Tuesday marked the start of World Space Week, an international celebration of science and technology.

This year's theme is space and sustainability.

The United Nations General Assembly declared in 1999 that World Space Week will be held annually from Oct. 4 to Oct. 10.

They chose these dates as Oct. 4 marks the launch date of the first human-made earth satellite, Sputnik 1.

Oct. 10 is the date of the signing of a treaty on peaceful uses and exploration of outer space.

Last year's World Space Week theme was women in space.