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Why are the fringes of the Sun’s atmosphere hotter than the surface? The Parker Solar Probe may tell us.

Bengaluru: Moving away from a source of heat should mean a gradual decrease in temperature, but it’s different for the Sun. Where the temperature on the surface of the star is estimated to be around 6,000°C, the outermost part of its atmosphere, the corona, can measure up to a million degrees Centigrade.

Just why it is so remains an enduring mystery for space explorers, but one they hope to solve with one of their most ambitious missions yet — NASA’s Parker Solar Probe, which took off from Florida Sunday.

The craft, the fastest ever built, is designed to get closer to the Sun than humanity has ever before. By November, NASA said, it will be at a distance of 24 million kilometres from the surface and in the corona, the Sun’s outer “aura” that is visible clearly to us during solar eclipses and extends thousands of kilometres from the surface.

This will be preceded by a brief dance with Venus, one of seven planned over seven years to enable Parker’s orbit to reach its target distance of just over six million kilometres from the Sun.

Nine years in the making

The Parker Solar Probe was announced in 2009 and built in collaboration with Johns Hopkins University, which will work on the data the craft provides.


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It is the first space mission named after a living person, astrophysicist and University of Chicago professor Eugene Parker, who first proposed the existence of solar win — described by Nasa as the constant flow of charged particles from the Sun — six decades ago.

Among other things, the probe will study the mechanisms behind the production and control of solar wind, solar magnetism, and the high-energy particles in the coronal plasma.

At its closest point of approach — six million km or 8.5 solar radii from the Sun — the Parker probe will be subject to extreme heat, over 475 times the amount we receive on Earth. So, the probe will be protected by a thermal shield at all times and always be in shadow. If not, it would completely burn up in a matter of seconds.

Naturally, the probe runs on solar energy. It is powered by two large solar panels. These would retract when the probe is close to the Sun, releasing two smaller secondary ones. The probe will fly at an average speed of 690,000 km per hour, fifteen times faster than a bullet.

Timeline and orbit

Staying within the corona for extended periods of time is dangerous for any craft that we can build today. The high heat will wear out the heat shields and the strong magnetic radiations will damage equipment over a period of time.

Thus, it is imperative for the craft to enter and exit the corona, to give itself a recovery period. Therefore, the craft cannot be in orbit around the Sun the way they are around planets.

What Parker will do instead is use the Venus’ gravity to go around the sun in a process known as ‘gravity assists’. In other words, it will be in an orbit that goes around the Sun and Venus, with the star on one end and the planet on the other. The probe is expected to have a lifespan of seven years, during which time it will make 24 orbits around the sun and Venus.

Animation of Parker Solar Probe trajectory
Animation of Parker Solar Probe’s trajectory | Wikimedia commons
With each subsequent orbit, the time taken to go around the Sun reduces as Venus moves in its own orbit, closer to the star.

The first Venus flyby is scheduled for 28 September.

An Indian mission in the wings

The Indian Space Research Organisation (Isro) also has its sights set on the corona, with plans to launch a craft, Aditya L-1, to study it.


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The five Lagrange Points | Wikimedia Commons

However, it has no intention of sending a craft so close to the Sun. Instead, Aditya-L1 will orbit a point in space between the Sun and Earth. This point is one of five ‘Lagrangian Points’, where the gravitational effects of the Sun and Earth (or any two bodies) act in such a way that the point is stable enough for an object to be present without tumbling away into space.

Lagrangian Points have the unique characteristic of being able to be orbited, despite just being empty space. Such an orbit is called Halo Orbit.

India’s 400-kg craft is expected to be launched in 2019-2020.

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