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Space Radiation Effects and Microelectromechanical Systems 91

geomagnetic fields extend to higher latitudes, the exposure to solar particles in the
polar regions will be reduced. This will be somewhat compensated by the enhanced
cosmic ray flux. Nevertheless, the total radiation exposure will not be as severe as
during a period of high solar activity.

5.1.3 INTERPLANETARY SPACE
Missions in interplanetary space consist of a number of phases, each with a different
radiation environment. The first phase typically involves placing the spacecraft into
geosynchronous orbit, which requires it to pass through the heart of the Earth’s
radiation belts, sometimes more than once. During this first phase, the spacecraft
will accumulate a significant radiation exposure. The spacecraft then spends some
time in GEO before being injected into its interplanetary orbit. Depending on how
long it spends in GEO, the spacecraft and the components on board could experi-
ence a substantial total dose due to the electrons in the outer region of the second
electron radiation belt. Once the spacecraft has left GEO and is traveling in
interplanetary space, it is exposed to radiation from the Sun and from cosmic
rays. The total radiation flux is then due to solar activity, the distance between
the Sun and the spacecraft, and cosmic ray contributions. The major danger to
spacecraft lies in solar particle events that, as noted previously, can lead to an
increase in the radiation flux to which a spacecraft is normally exposed by many
orders of magnitude.

5.1.4 PLANETARY MISSIONS
The radiation environment around other planets varies greatly, depending on the
strength of their magnetic fields and their distance from the Sun. For instance, the
Earth’s moon has no magnetic field and the radiation exposure there does not differ
significantly from that in interplanetary space. In contrast, Jupiter’s magnetic field
is much stronger than that of the Earth. In addition, Jupiter’s moons orbit within the
intense radiation belts, requiring any mission to one of Jupiter’s moons to use parts
with a high degree of immunity to radiation and, if necessary, to shield the parts as
well. Mars has a very weak magnetic field, which offers little ‘‘shielding’’ against
cosmic rays and solar particles. The absence of a magnetic field around Mars also
means that spacecraft in orbit around Mars will not encounter radiation belts, such
as those on Earth.

5.2 RADIATION EFFECTS

Before a device can be used in space it must be qualified to ensure that it will
survive the rigors of the space environment. Radiation qualification is one of many
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different qualification procedures that must be performed. Others include tempera-
ture, pressure, and vibration. In the absence of specific guidelines for qualifying
MEMS devices for a radiation environment, radiation test engineers make
use of standard radiation qualification procedures that have been developed for
microelectronics.

© 2006 by Taylor & Francis Group, LLC