#22.     WIND


The Wind spacecraft.

NASA's "Wind" spacecraft was launched 1 November 1994 by a Delta rocket from Cape Canaveral. It was designed to observe the solar wind approaching Earth, from a position near the Lagrangian point L1, and it is part of the International Solar-Terrestrial Physics (ISTP) Initiative.

That plan was later changed, when the solar observatory SOHO was placed in a similar orbit. At the time of this writing (10/97) "Wind" remains in an elongated elliptical orbit and continues to observe the solar wind from there. It still carries a large reserve of fuel for its rocket engine and may be sent in the future on a more extensive mission.

ISTP also includes NASA's Polar spacecraft (launched 24 February 1996), Japan's Geotail (1992) and SOHO (1995); other spacecraft will also be involved in ISTP research, including Russia's Interball (1995). The ISTP effort cuts across national boundaries. Two US-supplied intruments ride on Geotail; a French instrument and the first-ever Russian instrument to fly on a US spacecraft are both part of the Wind complement of instruments. Scientists all over the world plan are sharing ISTP data and collaborating in its analysis, often with the help of the World Wide Web.

Encounters with the Moon

The original plan called for "Wind" to approach its final station over two years, undergoing two close encounters with the Moon to boost its speed. A spacecraft approaching a stationary object--like a comet approaching the Sun--increases its speed as it approaches, but after it passes (assuming there has been no collision) it loses again all it had gained. If however the object is moving, the encounter is not symmetric, and in the end the spacecraft may have gained or lost speed, depending on its trajectory. The encounters of the space probes Voyager 1 and 2 with Jupiter not only helped them explore that giant planet and its magnetosphere, they also gave the Voyagers an extra boost which allowed them to continue to Saturn, where another boost helped Voyager 2 continue to Uranus, Neptune and beyond. "Wind" used the Moon's gravity in a similar manner, as did Geotail.

Instruments and Observations

Some of the instruments aboard Wind measure properties of the solar wind plasma--for instance, the speed of its flow, the flow's direction (it can vary by a few degrees) and the distribution of electron and ion energies. They also measure the proportions of various ions in the solar wind: protons and alphas form the great majority, but the stream also includes the rarer isotopes of "heavy" hydrogen and "light" helium, as well as carbon, oxygen and other elements. The variation of these proportions can shed light on processes in the Sun's corona, where the solar wind originates.

Radio wave receivers monitor emissions from the Sun and from space plasmas, and a magnetometer samples the interplanetary magnetic field (IMF) up to 44 times a second. Because the IMF is very weak (about 1/10,000 the Earth's surface field), the magnetic fields produced by electric currents on the spacecraft are strong enough to disturb its observation, and the magnetometer is therefore placed (here and on most deep-space spacecraft) at the end of a long boom, away from the interference.

"Wind" also carries two gamma ray detectors, to observe and time gamma ray bursts from distant space, probably beyond our galaxy (more on them in the section on high energy particles in the universe

Dateline December 1998

At the end of 1997, WIND rounded the L1 Lagrangian point and headed back to Earth. With the ACE spacecraft now positioned near L1, capable of routine monitoring of the solar wind, WIND with its unique capabilities can be positioned elsewhere, providing broader coverage of the solar wind as we approach the next sunspot maximum, around 2000-2001. The new set of orbits is achieved by flying near the moon and using its gravity to alter the spacecraft trajectory.

As of the end of 1998, the new mission is well under way, with "WIND" in its new "petal orbits" which explore the magnetosheath at points abreast of the magnetosphere but relatively distant from the ecliptic. For additional details and updates, see the home page of the "Wind" mission.

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Last updated: December 31, 1998