Page 98 - Intro to Space Sciences Spacecraft Applications

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Spacecraft Environment
the sun, the velocity of this stream increases as it leaves the sun (to a
point, after which it remains relatively constant).*
As mentioned earlier, the sun rotates while generating a substantial mag-
netic field. A characteristic of a plasma passing through a magnetic field is
that the field lines become trapped and move with the plasma. As the solar
wind propagates away from the sun, the solar magnetic field is dragged
along but, due to its magnetic properties and the rotation of the sun, also
affects the propagation of the plasma. The result is that the solar wind can-
not be described as propagating in a straight path away from the sun. Actu-
al propagation is three-dimensional and complex, making it difficult to
predict the actual environment that may appear in the vicinity of the earth.
On average, the velocity of the solar wind in the vicinity of the earth is
about 500 km/sec (supersonic in the definition of the term) and the densi-
ty is about five particles/cm3. Solar magnetic field strength is about 5 y,
approximately 1/10,000* the surface magnetic field strength of the earth.
As a final note, it is suspected that a boundary exists around 100 A.U.
away from the sun where the streaming solar wind encounters the interstel-
lar space environment defining a region surrounding the sun known as the
heZiosphere.t Some futuristic propulsion schemes propose to use the solar
wind, like a sailboat uses atmospheric winds, to travel between the planets.

Sunspots/Sohr Flares. The second major source of particle radiation is due
to disturbances on the solar surface. The processes which produce sunspots
and solar flares are intimately linked and must be described together.

Sunspots. Galileo, using the crude telescopes of his own design, was the
first to notice “blemishes” or spots on the surface of the sun. Even though
he had no idea of the processes that may have accounted for these spots,
he did keep a close enough watch on them to determine that the sun

*The Ulysses spacecraft, a joint NASABSA mission launched from the Space Shuttle in October
1990, used a gravity assist from Jupiter to change its orbital plane out of the ecliptic (the sun’s equa-
torial plane within which the planets more or less lie) to sample the interplanetary space environ-
ment over the sun’s poles. The results show a difference in the velocity/density relationship from the
ecliptic, which has yet to be fully explained.
___ - __
+Pioneer 10 (launched in December, 1973) and Voyager I and Voyager 2 (launched in September
1977 and August 1977, respectively) interplanetary spacecraft are on trajectories that will carry them
beyond the solar system. In 1996, Voyager 2 will be around 45 A.U. from the sun, and Pioneer 10
and Voyager I will both be approximately 60 A.U., all traveling in different directions. These still-
operating spacecraft continue to be monitored for information to help determine the extent and char-
acteristics of the heliosphere.