Ulysses (spacecraft)
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| Ulysses | |
 Ulysses spacecraft |
|
| Organization | NASA/ESA |
|---|---|
| Mission type | Orbiter |
| Flyby of | Jupiter |
| Satellite of | The Sun |
| Launch date | October 1990 |
| Launch vehicle | STS-41 |
Ulysses is a robotic space probe designed to study the Sun at all latitudes. The spacecraft, named for the Latin translation of "Odysseus", 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 is equipped with instruments to characterize fields, particles, and dust, and is powered by a radioisotope thermoelectric generator (RTG). The Ulysses mission is ongoing, still collecting valuable scientific readings to this day. However in February of 2008, what scientists had always known began to materialize. The power output from the RTG, which is generated by heat from the radioactive decay of plutonium-238, has dwindled to the point where it is insufficient to power some critical internal heaters to keep the spacecraft's attitude control fuel from freezing. See Extended Mission below.
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[edit] Mission
[edit] Planning
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 maneuvers in the 1970s. Those maneuvers 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.
[edit] Launch
Ulysses was launched from a Space Shuttle during low-Earth orbit. It was then propelled towards Jupiter by a combination of solid fuel motors.[1] The booster consisted of a two-stage Boeing IUS (Inertial Upper Stage), plus a McDonnell Douglas PAM-S (Payload Assist Module-Special) on a 70 rpm spin table. On leaving Earth, the spacecraft became the fastest ever artificially-accelerated object; the New Horizons probe has since set the new record.
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.
[edit] Jupiter swing-by
It arrived at Jupiter February 8th 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.
[edit] Solar northern polar regions
In 1994-95 it explored both the northern solar polar regions.
[edit] 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.[2]
[edit] Solar southern polar regions
In 2000-01 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.
[edit] Jupiter
Ulysses approached aphelion in 2003/2004 and made further distant observations of Jupiter.[3]
[edit] 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.
[edit] 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.[4]
[edit] Extended mission
Ulysses' mission has been extended until at least March 2009 so it can operate over the Sun's poles for the third time in 2007 and 2008. At some point, the power output from the spacecraft's RTG will be insufficient to operate science instruments and keep the attitude control fuel, hydrazine, from freezing.[5] Instrument power sharing is on-going. The most important instruments are always online, others are not. When the probe nears the sun, its power-hungry heaters are turned off and all instruments are on.[6]
On February 22, 2008, 17 years 4 months after the launch of the spacecraft, ESA and NASA announced that mission operations for Ulysses will likely cease within a few months. [7] [8] On April 12, 2008 NASA announced that the end date will be July 1, 2008. [9] The spacecraft has so far operated successfully for over four times its design life. This follows the failure of a component within the last remaining working chain of X-band communications sub-system on January 15, 2008. The other chain in the X-band sub-system had previously failed in 2003. [10]
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 is much weaker thereby reducing the achievable data rate. As the spacecraft travels on its outbound trajectory to the orbit of Jupiter, the downlink signal will eventually fall below the receiving capability of the even largest antennas (70m in diameter) of the Deep Space Network. Even before the downlink signal is lost, the attitude control fuel, hydrazine, on-board the spacecraft will likely be frozen due insufficient power from the RTG to keep it warm as the spacecraft moves away from the Sun on this part of its heliocentric orbit. Once the hydrazine freezes, the spacecraft can no longer be manoeuvred to keep its high gain antenna pointing towards Earth and the downlink signal will be lost in a matter of days. The failure of the X-band communications sub-system will actually hasten the date of this happening because the coldest part of the fuel pipework is routed over the X-band TWTAs which when one of them is operating keeps this part of the pipework sufficiently warm.
[edit] Results
During cruise phases, Ulysses is still providing unique data. As the only spacecraft out of the ecliptic with a gamma-ray instrument, Ulysses provides 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 comes from comparing: one of several spacecraft orbiting the Earth, an inner-Solar-system probe (to Mars, Venus, or an asteroid), and Ulysses. When Ulysses crosses the ecliptic twice per orbit, many GRB determinations lose accuracy.
[edit] Spacecraft
The spacecraft body is roughly a box, approximately 10 × 11 × 7 feet in size (3 × 3.3 × 2 m). The box mounts the 1.65 meter dish antenna and the RTG power source. The box is divided into noisy and quiet sections. The noisy section abuts the RTG; the quiet section houses the instrument electronics. Particularly "loud" components, such as the preamps for the radio dipole, are mounted outside the structure entirely, and the box acts as a Faraday cage.
Ulysses is spin-stabilized about the axis of the dish. The RTG, whip antennas, and instrument boom were placed to stabilize this axis. Spin is nominally 5 rpm. Inside the body is a hydrazine fuel tank. Hydrazine monopropellant is used for course corrections, and 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 detect orientation. For fine attitude control, the S-band antenna feed can be tipped slightly off-axis. A signal from Earth then pulses with the 5 rpm spin. The pulsing is deconvolved into orientation, a method called CONSCAN.
The spacecraft uses S-band for uplinked commands and downlinked telemetry, through dual redundant 5-watt transceivers. The spacecraft uses X-band for science return (downlink only), using dual 20W TWTAs. Both bands use the dish antenna; both are 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 peak DSN periods, the instruments record at lower resolution to reduce the load on the DSN.
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. Heating is minimized by the 1.3 AU perihelion, meaning that Ulysses always studies the sun from a greater distance than the Earth.
Overall weight at launch is 390 kg (814 pounds).
[edit] Instruments
Radio/Plasma antennas. Two beryllium-copper antennas unreel outwards from the body, perpendicular to the RTG and spin axis. Together this dipole spans 72 meters. A third antenna, of hollow beryllium-copper, deploys from the body, along the spin axis opposite the dish. It is a monopole antenna, 7.5 meters long. These measure radio waves generated by plasma releases, or the plasma itself as it passes over the spacecraft.
Experiment Boom. A third type of boom, shorter and much more rigid, extends from the last side of the spacecraft, opposite the RTG. This is 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 carries four types of instruments. A solid-state X-ray instrument is composed of two silicon detectors. It studies X-rays from solar flares and Jupiter's aurorae. The GRB experiment consists of two CsI scintillator crystals with photomultipliers. Two different magnetometers are mounted: a vector helium magnetometer and a fluxgate magnetometer. A two axis magnetic search coil antenna measures AC magnetic fields.
Body-Mounted Instruments. Detectors for electrons, ions, neutral gas, dust, and cosmic rays are mounted on the spacecraft body around the quiet section.
Lastly, the radio communications link can be used to search for gravity waves (through Doppler shifts) and to probe the Sun's atmosphere through occultation.
Total instrument mass is 55 kg.
[edit] References
- ^ http://www.esa.int/esaSC/SEMTDTUG3HF_index_0.html
- ^ Jones GH, Balogh A, Horbury TS (2000). "Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures". Nature 404 (6778): 574-6. PMID 10766233.
- ^ Ulysses - Science - Jupiter Distant Encounter Selected References
- ^ Neugebauer, Gloeckle; et al. (October 1 2007). "Encounter of the Ulysses Spacecraft with the Ion Tail of Comet McNaught". Astrophysical Journal 667: 1262-1266.
- ^ ESA Science & Technology: Ulysses Mission Extended
- ^ ESA Portal - Ulysses scores a hat-trick
- ^ Ulysses mission coming to a natural end. European Space Agency (2008-02-22). Retrieved on 2008-02-23.
- ^ "International Solar Mission to End Following Stellar Performance", NASA, 2008-02-22. Retrieved on 2008-02-23.
- ^ http://news.yahoo.com/s/ap/20080612/ap_on_sc/sci_solar_probe
- ^ February 2003 Operations. European Space Agency.
[edit] External links
- ESA Ulysses website
- ESA Ulysses mission operations website
- NASA/JPL Ulysses website
- Ulysses Measuring Mission Profile by NASA's Solar System Exploration
- ESA/NASA/JPL: Ulysses subsystems and instrumentation in high detail
- Where is Ulysses now!
- Max Planck Institute Ulysses website
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