Did You Know

Astronomers use light years as a measure of distance in the universe. One light year is the distance that light travels in one Earth year. The speed of light is 300 000 kilometres per second, so in one year light travels a distance of 9.5 million million kilometres. The nearest star to our solar system (apart from the Sun) is Proxima Centauri, at a distance of about 4.2 light years. In a scale model with the Sun and Earth 30 cm apart, Proxima Centauri would be 82 km away. Because Proxima Centauri is 4.2 light years away from Earth, it takes 4.2 years for light from that star to reach us. Thus when you look up into the night sky at this star, the light you see left the star 4.2 years ago. You are really looking into the past.

It takes about 8 minutes 20 seconds for light to travel from the Sun to Earth.

Table 3.1 Details of the Sun


2.0 x 1030 kg

Size relative to Earth

109 times larger


1.4 million km

Distance from Earth

150 million km


1410 kg/m3


3.9 x 1026 J/s

Surface temperature


Interior temperature

15 million°C

Equatorial rotation period

25 days


92% hydrogen, 7.8% helium

Surface gravity

290 N/kg (29 x Earth gravity)

Escape velocity

618 km/s

Photosphere thickness

400 km

Chromosphere thickness

2500 km

Core pressure

250 billion atmospheres

Sunspot cycle

11 years


4.5 billion years

The USA launched a number of unmanned solar probes between 1959 and 1968 as part of its Pioneer program. Many of these early probes have now completed their missions but still remain in orbit around the Sun. Missions such as Pioneers 10 and 11 showed that gravity assists were possible and that spacecraft could survive in high-radiation areas.

America's first space station, Skylab (launched in 1973), was used to study the Sun from Earth orbit. The space station included the Apollo Telescope Mount (ATM), which astronauts used to take more than 150 000 images of the Sun. Skylab was abandoned in February 1974 and re-entered the Earth's atmosphere in 1979.

Significant solar probes launched as joint USA and ESA projects include Ulysses (launched 6 October 1990) and SOHO (launched December 1995).

Figure 3.2 A spectacular photograph of solar flares and the Sun's surface as seen by the SOHO space probe in June 2000. Some of these flares extend more than 500 000 km into space. (Photo: NASA)

In 1998, NASA launched the TRACE (Transition Region and Coronal Explorer) satellite into Earth orbit to observe the Sun and study the connection between the Sun's magnetic field and the heating of its corona. The probe was launched from a Pegasus XL rocket dropped from a jet aeroplane flying high above the Pacific Ocean. TRACE's main instrument is a Cassegrain telescope, 30 cm in diameter and 160 cm long. TRACE is one of several small satellites in NASA's Small Explorer (SMEX) project.

The Genesis space probe launched by NASA in August 2001 collected samples of solar wind particles and returned them to Earth for analysis. Initial tests showed Genesis collected about 0.4 milligrams of solar particles, equal only to a few grains of salt. They are still being analysed.

Table 3.2 Significant probes sent to the Sun

Name of probe

Country of origin



pioneer 5

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