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MIT Space Plasma Group

Voyager Interstellar Mission

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The Heliosphere--A Short Review By Ian Axford and Steven Suess

Detection of Radio Emission from the Heliopause by the Voyager Plasma Wave Experiment

An Animation of the Motion of the Heliosphere Due to Changes In Solar Wind Pressure


Voyagers 1 and 2 were both launched in 1977. Voyager 1 encountered Jupiter in 1979, and Saturn in 1980. Voyager 2 encountered Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Both spacecraft are now headed out of the solar system, but are still within the solar wind. The solar wind consists primarily of protons and electrons flowing out from the sun. This wind blows out a bubble of sun-generated material in the surrounding interstellar medium, the matter between stars. The boundary of this bubble is called the heliopause, and the bubble itself is the heliosphere. Before the heliopause is reached, the solar wind passes through the termination shock, where the solar wind slows down and changes from supersonic to subsonic in preparation for reaching the heliopause. Neither of these boundaries has been explored.

As of 20 February 2004, Voyager 1 was 90.87 AU and Voyager 2 was 72.42 AU from the sun; estimates of the distance to the termination shock and heliopause range from 65-100 AU and 90-130 AU, respectively. The Voyager 1 and 2 spacecraft should reach both these boundaries before they run out of the hydrazine fuel used to stabilize the spacecraft. On the way out these spacecraft are making new measurements of the solar wind conditions far from the Sun.


The Voyager plasma instrument consists of four Faraday cup detectors which measure ion and electron current in the energy range 10-5959 eV/Z. Three of the cups look into the solar wind; the fourth looks sideways to the solar wind direction and is used for planetary encounters and detecting electrons. The currents obtained in the three detectors are analyzed to find the speed, density, temperature, flux, and dynamic pressure of the solar wind.

The instrument was designed and built under the direction of Herbert Bridge; the current P.I. is John Richardson at M.I.T.


A plot of 50-day averages of the solar wind speed, density, and temperature over the life of the Voyager 2 mission from 1977 to the present is available from the link below. The density shown on this plot is normalized to Earth by multiplying by the distance to Voyager in AU squared.

Voyager 2 Overview Plot

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For more information contact John Richardson at MIT (e-mail: jdr@space.mit.edu).

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