SUN EXPLORATION

The spacecraft exploration of the Sun, began in earnest with the United States' Pioneer spacecraft and the Earth orbiting satellites OSO (Orbiting Solar Observatory) and OGO (Orbiting Geophysical Observatory). The very first solar observing spacecraft was Pioneer 4, launched in 1959. From a solar orbit, Pioneer 4 studied solar flares and the Earth's magnetic field. Successive Pioneers studied the Earth's upper atmosphere, the magnetosphere, and collected data on solar radiation levels, the solar wind, and cosmic rays. Since 1959, more than 70 spacecraft have been used to investigate our nearest star

One of the most exciting periods in the study of the Sun came in 1973 and 1974, when astronauts aboard the Apollo programme space-station Skylab were able to carry out direct observations of solar phenomena in X-ray and ultraviolet wavelengths. The Skylab crews collected many thousands of impressive, and very useful images and monitor a wide range of solar activity including solar flares, prominences, bright-spots, coronal transients, and coronal holes. The Skylab mission advanced our understanding of the nature of the Sun dramatically.

A succession of less expensive unmanned probes followed Skylab, these included Explorer 49, (NASA ) in 1973; Helios-1 and 2, a joint NASA / German spacecraft in 1974 and 1976; and the Solar Maximum Mission (SMM) in 1980.

The objective of the Solar Maximum Mission (SMM) was to observe solar flares during a period of maximum solar activity. A mechanical fault at the beginning of the mission nearly put paid to SMM but was revived in a rescue mission (STS-41C in 1984). Shuttle astronauts made rendezvous with and repaired SMM, which went on to collect and return data until 1989.

Three more solar observing missions were launched during the 1990s: Ulysses (1990), Yohkoh (1991), and SOHO, (1995); in addition to the small Shuttle launched and retrieved solar satellites- SPARTANS 1 to 3, flown in 1993, 1994, and 1995.

Ulysses, a joint NASA / ESA mission, was designed to observe the environment at the polar regions of the Sun. It first flew outward to Jupiter in order to gain a gravitational boost and allow it to travel out of the ecliptic plane and over the poles of the Sun. Ulysses' first solar approach in June 1994; the spacecraft crossed the solar equator in February 1995, the north pole in June 1995, and another south polar pass was made in 2000. Bristling with instruments, among those carried by Ulysses were: magnetometres, a solar wind plasma experiment, an array of particle detectors, a radio and plasma-wave experiment, a solar-flare x-ray and cosmic gamma-ray burst experiment, a cosmic dust experiment, and coronal sounding experiment.

The SOHO (Solar and Heliospheric Observatory) spacecraft, operated by NASA and ESA, was equipped to observe velocity oscillations and radiance variations across the suns surface; their behavior enabling astronomers to build a better picture of the Sun's structure. Among SOHO's other tasks were to: study the Sun's outer atmosphere- the corona, and the solar wind generated there; to monitor the charged gases (plasma) close to the Sun; and monitor the solar wind in the proximity of the spacecraft.

SOHO is currently in a halo orbit about the Sun, at Earth's L1 Lagrange point, 1.5 million kilometres away from us; which allows it to make uninterrupted solar observations. A spin-off from solar observation are is discovery of comets when they pass close to the Sun, which would otherwise would probably have gone undetected. The LASCO coronagraph instrument aboard SOHO, masks the very bright visible surface in order to observe the space surrounding the Sun; to date more than 500 comets have been found in LASCO images, many by amateur observers searching the images which are made available on the internet.

New missions

The more recently launched Genesis (2001) collects particles of solar wind for return to Earth. The particles embed themselves in a wafer thin collector array which the spacecraft unfurled when it reached the L1 Langrage point. After two years collecting Genesis will return to Earth, the sample capsule re-entering the atmosphere for a mid-air recovery. The particles will undergo very detailed analysis - in comparing them with the known composition of the planets and asteroids, it is hoped these bits of the Sun will shed light on the formation and early development of the Sun and the Solar System.

Another new mission is HESSI (High Energy Solar Spectroscopic Imager). Successfully launched in 2003, HESSI's science objectives are to study solar flares (made up of X-rays and other emissions), and their behavior - in particular, to explore the poorly understood mechanism for the gas reaching very high temperatures in the solar atmosphere. HESSI will acquire simultaneous high-resolution X-ray images, gamma-ray data and high-resolution spectroscopy; which will be combined to build a detailed energy spectrum across images of the energy release sites.

Future solar observation missions include Solar-B, a Japanese Space Agency (ISAS) mission which will continue the work of Yohkoh (Solar-A). Solar-B would carry optical, extreme ultra-violet and X-ray sensors. Its main science goal, to explore the Sun's magnetic field and corona and determine how changes in the Sun's magnetic field are related to changing solar output.

STEREO (Solar TErrestrial RElations Observatory), scheduled for launch in 2005, comprises two almost identical spacecraft which will be used to obtain 3D stereoscopic data of the violent eruptions from the solar surface known as CMEs (coronal mass ejections). The STEREO instruments will be used in conjunction with Earth-based detectors and monitor the build up, the release of energy and the form and trajectory of CMEs in three-dimensional views. A better understanding of the behavior of the CMEs and is very important, as when they reach Earth they generate electromagnetic storms detrimental to satellites, communication and navigation systems, and can cause electrical power failures.