The sun in sight
IN a few weeks, we shall see the launch of the most daring of all spacecraft designed to study the sun.
It is the Parker Solar Probe, and it will study the sun from a closer distance than any previous space missions.
Its goal will be to learn much more about the sun’s superheated corona (its outer atmosphere), its magnetic field, and the stream of charged particles, called the solar wind, coming from the sun.
The probe is named after physicist Eugene Newman Parker, who some 60 years ago put forward important ideas about the corona and the solar wind.
The sun is the only star that we can study at close range — it is only 150 million kilometres from us. This contrasts with the 40 trillion kilometre distance to the nearest star system to the sun: the triple-star Alpha Centauri system, of which the two brightest form one of the “pointers” to the Southern Cross.
The probe will eventually approach the sun to within about 6 million kilometres, getting to a point 96 per cent of the way from the Earth to the sun.
It may seem that getting a spacecraft to travel study the sun at close quarters would be a relatively easy task, given that the sun’s gravitational field is the strongest in the solar system. However, it’s not that simple to get there, and working out how to do this has been a major problem.
The difficulty starts with the fact that a space probe at launch is already travelling at about 30 kilometres per second — the Earth’s orbital speed around the sun. To get the craft to be in the right place for its solar studies turns out to be a rather complex problem.
The solution is to have an amazing series of fly-bys of Venus. The probe will fly past that planet seven times between late 2018 and late 2024, using the gravity of Venus to repeatedly change its velocity so that it eventually settles into an orbit that will take it only about 88 days to complete each passage around the Sun. This is only about a quarter of the Earth’s orbital period, and is about equal to that of Mercury.
However, the craft will be on a far more elliptical (ovalshaped) orbit than Mercury, allowing it to pass much closer to the sun than Mercury does.
Heading so close to the sun means that the probe will need to withstand extreme heat. At that distance, over half a megawatt is incident on every square metre. A special shield made of a lightweight carbon composite, 11.4cm thick, on the sunward side of the craft will provide protection.
As the probe makes each pass through its closest point to the sun in the outer part of the corona, it will have a splendid opportunity to make measurements of the corona and the magnetic field, supplying astronomers with a great deal of information.
Although we can study the corona remotely using special instruments, it cannot be seen under normal circumstances — and in any case, do remember that looking at the sun at any time can cause eye damage.
However, it is visible to the unaided eye during a total solar eclipse, when the moon briefly covers the disc of the sun completely. These events are seen about every year and a half, on average, along a narrow path across some part of the world.
The next one visible from Tasmania will not be until the year 2131; if you plan to be around at that time, the place to be will be the far North-West.
The solar wind is a major topic of study. It is responsible for our “space weather”, which causes geomagnetic storms that can involve disruptions to communications, spacecraft electronics and our electrical energy supplies, but also wonderful displays of the aurora, as Tasmanians well know.
Although we shall have to wait quite some time before we have the results from the Parker Solar Probe, they will be very exciting. Its many measurements will help us to understand more about our sun, and indeed stars in general.