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Mission to Asteroid, Mars, and Comet Delayed
April 20, 1998
Software Troubles and Late Electronics System
Force NASA To Postpone Deep Space 1
The planned July 1998 launch of NASA's Deep Space
1 technology validation mission from Cape
Canaveral, Florida, has been rescheduled for
October.
The delay is due to a combination of late delivery
of the spacecraft's power electronics system and
an ambitious flight software development schedule,
which together leave insufficient time to test the
spacecraft thoroughly for a July launch.
The power electronics system regulates and
distributes power produced by not only the solar
concentrator array, a pair of experimental solar
panels composed of 720 cylindrical Fresnel lenses,
but also by an on-board battery. Among many other
functions, it helps the solar array to operate at
peak efficiency, and ensures that the battery is
able to cover temporary surges in power needed so
that the ion propulsion system (which needs
electricity for its basic operations) receives a
steady power supply.
"With a new launch date for this bold mission, we
can be more confident that we will be ready to
fully exercise our payload of important
technologies," explained Chief Mission Engineer
Marc Rayman of NASA's Jet Propulsion Laboratory in
Pasadena, California. "The entire DS1 team looks
forward to this opportunity to make a significant
contribution to science missions of the future
through the capabilities we are testing on DS1."
Deep Space 1 is the first launch in NASA's New
Millennium program, a series of missions designed
to test new technologies so that they can be
confidently used on science missions of the 21st
century. Among the 12 technologies the mission is
designed to validate are ion propulsion,
autonomous optical navigation, a solar power
concentrator array and an integrated camera and
imaging spectrometer.
The earlier July launch period for DS1 allowed it
to fly a trajectory encompassing flybys of an
asteroid, Mars, and a comet. By the end of May,
the mission design team is scheduled to finalize
new target bodies in the solar system for DS1 to
encounter based on an October launch date.
Editor's note: Deep Space 1 will no longer visit
asteroid 3352 McAuliffe, Mars, and comet
West-Kohoutek-lkemura. The launch delay was
announced after this article went to press.
Mission planners will announce the new targets for
this mission by the end of May. The full text and
graphics for this article will appear in the
May/June 1998 issue of The Planetary Report. This
publication goes out to all members of the
Planetary Society. If you're not already a member,
we encourage you to join.
Deep Space 1: Exploration Technology for the 21st
Century
by Robert M. Nelson
and
Marc D. Rayman
This summer NASA takes a revolutionary
step when it launches Deep Space 1
(DS1). During its flight, the spacecraft
will visit asteroid 3352 McAuliffe, the
planet Mars, and comet
West-Kohoutek-lkemura. But its primary
goal is not to study these fascinating
bodies; rather, as a member of the New
Millennium program, its job is to pave
the way for future, even more exciting,
space science missions.
NASA has already flown missions to
asteroids, comets, and Mars, so what
makes DS1 unusual? It will demonstrate a
dozen technical innovations that will
serve as foundation technologies for the
next generation of deep-space missions.
Foremost among these new technologies
will be solar electric propulsion (SEP),
which will enable a whole class of
ambitious missions that are simply
impractical or unaffordable, with the
standard chemical propulsion available
today.
A Test Drive
DS1 will be launched from Cape Canaveral
on the first Delta 7326 rocket, a
low-cost member of the Delta 11 family.
DS1 is so small that even this
economy-class launch vehicle will be
able to carry a second spacecraft --
SEDSAT-1, an Earth orbiter built at the
same time by students at the University
of Alabama in Huntsville.
Once in space, DS1 will be checked out
and certified by the mission operations
team, and then the SEP system will begin
thrusting. Instead of burning a strong,
short pulse of chemical propellant,
followed by a long interplanetary
cruise, the SEP system will sustain a
tenuous but very high-velocity stream of
ionized xenon. This stream will create a
gentle, steady thrust that will propel
the spacecraft almost continuously
during interplanetary cruise.
Although the thrust of SEP is small, its
advantage accrues because the exhaust
velocity of the ion rocket is many times
greater than the exhaust velocity of a
conventional chemical system. The bottom
line is that SEP requires far less
propellant than a chemical rocket to
deliver the same payload mass to a
target, It takes time for the gentle
thrust to build up high spacecraft
velocity, so SEP is appropriate only for
missions requiring high energy or long
trips.
Within a month of launch, DS1 will have
accomplished most of its major
objectives, and we will have assessed
its payload of advanced technologies. If
a technology fails during the flight,
even if it causes the loss of the
spacecraft, we may still regard the
mission as a success if it achieves the
program goal of reducing the risk for
future science missions. It is in these
future missions that the real science
return of DS1 will be found. But this
high-risk project will attempt to return
science during its test flight....
The flight of DS1 will test new autonomy
technologies, solar concentrator arrays,
and a variety of telecommunications and
microelectronics devices. Autonomy,
which in this case means the ability of
the spacecraft to make its own
decisions, can help reduce the heavy
burden on NASA's Deep Space Network
(DSN). As more and more probes are sent
into space in the coming years, it will
be harder for the DSN to communicate
with all of them as frequently as it has
done in the past. With autonomy
technologies allowing spacecraft to
operate for longer times without
detailed instructions from Earth, the
precious resources of the DSN can go
further. In addition, by placing more
responsibility on the spacecraft, we
reduce delays caused by signal travel
times and limited communications rates.
Despite the potential advantages, it is
easy to see that onboard decision-making
systems entail risk for the first user.
If the autonomy systems on DS1 perform
as planned, future mission teams can be
more confident about leaving important
decisions to the spacecraft.
One of the powerful autonomy
technologies on DS1 is the navigation
system. It uses images of main-belt
asteroids viewed against the background
stars to compute the spacecraft's
position. As the spacecraft travels,
foreground objects (the asteroids) will
appear to move relative to the
background stars. The apparent shift, or
parallax, gives the navigation system
information from which to triangulate
the spacecraft position. The navigation
system then uses positions calculated at
earlier times to determine trajectory,
making allowances for SEP thrusting,
gravitational pulls of the Sun and
planets, and other forces. If the
navigation system finds that it is off
course, it can make a course correction
by adjusting the direction or duration
of SEP thrusting....