Why ISRO Launches Its Rockets from Sriharikota

If you have ever watched an ISRO launch on TV, you have seen the same scene: a rocket rising from a flat, sandy strip of coast with the sea right behind it. That place is Sriharikota, a barrier island on the Andhra Pradesh coast, north of Chennai and close to the Tamil Nadu border. It is home to the Satish Dhawan Space Centre, usually shortened to SDSC SHAR.

The location was not an accident. Picking a launch site is a physics problem and a safety problem at once, and Sriharikota solves both rather well.

The free push from Earth's spin

The Earth spins from west to east, once a day. A rocket waiting on the pad is already travelling east at a healthy speed before its engines fire.

How fast depends on where you are. At the equator, the ground covers the full circumference of the Earth, about 40,000 km, every day, which works out to roughly 1,670 km/h. As you move towards the poles, the circle you travel each day gets smaller, so your speed drops. At the poles it is zero.

A simple rule gives a good estimate: the speed is the equator figure multiplied by the cosine of your latitude. Here is how that plays out for a few well-known launch sites, with rounded numbers:

Why does this matter? To stay in a low orbit, a satellite has to move at roughly 28,000 km/h. Every bit of speed the rocket gets for free is speed it doesn't have to create by burning propellant. A head start of 1,600 km/h is nowhere near the whole job, but it means slightly less fuel, or a slightly heavier satellite. Small margins add up.

The catch: the push only helps rockets heading east, the way the Earth turns.

Sea to the east, where the pieces fall

A rocket is built in stages. Each stage burns, empties and drops away, and the spent hardware falls back to Earth somewhere along the flight path. The payload fairing, the nose shell that protects a satellite on the way through the atmosphere, also falls away early. You don't want any of this landing on a town.

From Sriharikota, a rocket heading east is over the Bay of Bengal within moments of lifting off. Spent stages drop into open sea, far from homes. The flight path towards geostationary transfer orbit, used for communication satellites such as the GSAT series, runs east over water for a long way.

That's why so many launch sites, from Cape Canaveral to Kourou, sit on an east-facing coast. A site deep inland would drop hardware over farmland on every launch.

Why being near the equator helps geostationary satellites

There's a second, less obvious benefit of a fairly low latitude. Communication and weather satellites often go to geostationary orbit, which sits directly above the equator, about 36,000 km up. A satellite there goes round at the same rate as the Earth turns, so it seems to hang still over one spot.

A rocket launched from a given latitude puts its satellite into an orbit tilted at least that much to the equator. Before the satellite can settle over the equator, that tilt has to be removed, and changing the angle of an orbit is expensive in fuel. Start from 13.7° and there is less tilt to remove than from 28.5°, and far less than from 46°. Fuel saved here can mean a longer working life, because the same fuel is later used to keep the satellite in position.

Going south: polar and sun-synchronous launches

Not every satellite goes east. Earth-observation and mapping satellites, including many launched on the PSLV, use polar or sun-synchronous orbits. These pass close to both poles, so as the Earth turns underneath, the satellite gradually sees the whole planet. A sun-synchronous orbit is a special near-polar orbit that crosses each place at about the same local time of day, which keeps the lighting consistent between images.

To reach these orbits, rockets from Sriharikota head roughly south, and again the stages fall into the sea. But there's a complication. A path straight south from Sriharikota would take the rocket over Sri Lanka. To avoid flying over another country, the rocket follows a path that bends around it, often called a dogleg manoeuvre. That turn costs some performance.

Polar launches also get no help from the Earth's spin, since they aren't heading east. Sun-synchronous orbits are actually tilted slightly backwards against the spin, so the rocket has to cancel a little of that free eastward speed. That's why the same rocket can lift more to an eastward orbit than to a polar one.

Why an island?

Sriharikota is a long, narrow island with the Bay of Bengal on its eastern side and Pulicat Lake, a large brackish lagoon, on its western side. That geography gives ISRO several things a launch site needs.

• Space and a safety buffer. Rockets carry tonnes of propellant, and if something goes wrong near the pad, it has to happen far from people. A large, thinly populated island makes it easier to keep a wide safety zone.
• Easier control of the area. With water on two sides, the site is simpler to secure and to clear before a launch.
• Sea in the right directions. Open water to the east for eastward launches, and to the south for polar ones.

The site was chosen in the late 1960s, when India's space programme was moving beyond the small sounding rockets launched from Thumba, near Thiruvananthapuram. Thumba was picked because it lies very close to the magnetic equator, ideal for studying the upper atmosphere, but it was never suited to large orbital rockets. The centre was later named after Satish Dhawan, who led ISRO through much of its formative period.

There are trade-offs. The east coast gets cyclones, especially in the months after the monsoon, and launches are sometimes rescheduled for weather. Lightning, strong upper-level winds and heavy rain all play a part in the final go or no-go decision. No site is perfect; this one gets the big things right.

The new small-rocket site in Tamil Nadu

ISRO has been developing a second launch site at Kulasekarapattinam, on the coast of Thoothukudi district in southern Tamil Nadu. It is meant mainly for small launchers such as the SSLV, the Small Satellite Launch Vehicle.

The logic follows from everything above. Being further south and west, a rocket from there can head south towards polar orbits over open ocean without the detour around Sri Lanka. For a small rocket, avoiding that turn is a meaningful share of what it can carry. Projects like this take years, so for the current status, go by ISRO's own announcements rather than older news reports.

What to remember next time you watch a launch

• The rocket starts with a free eastward speed of about 1,600 km/h, thanks to the Earth's spin and Sriharikota's fairly low latitude.
• Eastward launches, for communication satellites, fly out over the Bay of Bengal, so spent stages fall into the sea.
• Southward launches, for polar and sun-synchronous orbits, also go over water but have to bend around Sri Lanka.
• East coast weather is the main thing that pushes launch dates around.

If you'd like to see a launch in person, ISRO has run a public viewing gallery at Sriharikota for some launches, with registration online. Arrangements vary from launch to launch, so check ISRO's official website and social media accounts well in advance.