TEMPEST: A MIDEX Mission Devoted to the Three Gem "Campaigns"
C. T. Russell, J. L. Burch, J. F. Fennell, D. A. Gurnett, B. Hillard,
W. S. Kurth, R. E. Lopez, J. G. Luhmann, G. Paschmann, P. H. Reiff
Introduction
The Geospace Environmental Modeling (GEM) program consists
effectively of three campaigns: a substorm campaign, a boundary layer
campaign and space weather campaign whose goal is to unite and integrate
our understanding of the solar terrestrial activity chain in one unified
predictive model. Progress in these areas requires in depth analysis of
existing data, improvements in our numerical modeling efforts and the
acquisition of new data. While the ISTP program will bring new
observations it will not provide the measurements in some of the critical
regions of the magnetosphere to address these major GEM objectives. We
need measurements, and many of them, in the current disruption region
and the reconnection region. We need measurements of the thickness of
the current sheet at all phases of the substorm. We need to map the
plasma populations at low altitudes unambiguously to their associated
equatorial populations. We need to investigate the acceleration region
above the auroral zone at all altitudes up to the magnetopause. We need
to measure the heart of the radiation belts and the equatorial
magnetosphere from low altitudes to high. We need to study the
dynamics of the magnetopause and the constitution of its boundary layers
at all latitudes up to and beyond the cusp, not just in the equatorial plane.
Fortunately such a mission is now both technologically and economically
practical and we have prepared a proposal for NASA's MIDEX
Announcement of Opportunity. We have called this mission TEMPEST:
Twin Electric Magnetospheric Probes Exploring on Spiral Trajectories.
The Tempest Mission
TEMPEST, consisting of two low-mass spacecraft launched by
Pegasus rockets into orthogonal 400- and 600-km-altitude orbits (high and
low inclination, respectively), will carry a small complement of basic
particles and field instruments. Xenon ion engines powered by solar
arrays take these spacecraft on a spiral trajectory from their initial orbits
completely through the magnetosphere to 15 RE (circular) in 2 yr. The
low-inclination spacecraft will fly through the heart of the equatorial
magnetosphere where the ring current is formed, the killer electrons are
found, and the crosstail current is disrupted at the onset of a substorm.
The high-inclination spacecraft will spiral upward through the acceleration
region on auroral field lines, mapping the plasma populations from low to
high altitudes and determining where and how these particles are
energized. Both missions provide complete coverage of magnetospheric
regions heretofore inadequately explored and provide unequivocal
identification of access, acceleration, transport and loss mechanisms for
energetic charged particles in the magnetosphere.
Initially the satellites will be configured so that the low-altitude,
high-latitude data can be compared with simultaneous data in the
equatorial plane. During this phase the low-altitude spacecraft measures
waves and particle distributions where the "loss cone" can be well
resolved while the high-altitude spacecraft measures the full equatorial
distributions on the same field lines in the only region in which they can
be measured. Later the high-inclination spacecraft joins the low-
inclination spacecraft at high altitudes and they spiral outward in
orthogonal orbits through the regions of current disruption region and the
near-Earth neutral point, making close encounters twice per orbit. This
orbit also carries the spacecraft through the magnetopause (in orthogonal
planes) on the dayside of the orbit.
TEMPEST Trajectory
The TEMPEST Mission consists of two spacecraft to be launched
4 months apart by two Pegasus launch systems based at the Wallops Flight
Facility in Virginia. The first vehicle, the low-inclination spacecraft, will
be launched into a 600-km-altitude circular orbit at a 30o inclination.
Roughly 4 months later, the high-inclination spacecraft will be launched
into a 400-km-altitude orbit at an orbital inclination of 70 degrees. After a brief
checkout period, both spacecraft initiate operation of the ion propulsion
system, beginning the spiral to higher altitudes.
The ion thrusters will perform orbit-raising operations only when
in full sunlight. Operations also include inclination changes over the
initial phases of the mission. Although scientific data will be gathered
throughout all phases of the mission, specific coast periods have been built
into the time line to allow for collection over orbits without significant
thruster operation. While the spacecraft are at low altitudes, these coast
periods are kept at a minimum to decrease time spent in the radiation
belts. At high altitudes, up to 95 percent of the time will be spent in
coast to allow for science gathering without thruster operation.
The low inclination spacecraft reaches 0 degrees inclination at an orbital
distance of 8 Earth radii (RE) after launch. This vehicle nominally spends
1 yr between 8 and 15 RE at 0o inclination collecting data. Approximately
89 percent of this time is spent coasting. Four months after the low-
inclination spacecraft reaches 8 RE, the high-inclination spacecraft arrives
at this orbital radius at 90 degrees inclination, 15 months after its launch. It then
spends 8 months between 8 and 15 RE at 90 degrees inclination coasting for about
83 percent of the time.
The TEMPEST mission ends when both spacecraft reach 15 RE,
for a total trip time of 2 yr, 3 months for the low-inclination spacecraft
and 1 yr, 11 months for the high-inclination vehicle.
TEMPEST Instrumentation
The TEMPEST spacecraft carries 5 instruments: a plasma analyzer
(MOSS); an energetic particle instrument (TEPS); an Electric Field
Experiment (EFE); a spacecraft interactions package (SIP) and a
magnetometer (MAG). The plasma analyzer consists of three miniaturized
optimized smart sensors (MOSS), two for ions and one for electrons. One
of the ion sensors has a time of flight section that provides a mass
resolution of 2 at 10% of peak over a mass range of 1-200 amu. The
field of view of the mass analyzers is 360 degrees x 10 degrees and with
electrostatic
deflection can cover 360 degrees x 90 degrees or 2.8 steradians each. Together the ion
analyzers cover nearly the full 4 steradians.
The energetic particle package will measure electrons from 20 keV
to 10's of MeV and ions from 20 keV to 100's of MeV. The Energetic
Proton Spectrometer and the Energetic Electron Spectrometer each have
fields of view of 180 degrees x 12 degrees and 5 pixels in the FoV. The Ultra
Relativistic Electron Detector has a 50 degree conical FoV.
The electric field experiment measures both the static and
oscillating electric field on the high inclination spacecraft and the
oscillating field on the low inclination spacecraft. At high altitudes the
electric field will be deduced from the plasma drift velocity. The high
inclination spacecraft carries 4 antenna elements whose potential
differences will be measured in a pairwise fashion. The low inclination
spacecraft will have a single pair of elements. The analyzer includes a
high frequency receiver from 0.1 to 1 MHz; a swept frequency receiver
from 50 Hz to 100 kHz and a low frequency digital receiver from 0 to 50
Hz. A wideband receiver can be used on command to provide waveform
measurements up to 10 kHz.
The spacecraft interactions package will study spacecraft charging
from auroral to geosynchronous plasmas on the high inclination
spacecraft. Langmuir probe measurements to provide the vehicle potential
and the plasma density in the inner magnetosphere will be made on both
spacecraft. In the active experiment on the high inclination spacecraft, a
series of positive voltages will be applied to a small plate on the front of
the spacecraft and the resulting current measured.
The magnetometer will measure the magnetic field and currents at
all altitudes with high amplitude and temporal resolution using dual
magnetometers mounted on a 3m boom.
Figure 1 shows the high-inclination spacecraft in its deployed state.
Figure 2 shows the undeployed spacecraft in the Pegasus shroud.
Can TEMPEST be done with Chemical Propulsion?
The use of a Pegasus launch vehicle is enabled by ion electric
propulsion.
Figure 3 shows a mass comparison for the proposed
TEMPEST low-inclination mission performed by two spacecraft: the
proposed vehicle using ion electric propulsion and the comparison using
chemical propulsion. To map the magnetosphere with near circular orbits,
200 Hohmann transfers are assumed for the chemically propelled
spacecraft. The same spacecraft bus as the ion propulsion vehicle is
assumed with the same scientific payload. The ion propulsion system is
replaced with a 310-sec specific impulse, 450-N-thrust bipropellant
propulsion system, and a 2-kW power system is replaced by a 350-W
power system. The chemical system tankage fraction is assumed to be
0.053. The chemical spacecraft dry contingency is set to 15 percent. The
difference in mass is dramatic. The total launch mass is almost eight
times larger for the chemical TEMPEST. The huge increase in fuel mass
and tankage far outweighs the reduced propulsion/power system mass.
Not even the Delta 7320 is capable of launching the chemical TEMPEST.
In addition, science mission flexibility would be severely impacted;
revisiting portions of the magnetosphere would require exorbitant amounts
of fuel.
Solar Electric Propulsion
The development of ion engines began at NASA Lewis Research
Center, the Jet Propulsion Laboratory and TRW in the 1960s and has
continued since that time. The original thrusters used mercury propellant
but more recently the inert gas xenon has been used instead. Also the
thrusters have been derated to operate at a lower thrust than they might
ultimately achieve. Thus once solar electric propulsion missions become
common place we may expect significant improvements in performance
above that planned for the initial generation of missions. Three factors
have sparked renewed interest in solar electric propulsion (SEP). First,
recent mission studies have shown that solar electric propulsion was
compatible with small missions and that it provides life-cycle cost
reductions. For example savings in the launch vehicle cost because a
solar electric propulsion system is lighter than a corresponding chemical
system and savings in operations cost due to shorter trip times for some
missions can far outweigh the extra cost of the solar electric propulsion
unit. The second important factor sparking renewed interest in SEP is the
growth in the potential commercial market. Large communication
satellites continue to be placed in geosynchronous orbit and they require
propulsion systems for long term station keeping. Solar electric
propulsion is ideal for this application. The third important factor is the
degree of technical readiness for flight that solar electric propulsion has
achieved. Two of NASA's Offices have been cofunding the development
of solar electric propulsion under the NSTAR program which stands for
NASA Solar Electric Propulsion Technology Applications Readiness
program. The NSTAR program has already initiated ground testing of the
power processing units and thrusters and a space test is planned possibly
as early as 1998.
A xenon ion SEP system works as follows. Solar panels generate
about 2.5 kw of power for an Explorer class spacecraft. This power is
used to ionize xenon gas. This ionized plasma is accelerated and focused
electrostatically exiting the ion engine at exhaust velocities of thousands
of meters per second. At the exit plane electrons are added to the ion
beam to make it a neutral plasma. A thruster with an exit plane aperture
of 30 cm can generate 90 millinewtons of thrust with a 1.8 kW power
input. A single thruster can operate continuously for at least one year.
Solar electric propulsion provides ten times more thrust per kilogram of
fuel than does chemical propulsion and can maintain thrusting over a
much longer interval. The ability to thrust continuously and efficiently
enables distant targets to be reached more quickly.
The color photographs show models of an early concept of the
TEMPEST spacecraft.
Photo 1
Photo 2
Conclusions
Solar electric propulsion is now ready for use on scientific space
missions. It enables missions to explore the entire magnetosphere from
low altitudes to the distant tail and at all inclinations for an affordable
price. The TEMPEST mission uses this technology to provide a complete
survey of magnetospheric regions heretofore inadequately explored. It
will provide unequivocal identification of access, acceleration, transport
and loss mechanisms for charged particles in the magnetosphere. The
measurements will enable us to test each of the major paradigms of the
substorm onset, to determine how the ring current forms, and how killer
electrons are energized. It will provide the grist for future models of the
magnetospheric environment. It will provide critical measurements of the
magnetopause boundary layer plasma at all latitudes and it will enable us
to determine what distinguishes storms from substorms.
Back to the GEM online poster page
Last modified: July 05, 1995
