Ulysses was a
robotic space
probe designed to study the
Sun.
The
spacecraft, named for the Latin translation of
"Odysseus" after Dante's Inferno, was launched October 6, 1990 from
the Space Shuttle Discovery (mission STS-41) as a joint venture of NASA
and the
European Space
Agency
. It was originally scheduled for launch in
1986 aboard the Space Shuttle Challenger. The spacecraft's mission
was to study the Sun at all latitudes, equipped with instruments to
characterize fields, particles, and dust, and was powered by a
radioisotope
thermoelectric generator (RTG).
By February 2008, the power output from the
RTG, which is
generated by heat from the radioactive decay of
plutonium-238, had decreased enough to leave
insufficient power for internal heaters to keep the spacecraft's
attitude control hydrazine fuel
from freezing.The end of mission was at one point scheduled for
July 1, 2008, but mission scientists came up with a method to keep
the fuel liquid by conducting a short thruster burn every two
hours, allowing the mission to continue.The cessation of mission
operations and deactivation or hibernation of the spacecraft was
determined by the inability to prevent attitude control fuel from
freezing.The last day for mission operations on Ulysses was 30 June
2009.This was a full year after the most recent previously
announced mission end date. The scheduled end of mission in 2009
was the fourth time that the end of the spacecraft's mission had
been scheduled.The last scheduled ground station pass of the
mission was over the Madrid Deep Space Network 70m ground station
(DSS-63) from around 15:35 to 20:20 UTC. There were no
decommissioning engineering tests on the spacecraft.
Mission
Planning
Illustration of Ulysses after
deployment
Ulysses' second orbit
Until Ulysses, the Sun was only observed from low solar latitudes.
The Earth's orbit defines the
ecliptic
plane, which differs from the Sun's equatorial plane by only 7.25
degrees. Even spacecraft directly orbiting the Sun do so in planes
close to the ecliptic because a direct launch into a high
inclination solar orbit would require a prohibitively large launch
vehicle.
Several spacecraft (
Mariner 10,
Pioneer 11, and
Voyagers 1 and 2) had performed
gravity assist manoeuvres in the 1970s. Those
manoeuvres were to reach other planets also orbiting close to the
ecliptic, so they were mostly in-plane changes. However, gravity
assists are not limited to in-plane maneuvers; a suitable flyby of
Jupiter could produce a significant plane
change. An Out-Of-The-Ecliptic mission (OOE) was thereby proposed.
See article Pioneer H.
Originally, two spacecraft were to be built by NASA and ESA, as the
International Solar Polar Mission. One would be
sent over Jupiter, then under the Sun. The other would fly under
Jupiter, then over the Sun. This would provide simultaneous
coverage. Due to cutbacks, the US spacecraft was canceled in 1981.
One spacecraft was designed, and the project recast as
Ulysses, due to the indirect and untried flight path. NASA
would provide the Radioisotope Thermoelectric Generator (RTG) and
launch services, ESA would build the spacecraft assigned to Astrium
GmbH, Friedrichshafen, Germany (formerly Dornier Systems). The
instruments would be split into teams from universities and
research institutes in Europe and the United States. This process
provided the 10 instruments on board.
The changes delayed launch from February 1983 to May 1986 where it
was to be deployed by the
Space Shuttle Challenger,
however, the
Challenger disaster pushed
the date to October 1990.
Launch
Ulysses was deployed into low-Earth orbit from the Space
Shuttle Discovery. From there, it was propelled on a trajectory to
Jupiter by a combination of solid rocket motors. This upper stage
consisted of a two-stage
Boeing IUS (Inertial Upper Stage), plus a
McDonnell Douglas PAM-S (
Payload Assist Module-Special). The
IUS was inertially stabilised and actively guided during its burn.
The PAM-S was unguided and it and the Ulysses spacecraft was spun
up to 80 rpm for stability at the start of its burn. On burnout of
the PAM-S, the motor and spacecraft stack was yo-yo despun (weights
deployed at the end of cables) to below 8 rpm prior to separation
of the spacecraft. On leaving Earth, the spacecraft became the
fastest ever artificially-accelerated object, and held that title
until the
New Horizons probe was
launched.
On its way to Jupiter the spacecraft was in an elliptical
Hohmann transfer orbit with
perihelion near 1 AU and aphelion near 5 AU, Jupiter's distance
from the Sun. At this time Ulysses had a low orbital inclination to
the ecliptic.
Jupiter swing-by
It arrived at Jupiter February 8, 1992 for a
swing-by maneuver that increased its
inclination to the
ecliptic by 80.2
degrees. The giant planet's gravity bent the spacecraft's flight
path downward and away from the ecliptic plane. This put it into a
final orbit around the Sun that would take it past the Sun's north
and south poles.The size and shape of the orbit were adjusted to a
much smaller degree so that aphelion remained at approximately 5
AU, Jupiter's distance from the Sun, and perihelion was somewhat
greater than 1 AU, the Earth's distance from the Sun.
Solar northern polar regions
Between 1994 and 1995 it explored both the southern and
northern solar polar regions, respectively.
Comet Hyakutake
On May 1, 1996, the spacecraft unexpectedly crossed the ion tail of
Comet Hyakutake (C/1996 B2),
revealing the tail to be at least 3.8 AU in length.
Solar southern polar regions
Between 2000 and 2001 it explored the
southern
solar polar regions, which gave many unexpected results. In
particular the southern
magnetic pole was
found to be much more dynamic and without any fixed clear location.
It is, of course, wrong to say that the Sun has no magnetic south
pole. The Sun is not a
magnetic
monopole; the pole is merely more diffusely located than the
north pole.
Jupiter
Ulysses approached
aphelion in
2003/2004 and made further distant observations of Jupiter.
Comet McNaught-Hartley
Encounter with a comet tail happened again in 2004 when Ulysses
flew through the ion tailings of
Comet McNaught-Hartley. A
coronal mass ejection carried the
cometary material to Ulysses.
Comet McNaught
In 2007 Ulysses passed through the tail of
Comet McNaught. The results were surprisingly
different from its pass through Hyakutake's tail, with the measured
solar wind velocity dropping from approximately 700 kilometers per
second to less than 400 kilometers per second.
Extended mission
ESA's Science Programme Committee approved the fourth extension of
the Ulysses mission to March 2009 thereby allowing it to operate
over the Sun's poles for the third time in 2007 and 2008. After it
became clear that the power output from the spacecraft's RTG would
be insufficient to operate science instruments and keep the
attitude control fuel,
hydrazine, from freezing, instrument power sharing
was initiated. Up until then, the most important instruments had
been kept online constantly, whilst others were deactivated. When
the probe neared the Sun, its power-hungry heaters were turned off
and all instruments were turned on.
On
February 22, 2008, 17 years 4 months after the launch of the
spacecraft, ESA
and NASA announced
that mission operations for Ulysses would be likely to cease within
a few months. On April 12, 2008 NASA announced that the end
date will be July 1, 2008. The spacecraft operated successfully for
over four times its
design life. A
component within the last remaining working chain of
X-band downlink sub-system failed on January 15,
2008. The other chain in the X-band sub-system had previously
failed in 2003.
Downlink to
Earth
resumed on
S-band, but the
beamwidth of the
high
gain antenna on S-band is not as narrow as on X – so the
downlink signal was much weaker, thereby reducing the achievable
data rate. As the spacecraft traveled on
its outbound
trajectory to the orbit of
Jupiter, the downlink signal would have eventually fallen below the
receiving capability of even the largest antennas (70m in diameter)
of the
Deep Space Network. Even
before the downlink signal was lost due to distance, the
hydrazine attitude control fuel on-board the
spacecraft was considered likely to
freeze,
as the
radioisotope
thermal generators failed to generate enough power for the
heaters to combat the cold of space. Once the hydrazine froze, the
spacecraft would no longer be able to maneuver to keep its high
gain antenna pointing towards Earth, and the downlink signal would
then be lost in a matter of days. The failure of the X-band
communications sub-system hastened this, because the coldest part
of the fuel pipework was routed over the X-band
TWTAs which, when one of them was operating, kept this
part of the pipework sufficiently warm.
The previously announced mission end date of July 1, 2008 came and
went but mission operations continued albeit in a reduced capacity.
The availability of science data gathering was limited to only when
Ulysses is in contact with a ground station due to the
deteriorating S-band downlink margin no longer being able to
support simultaneous real-time data and tape recorder playback.
When the spacecraft was out of contact with a ground station, the
S-band transmitter was switched off and the power was diverted to
keep the internal heaters to add to the warming of the hydrazine.
On June 30, 2009 ground controllers sent commands to switch to its
low gain antennae which ceased communications with the spacecraft
along with previous commands to schedule the shut down of its
transmitter entirely.
Results
During cruise phases,
Ulysses provided unique data. As the
only spacecraft out of the ecliptic with a
gamma-ray instrument,
Ulysses was an
important part of the
InterPlanetary Network (IPN). The IPN
detects
gamma ray bursts (GRBs);
since gamma rays cannot be focused with mirrors, it was very
difficult to locate GRBs with enough accuracy to study them
further. Instead, several spacecraft can locate the burst through
triangulation (or, more specifically,
multilateration). Each spacecraft
has a gamma-ray detector, with readouts noted in tiny fractions of
a second. By comparing the arrival times of gamma showers with the
separations of the spacecraft, a location can be determined, for
follow-up with other telescopes. Because gamma rays travel at the
speed of light, wide separations are needed. Typically, a
determination came from comparing: one of several spacecraft
orbiting the Earth, an inner-Solar-system probe (to
Mars,
Venus, or an
asteroid), and
Ulysses. When
Ulysses crossed the ecliptic twice per orbit, many GRB
determinations lost accuracy.
Additional discoveries:
- Ulysses discovered that the Sun's magnetic field
interacts with the Solar System in a more complex fashion than
previously assumed.
- Ulysses discovered that dust coming into the Solar System from
deep space was 30 times more abundant than previously
expected.
- In 2007-2008 Ulysses determined that the magnetic
field emanating from the Sun's poles is much weaker than previously
observed.
- That the solar wind has "grown
progressively weaker during the mission and is currently at its
weakest since the start of the Space Age."
Spacecraft
Ulysses spacecraft
The spacecraft body is roughly a box, approximately 3.2 × 3.3 × 2.1
m in size. The box mounted the 1.65 m
dish
antenna and the
RTG power source. The
box was divided into noisy and quiet sections. The noisy section
abutted the RTG; the quiet section housed the instrument
electronics. Particularly "loud" components, such as the preamps
for the radio dipole, were mounted outside the structure entirely,
and the box acted as a
Faraday
cage.
Ulysses is spin-stabilised about its z-axis which roughly
coincides with the axis of the dish antenna. The RTG, whip
antennas, and instrument boom are placed to stabilize this axis.
Spin is nominally 5 rpm. Inside the body is a
hydrazine fuel tank. Hydrazine
monopropellant was used for course
corrections inbound to Jupiter, and is now used exclusively to
repoint the spin axis (and thus, the antenna) at Earth. The
spacecraft is controlled by eight thrusters, in two blocks.
Thrusters are pulsed in the time domain to perform rotation or
translation. Four Sun sensors detected orientation. For fine
attitude control, the S-band antenna feed is mounted slightly
off-axis. This offset feed combined with the spacecraft spin
introduces an oscillation to an S-band radio signal transmitted
from Earth when received on-board the spacecraft. The amplitude and
phase of this oscillation is proportional to the oriention of the
spin axis relative to the Earth direction. This method of
determining the relative orientation is called CONSCAN and was
widely employed in early infra-red guided missiles.
The spacecraft uses S-band for uplinked commands and downlinked
telemetry, through dual redundant 5-watt transceivers. The
spacecraft used
X-band for science return
(downlink only), using dual 20W
TWTAs until the
failure of the last remaining TWTA in January 2008. Both bands use
the dish antenna with prime-focus feeds, unlike the
Cassegrain feeds of most other
spacecraft dishes.
Dual tape recorders, each of approximately 45 megabit capacity,
store science data between the nominal 8-hour communications
sessions during the prime and extended mission phases.
The spacecraft is designed to withstand both the heat of the inner
Solar System and the cold at Jupiter distance. Extensive blanketing
and electric heaters protect against cold.
Total mass at launch was 366.7 kg of which 33.5 kg was
hydrazine (used for attitude control and orbit correction).
Instruments
Ulysses instruments
Ulysses radial boom test
Radio/Plasma antennas. Two
beryllium-copper antennas unreeled outwards
from the body, perpendicular to the RTG and spin axis. Together
this
dipole spanned 72 meters. A third
antenna, of hollow beryllium-copper, deployed from the body, along
the spin axis opposite the dish. It was a
monopole antenna, 7.5 meters long. These
measured radio waves generated by plasma releases, or the plasma
itself as it passed over the spacecraft. This receiver ensemble was
sensitive from dc to 1 MHz.
Experiment Boom. A third type of boom, shorter and
much more rigid, extended from the last side of the spacecraft,
opposite the RTG. This was a hollow carbon-fiber tube, of
50 mm diameter. It can be seen in the photo as the silver rod
stowed alongside the body. It carried four types of instruments. A
solid-state
X-ray instrument, which was
composed of two
silicon detectors to study
X-rays from
solar flares and Jupiter's
aurorae. The GRB experiment
consisted of two
CsI scintillator
crystals with photomultipliers. Two different
magnetometers were mounted: a vector helium
magnetometer and a fluxgate magnetometer. A two axis magnetic
search coil antenna measured AC magnetic fields.
Body-Mounted Instruments. Detectors for
electrons,
ions, neutral gas,
dust, and
cosmic
rays were mounted on the spacecraft body around the quiet
section.
- SWOOPS (Solar Wind Observations Over the Poles of the
Sun) measures positive ions and electrons.
Lastly, the radio communications link could be used to search for
gravitational waves (through
Doppler shifts) and to probe the Sun's
atmosphere through
occultation. No
gravitational waves were detected.
Total instrument mass is 55 kg.
References
- "The project was renamed at ESA's request in honour not only of
Homer's mythological hero but with reference to Dante's description
in the Inferno of Ulysses' urge to explore an uninhabited world
behind the Sun" in Jane's Spaceflight Directory 1988, ISBN
0-710-60860-8
- ESA Portal Sun to set on Ulysses solar mission on 1
July
- ESA Portal Ulysses hanging on valiantly
- Solar wind blows at 50-year low 2008-09-24,
Jonathan Amos, BBC News Online. Retrieved 2008-09-28
- The odyssesy concludes ...
- ESA - Space Science - Sun to set on Ulysses solar
mission on 1 July
- Ulysses - Science - Jupiter Distant Encounter
Selected References
- ESA Science & Technology: Ulysses Mission
Extended
- ESA Portal Ulysses scores a hat-trick
-
http://news.yahoo.com/s/ap/20080612/ap_on_sc/sci_solar_probe
- Ulysses Mission Ops - No more data
playback
- NASA : International Mission Studying Sun to
Conclude
- Unified Radio and Plasma Wave Investigation,
JPL
- Goldstein, Bruce. SWOOPS/Electron User Notes, Jet
Propulsion Laboratory
- The Gravity Wave Experiment, Astronomy and
Astrophysics
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