Ulysses
The mission to the Sun's poles
In the early 1970s, NASA and the European Space Agency (ESA) began planning a mission that would send two spacecraft through Jupiter’s gravitational field and on to the Sun, where they would pass over the north and south poles, mapping previously unstudied regions of the Sun and gathering vital data about solar wind, plasma, and the Sun’s magnetic field. The mission would require what is known as an “out-of-ecliptic” orbit, meaning that a spacecraft would operate outside the ecliptic plane in which most major bodies orbit the Sun. Scientists began studying out-of-ecliptic missions at the dawn of the space age, but it took decades to develop the technology to navigate the vast distances required and effectively utilize gravity assists.
The mission, which was eventually referred to as the International Solar Polar Mission (ISPM), was approved in 1976, the payload approved in 1977, and a provisional launch date set for February 1983. The primary objectives were to investigate, as a function of solar latitude, the properties of the solar wind and the interplanetary magnetic field, of galactic cosmic rays and neutral interstellar gas, and to study energetic particle composition and acceleration. Secondary objectives included interplanetary and planetary physics investigations during the initial Earth-Jupiter phase and investigations in the Jovian magnetosphere.
The original mission was planned for two spacecraft – to a different design, one built by ESA and the other by NASA – to make simultaneous close passes over opposite poles of the Sun. According to the September 1980 flight manifest, NASA Solar Polar probe was scheduled to be launched on STS-35, then OV-104 Atlantis’ maiden voyage, in March 1985, followed by the ESA Solar Polar probe aboard Challenger STS-36 in April 1985. Both spacecraft were to be mounted to a three-stage planetary variant of the IUS. To achieve the desired high-inclination solar orbits, they were to make a detour via Jupiter. The two spacecraft were to fly in formation until they were through the asteroid belt, then they were to slowly diverge in order to make opposing polar passes of the giant planet so that the resulting gravitational slingshots would hurl one above, and the other below the ecliptic plane to facilitate simultaneous study of both of the Sun’s poles.
Due to cutbacks, NASA cancelled its spacecraft in 1981. However, ESA decided to go ahead with a single-spacecraft version of the mission. The spacecraft was provided by ESA and built by Dornier Systems in Germany. Subcontractors included firms in Austria, Belgium, Denmark, France, Italy, the Netherlands, Spain, Sweden, Switzerland, the United Kingdom and the United States. NASA provided the radioisotope thermoelectric generator that powered the spacecraft and launch services, including the two-stage Inertial Upper Stage (IUS) and Payload-Assist Module (PAM-S) that put the spacecraft on a course for Jupiter. In 1984, the spacecraft was named Ulysses, an allusion to the protagonist of Homer's poem the Odyssey, who travelled great distances and explored uncharted territory, and to Dante's character in the Inferno.
Ulysses was at Kennedy Space Center awaiting a May 1986 launch aboard the Space Shuttle Challenger on STS-61-F, but it was dismantled and returned to Europe after the Challenger disaster in January 1986. Following the investigation and changes to the Shuttle program, it was slated to launch in 1990, aboard STS-41 mission.
The total spacecraft mass was 366.7 kg at launch, including 33.5 kg of fuel for orbit and attitude adjustments. It had a box-type structure with two overhanging balconies and a single aluminium honeycomb equipment platform with dimensions 3.2 m × 3.3 m × 2.1 m. The 55.1 kg science payload included two magnetometers, two solar wind plasma instruments, a unified radio/plasma wave instrument, three energetic charged particle instruments, an interstellar neutral gas sensor, a solar X-ray/cosmic gamma-ray burst detector, and a cosmic dust sensor. The spacecraft had a 72.5 m radial dipole antenna, 7.5 m axial monopole antenna, and a 5.6 m long radial magnetometer boom. The communications systems were also used to study the solar corona and to search for gravitational waves. Ulysses was powered by a single radioisotope generator, with 285 W at start and 244 W at the end of the first solar cycle. It was spin stabilized at a rate of 5 rpm and its high-gain antenna with a 1.65 m diameter parabolic dish pointed continuously to the Earth. Overall program costs, through end of the primary mission, were estimated at $500 million for NASA and $250 million for ESA.
There were 12 experiments returned:
1. Magnetometer (VHM/FGM) — determine the features and gradients of magnetic fields
2. Solar Wind Plasma Experiment (SWOOPS) — characterise interplanetary plasma in three dimensions out to Jupiter
3. Solar Wind Ion Composition Instrument (SWICS) — determine uniquely the elemental and ionic-charge composition, temperatures and mean speeds of all major solar-wind ions
4. Unified Radio and Plasma Wave Instrument (URAP) — determine the direction, angular size and polarisation of radio sources
5. Energetic Particle Instrument (EPAC) — measure the fluxes, angular distributions, energy spectra and composition of ions
6. Interstellar Neutral-Gas Experiment (GAS) — measure in-situ the properties of the local interstellar gas
7. Low-Energy Ion and Electron Experiment (HI-SCALE) — make measurements of interplanetary ions and electrons
8. Cosmic Ray and Solar Particle Instrument (COSPIN) — measure the three-dimensional anisotropies of protons and helium at low energies
9. Solar X-ray and Cosmic Gamma-Ray Burst Instrument (GRB) — study gamma-ray bursts, soft gamma repeaters and solar flares
10. Dust Experiment (DUST) — provide direct observations of dust grains in interplanetary space and investigate their physical and dynamic properties
11. Coronal-Sounding Experiment (SCE) — study the solar atmosphere using established coronal-sounding techniques
12. Gravitational Wave Experiment (GWE) — search for gravitational waves
The initial plan has been for NASA’s spacecraft to carry a camera to record the solar disk and the corona, but this could not be transferred, because it would have made the remaining spacecraft too large for even the enhanced IUS, so Ulysses has no imaging capability.
Ulysses was launched on October 6, 1990 at 11:47:16 UTC, from Kennedy Space Center Launch Complex 39B, Florida, aboard Space Shuttle Discovery, mission STS-41. The crew were: Commander Richard N. Richards, Pilot Robert D. Cabana and Mission Specialists Bruce E. Melnick, William Shepherd and Thomas Akers. Early that afternoon, the crew opened the Payload Bay doors and began the complex process that released Ulysses to begin its unprecedented, nearly 19-year voyage to the Sun via Jupiter. About 7.5 hours after launch Ulysses was sent on its way into heliocentric orbit via a combined IUS/PAM-S upper stage rocket combination. Escape velocity was about 15.4 kilometers per second, higher than had been achieved by either of the Voyagers or Pioneers, and the fastest velocity ever achieved by a human-made object at the time.
After a midcourse correction July 8, 1991, Ulysses passed within about 378,400 kilometers of Jupiter at 12:02 UT on February 8, 1992. The giant planet's gravity bent the spacecraft's flight path southward and away from the ecliptic plane. This put the probe into its final orbit around the Sun, that took it past the Sun's south and north poles.
Ulysses was only the fifth spacecraft to encounter Jupiter. Thirteen years had elapsed since the last spacecraft passed through its complex and dynamic magnetosphere. The first four encounters, by Pioneer 10 in 1973, Pioneer 11 in 1974 and Voyagers 1 and 2 in 1979, were designed specifically to study Jupiter and left an impressive legacy of scientific information regarding Jupiter’s atmosphere and magnetosphere.
If the encounter is defined as the interval between the first inbound bow shock crossing to the last outbound crossing, it lasted 13.6 days. The spacecraft was continuously inside the magnetosphere for 7.4 days. Following closest approach, Ulysses traversed the Io Plasma Torus in basically a north-south direction. Although Ulysses was a heliospheric mission, the experiments were well suited to an investigation of Jupiter’s magnetosphere and have returned much new and useful information. In particular, while outbound from Jupiter, Ulysses passed through a previously unexplored sector of the magnetosphere near a local time of 18 hours (dusk).
Ulysses trajectory at Jupiter flyby. Credit: ESA
After a 17-day period passing through and studying the
Jovian system, the spacecraft headed downwards and back toward
the Sun. It entered the orbit with a perihelion 1.34 au (200
million km), aphelion 5.4 au (809.5 million km), inclination
79.4° with respect to the ecliptic plane, and period 2,264.26
days (6.2 years). From about mid-1993 on, Ulysses was constantly
in the region of space dominated by the Sun’s southern pole, as
indicated by the constant negative polarity measured by the
magnetometer. South polar observations extended from June 26 to
November 6, 1994, when the vehicle was above 70 degrees solar
latitude. It reached a maximum of 80.2 degrees in September
1994. Its instruments found that the solar wind blows faster at
the south pole than at the equatorial regions.
Flying up above the solar equator March 5, 1995, Ulysses
passed over the north polar regions between June 19 and Sept.
30, 1995, (maximum latitude of 80.2 degrees). Closest approach
to the Sun was March 12, 1995, at a range of about 200 million
kilometers. ESA officially extended Ulysses’ mission Oct. 1,
1995, renaming this portion as the Second Solar Orbit. The spacecraft made a second pass over the solar
south pole between September 2000 and January 2001 and the
northern pole between September and December 2001. At the time,
the Sun was at the peak of its 11-year cycle; Ulysses found that
the southern magnetic pole was much more dynamic than the north
pole and lacked any fixed clear location.
Three times during its mission, the spacecraft unexpectedly passed through comet tails — the first time in May 1996 (Comet C/1996 B2 Hyakutake), the second time in 1999 (Comet C/1999 T1 McNaught-Hartley), and the third time in 2007 (Comet C/2006 P1 McNaught).
Ulysses explored the solar wind from all angles, producing the first three-dimensional picture of the heliosphere. It found that the wind from the cooler regions close to the Sun's poles fans out to fill two thirds of the heliosphere, and blows at a uniform speed of 750 kilometres per second, much faster than the 350 kilometres per second wind that emerges from the Sun’s equatorial zone. The spacecraft discovered interstellar dust and made the first direct measurements of interstellar helium atoms in the solar system. Ulysses’ principal findings include data that showed that there is a weakening of the solar wind over time (which was at a 50-year low in 2008), that the solar magnetic field at the poles is much weaker than previously assumed, that the Sun’s magnetic field “reverses” in direction every 11 years, and that small dust particles coming from deep space into the solar system are 30 times more abundant than previously assumed.
References:
Asif A. Siddiqi. Beyond
Earth: A Chronicle of Deep Space Exploration,
1958-2016. Washington, DC: NASA History Program
Office, 2018. ISBN 978-1-62683-042-4
NASA
Solar System Exploration: Ulysses
NASA. NSSDCA: Ulysses
ESA: Ulysses
overview
© 2025, Andrew Mirecki
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