From UPSC perspective, the following things are important :
Prelims level : SSLV, PSLV, GSLV
Mains level : Not Much
The Indian Space Research Organisation (ISRO) has said that the satellite onboard its’ maiden Small Satellite Launch Vehicle “are no longer usable” after the SSLV-D1 placed them in an elliptical orbit instead of a circular one.
What is SSLV?
- The SSLV is a small-lift launch vehicle being developed by the ISRO with payload capacity to deliver:
- It would help launching small satellites, with the capability to support multiple orbital drop-offs.
- In future a dedicated launch pad in Sriharikota called Small Satellite Launch Complex (SSLC) will be set up.
- A new spaceport, under development, near Kulasekharapatnam in Tamil Nadu will handle SSLV launches when complete.
- After entering the operational phase, the vehicle’s production and launch operations will be done by a consortium of Indian firms along with NewSpace India Limited (NSIL).
- Height: 34 meters
- Diameter: 2 meters
- Mass: 120 tonnes
- It will be a four stage launching vehicle.
- The first three stages will use Hydroxyl-terminated polybutadiene (HTPB) based solid propellant, with a fourth terminal stage being a Velocity-Trimming Module (VTM).
SSLV vs. PSLV: A comparison
- The SSLV was developed with the aim of launching small satellites commercially at drastically reduced price and higher launch rate as compared to Polar SLV (PSLV).
- The projected high launch rate relies on largely autonomous launch operation and on overall simple logistics.
- To compare, a PSLV launch involves 600 officials while SSLV launch operations would be managed by a small team of about six people.
- The launch readiness period of the SSLV is expected to be less than a week instead of months.
- The SSLV can carry satellites weighing up to 500 kg to a low earth orbit while the tried and tested PSLV can launch satellites weighing in the range of 1000 kg.
- The entire job will be done in a very short time and the cost will be only around Rs 30 crore for SSLV.
Significance of SSLV
- SSLV is perfectly suited for launching multiple microsatellites at a time and supports multiple orbital drop-offs.
- The development and manufacture of the SSLV are expected to create greater synergy between the space sector and private Indian industries – a key aim of the space ministry.
Back2Basics: Various Orbits of Satellites
 Geostationary orbit (GEO)
- Satellites in geostationary orbit (GEO) circle Earth above the equator from west to east following Earth’s rotation – taking 23 hours 56 minutes and 4 seconds – by travelling at exactly the same rate as Earth.
- This makes satellites in GEO appear to be ‘stationary’ over a fixed position.
- In order to perfectly match Earth’s rotation, the speed of GEO satellites should be about 3 km per second at an altitude of 35 786 km.
- This is much farther from Earth’s surface compared to many satellites.
- GEO is used by satellites that need to stay constantly above one particular place over Earth, such as telecommunication satellites.
- Satellites in GEO cover a large range of Earth so as few as three equally-spaced satellites can provide near-global coverage.
 Low Earth orbit (LEO)
- A low Earth orbit (LEO) is, as the name suggests, an orbit that is relatively close to Earth’s surface.
- It is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth – which is low compared to other orbits, but still very far above Earth’s surface.
- Unlike satellites in GEO that must always orbit along Earth’s equator, LEO satellites do not always have to follow a particular path around Earth in the same way – their plane can be tilted.
- This means there are more available routes for satellites in LEO, which is one of the reasons why LEO is a very commonly used orbit.
- It is most commonly used for satellite imaging, as being near the surface allows it to take images of higher resolution.
- Satellites in this orbit travel at a speed of around 7.8 km per second; at this speed, a satellite takes approximately 90 minutes to circle Earth.
 Medium Earth orbit (MEO)
- Medium Earth orbit comprises a wide range of orbits anywhere between LEO and GEO.
- It is similar to LEO in that it also does not need to take specific paths around Earth, and it is used by a variety of satellites with many different applications.
- It is very commonly used by navigation satellites, like the European Galileo system of Europe.
- It uses a constellation of multiple satellites to provide coverage across large parts of the world all at once.
 Polar Orbit
- Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth’s poles.
- Satellites in a polar orbit do not have to pass the North and South Pole precisely; even a deviation within 20 to 30 degrees is still classed as a polar orbit.
- Polar orbits are a type of low Earth orbit, as they are at low altitudes between 200 to 1000 km.
 Sun-synchronous orbit (SSO)
- SSO is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun.
- This means they are synchronised to always be in the same ‘fixed’ position relative to the Sun.
- This means that the satellite always visits the same spot at the same local time.
- Often, satellites in SSO are synchronised so that they are in constant dawn or dusk – this is because by constantly riding a sunset or sunrise, they will never have the Sun at an angle where the Earth shadows them.
- A satellite in a Sun-synchronous orbit would usually be at an altitude of between 600 to 800 km. At 800 km, it will be travelling at a speed of approximately 7.5 km per second.
 Transfer orbits and geostationary transfer orbit (GTO)
- Transfer orbits are a special kind of orbit used to get from one orbit to another.
- Often, the satellites are instead placed on a transfer orbit: an orbit where, by using relatively little energy from built-in motors, the satellite or spacecraft can move from one orbit to another.
- This allows a satellite to reach, for example, a high-altitude orbit like GEO without actually needing the launch vehicle.
- Reaching GEO in this way is an example of one of the most common transfer orbits, called the geostationary transfer orbit (GTO).