Context

• The Indian Space Research Organisation (ISRO) launched its first space mission of the year. An earth observation satellite named EOS-04 along with two other smaller satellites were launched into space using a PSLV (Polar Satellite Launch Vehicle) rocket. This was the 54th flight of the PSLV rocket.

Launch vehicles and satellites

• Satellites don’t go into the space on their own.
  • They have to be carried there by launch vehicles, or rockets, like the PSLV. The rockets have powerful propulsion systems that generate the huge amount of energy required to lift heavy objects like satellites into space, overcoming the gravitational pull of the earth.
• Satellites, or payloads as they are often called, sit inside the rocket, and are ejected once they reach near their intended orbit in space.

  

  • Most satellites have small propulsion systems and carry small amounts of fuel, because they encounter very little drag, or force, in outer space.
• Both rockets and satellites are spacecraft, which is a generic term used for any object that is sent into space.
• Satellites carry one or more instruments that do the scientific work for which they are sent into space. Their operational life sometimes extends up to decades.
• But rockets, or launch vehicles, become useless after the launch. Their only job is to take the satellites to their intended orbits.
• Rockets have several detachable energy-providing parts.
• • They burn different kinds of fuels to power the rocket.
  • Once their fuel is exhausted, they detach from the rocket and fall off, often burning off in the atmosphere due to air-friction, and getting destroyed.
  • Only a small part of the original rocket goes till the intended destination of the satellite.
  • Once the satellite is finally ejected, this last part of the rocket either becomes part of space debris, or once again burns off after falling into the atmosphere.

Types of launch vehicles

• PSLV is not the only launch vehicle that ISRO uses.
• There are several kinds of rockets that are used.
• The difference is mainly in the power that they can generate, the weight they can carry, and the distance they can travel into space, all of which are related.
• It all comes down to the energy that is generated, which is a function of how efficient the engine and the fuel is.
• Several satellites need to be deposited only in the lower earth orbits, which starts from about 180 km from earth’s surface and extends up to 2,000 km.
• Most of the earth-observation satellites, communication satellites, and even the International Space Station, a full-fledged laboratory in space that hosts astronauts, function in this space.
• It takes smaller amount of energy to take the satellites to the low-earth orbits, and accordingly smaller, less powerful, rockets are used for this purpose.
• But there are other satellites which need to go much deeper in space. Geostationary satellites, for example, have to be deposited in orbits that are about 36,000 km from earths’ surface.
• The planetary exploration missions also need their rockets to leave them much deeper in space. For such space missions, much more powerful rockets are used.
• In general, there is a trade-off between the weight of the satellite that needs to be launched, and the distance it needs to be taken to.
• The same rocket can take smaller satellites much deeper into space compared to a heavier satellite.

Launch vehicles used by ISRO

• ISRO currently uses two launch vehicles – PSLV and GLSV (Geosynchronous Satellite Launch Vehicle), but there are lots of different variants of these.
  • PSLV is the most reliable rocket used by ISRO till date, with 52 of its 54 flights being successful.
  • GSLV is much more powerful rocket, meant to carry heavier satellites much deeper into space.
    • Till date, GSLV rockets have carried out 18 missions, of which four ended in failure.
• The first rocket developed by ISRO was simply called SLV, or satellite launch vehicle.
• It was followed by the Augmented Satellite Launch Vehicle or ASLV. These two could carry small satellites, weighing up to 150 kg, to the lower earth orbits.
  • ASLV operated till the early 1990s before PSLV came on the scene.
  • PSLV’s first launch was in 1994, and it has been ISRO’s main rocket ever since.
• Today’s PSLV, however, is vastly improved and several times more powerful than the ones used in the 1990s.
• GSLV carried the Chandrayaan-2 mission, and is slated to take the Gaganyaan manned space mission as well.
  • Its Mk-III version can carry satellites weighing up to 4,000 kg to the geosynchronous transfer orbit, close to 36,000 km from earth’s surface.
  • It can take 10,000-kg satellites to the lower earth orbits. Mk-III versions have made ISRO entirely self-sufficient for launching its satellites.
  • Before this, it used to depend on the European Arianne launch vehicle to take its heavier satellites into space.
• ISRO has also developed a launch vehicle specifically for small and micro-satellites.
  • It is called Small Satellite Launch Vehicle, or SSLV, and is targeted at rising global demand for the launch of such satellites.
  • SSLV is meant to offer cost-effective launch services for satellites up to 500 kg.
  • The inaugural SSLV launch is slated for next month. It is supposed to carry an indigenous earth observation satellite EOS-03 into space.

Reusable rockets

• The future rockets are meant to be reusable. Only a small part of the rocket would be destroyed during the mission. The bulk of it would re-enter the earth’s atmosphere and land very much like an airplane, and can be used in future missions.
• Reusable rockets would cut down on costs and energy, and also reduce space debris, which is becoming a serious problem because of the large number of launches.
• Fully-reusable rockets are still to be developed, but partially-reusable launch vehicles are already in use.
• ISRO has also developed a reusable rocket, called RLV-TD (Reusable Launch Vehicle Technology Demonstrator) which has had a successful test flight in 2016.

Back to basics

Why don’t satellites fall out of the sky?

• A satellite is a type of machine that orbits Earth, taking pictures and collecting information. There are thousands of satellites orbiting Earth right now.

How do they all stay up there—and why don’t they just fall out of the sky?

• If you throw a ball into the air, the ball comes right back down. That’s because of gravity—the same force that holds us on Earth and keeps us all from floating away.
• To get into orbit, satellites first have to launch on a rocket. A rocket can go 25,000 miles per hour! That’s fast enough to overcome the strong pull of gravity and leave Earth’s atmosphere. Once the rocket reaches the right location above Earth, it lets go of the satellite.
• The satellite uses the energy it picked up from the rocket to stay in motion. That motion is called momentum.

But how does the satellite stay in orbit? Wouldn’t it just fly off in a straight line out into space?

• Not quite. You see, even when a satellite is thousands of miles away, Earth’s gravity is still tugging on it. That tug toward Earth–combined with the momentum from the rocket… …causes the satellite to follow a circular path around Earth: an orbit.
• When a satellite is in orbit, it has a perfect balance between its momentum and Earth’s gravity. But finding this balance is sort of tricky.
• Gravity is stronger the closer you are to Earth. And satellites that orbit close to Earth must travel at very high speeds to stay in orbit.

Source: IE & Scijinks

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