This is the story behind NASA's successful exploration of the solar
system. For a half century, NASA has sent one probe after another
into space, achieving scientific and technological successes along
the way and adding to humanity's knowledge of the solar system.
NASA has reaped these great rewards thanks to a small investment in
deep-space navigation. With rare exceptions, navigation's great
achievements-and scientific accomplishments-have gone unobserved.
The failures of the Mars Climate Orbiter and Mars Polar Lander shed
navigation (wrongly) in a negative light. Yet, the indispensable
role of navigators behind NASA's many successes over the past half
century has not come to light-until now. The institutional home of
deep-space navigation is the NASA's Jet Propulsion Laboratory. JPL
navigation originated long before it became part of NASA, when the
lab developed and tested missiles for the nation's military space
effort. From the start, deep-space navigation was an endeavor built
on science and mathematics and dependent on the Deep Space Network
for tracking spacecraft and on digital computers and software for
processing data. Navigation is multidisciplinary. It involves
astronomy and radio astronomy, geodesy and geophysics, cartography
and meteorology, ionospheric physics and radio science. Navigators
interact with a panoply of institutions, whether the Bureau
International de l'Heure, the International Polar Motion Service,
the International Union of Geodesy and Geophysics, and the
International Astronomical Union. JPL navigation started as a
consumer of Naval Observatory data, practices, and ephemerides, but
in the 1970s became the source of ephemerides, constants, models,
and ephemerides for the world's almanac offices. Navigators have
been project scientists on many NASA missions, performing
experiments in celestial mechanics and gravitational fields,
undertaking radio occultations, and testing Einstein's General
Theory of Relativity. They also have made a number of memorable
scientific discoveries: mascons on the Moon and Mars, volcanism on
Io, and over a dozen satellites of the outer planets. The nature of
deep-space navigation began to change with the introduction of
optical navigation, which uses a probes science camera and
telemetry to determine its position relative to a planet, moon, or
asteroid. Deep-space navigation began-and remains-an activity
carried out entirely on Earth. Radio signals from the Deep Space
Network constituted the only information processed. Optical
navigation began to move some of the process-and equipment-to
spacecraft. At first, image processing was a labor-intensive and
computer-intensive effort. Software improvements and advances in
spacecraft computers paved the way for autonomous navigation, which
transferred many basic navigation processes to the onboard
computer: estimating trajectories, planning and executing imaging,
and analyzing pictures. The most recent evolutionary stage has been
the merger of this autonomous software with spacecraft guidance and
control functions.
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