientific papers, making it one of the most productive scientific instruments ever built.

The telescope rotates around Earth at 28,000 kilometers an hour and takes 96 minutes to complete one orbit. The telescope was named after astronomer Dr. Edwin Hubble, who in 1929 discovered that the universe was expanding. This notion built the Big Bang theory.

The telescope looks like a big shiny, silver bucket with two gangly solar arms and a couple of dish antennae. It has the size of a school bus and weighs about as much as two elephants.

Flying about 340 miles over Earth, far from our light-polluted cities, Hubble has an unobstructed view of the universe. It was launched on April 24, 1990 aboard the space shuttle Discovery from Kennedy Space Center, Florida.

The telescope is also behind the discovery of Hubble Deep Field, tiny speck of sky that revealed a zoo of about 3,000 galaxies as old as 10 billion years as well as supermassive black holes.

Zoltan Levay who works in the Office of Public Outreach at the Space Telescope Science Institute at Johns Hopkins University gave an interview for a new book, marking the 25th anniversary of Hubble's launch. The book is 'Expanding Universe: Photographs from the Hubble Space Telescope'.

He said the hardest part of his job was selecting the best photos from the thousands created by Hubble.