What does the fourth stage of the PSLV do?

Among the rockets that India has built, one has earned a reputation for reliability above all: the Polar Satellite Launch Vehicle, or PSLV.

In its most powerful version, the PSLV stands about 44 metres tall, is 2.8 metres wide and weighs about 320 tonnes at lift-off. It has four stages, each of which fires in turn and then falls away. The first stage gives the huge push needed to lift the rocket off the ground; the fourth and last carries the satellites to the exact orbits they need.

What makes the PSLV special is that it can carry several satellites at once and place them in different orbits. In 2008 it launched ten satellites together. In 2016 it placed eight satellites in two different orbits in a single mission. And in one flight, known as PSLV-C37, it carried a Cartosat-2 series satellite along with 103 smaller co-passenger satellites: 104 in all.

The rocket has also carried some of India's most important missions, including Chandrayaan-1 to the Moon, the Mars Orbiter Mission and the Aditya-L1 solar observatory. For a machine that must work perfectly the first time, every time, it has become one of the most trusted in the world.