The Ulysses Mission - Ulysses

The Ulysses Mission

Exploring Space over the Sun's Poles

[ Terminology | Introduction | Scientific goals | References ]
[ Articles written for Ulysses pages on ESA Science & Technology website ]

The main scientific goal of the joint ESA-NASA Ulysses deep-space mission is to make the first-ever measurements of the unexplored region of space above the Sun's poles.

Terminology explained

Before continuing with a more detailed description of the scientific questions being addressed by Ulysses, here is an explanation of some frequently used terms.

• Solar Wind
• The streams of charged particles (mainly protons and electrons) flowing continuously away from the Sun in all directions at speeds of 300-800 km/sec (~ 1 million mph!).
• Heliosphere
• The immense magnetic bubble carved out in space by the solar wind that defines the extent of the Sun's influence.
• Cosmic Rays
• High-speed particles - either atomic nucei or electrons - that travel throughout the solar system. Some particles originate at the Sun, but most come from sources outside the heliosphere and are known as galactic cosmic rays.
• Interplanetary Magnetic Field
• The magnetic field that exists throughout the heliosphere. The IMF is formed by the radially expanding solar wind that drags the solar magnetic field outward to the boundary of the heliosphere. Solar rotation winds the IMF lines of force into a spiral.
• Heliospheric Magnetic Field
• See Interplanetary Magnetic Field.
• Plasma
• The state of matter in which neutral atoms are separated into charged components (ions and electrons), and in which the number of particles is so great that the ion-electron "gas" can be treated as a fluid.
• Solar Corona
• The outer part of the Sun's "atmosphere", the corona can only be seen during total solar eclipses, appearing as a diaphanous halo around the Sun's disk.
• Shock
• An irreversible wave generated by plasma flow which causes a transition from supersonic to subsonic flow. Shocks are characterized by increased density, pressure, magnetic and electric field strength, and changes in flow speed.

See also the Cosmic and Heliospheric Learning Center Home Page at NASA/GSFC for more excellent descriptions of space physics terminology.

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Introduction

The Ulysses scientific investigations encompass studies of the heliospheric magnetic field, heliospheric radio and plasma waves, the solar wind plasma including its minor heavy ion constituents, solar and interplanetary energetic particles, galactic cosmic rays and the anomalous cosmic ray component. Other investigations are directed towards studies of cosmic dust and interstellar neutral gas, as well as solar x-rays and cosmic gamma-ray bursts. Radio science experiments to probe the solar corona and to conduct a search for gravitational waves have also been carried out. An overview of the scientific investigations is given in Table 1.

Because direct injection into a solar polar orbit from the Earth is not feasible, a gravity-assist is required to achieve a high-inclination orbit. As a result, Ulysses was launched at high speed towards Jupiter in October 1990, after being carried into low-Earth orbit by the space shuttle Discovery. Following the fly-by of Jupiter in February 1992 /3/, the spacecraft is now travelling in an elliptical, Sun-centred orbit inclined at 80.2 degrees to the solar equator. In the normal operating mode, the scientific data acquired by the Ulysses instruments are stored by a tape recorder on board the spacecraft for approximately 16 hours and downlinked to the NASA Deep Space Network once a day together with the real time data during a nominal 8-hour tracking pass. The coverage to date has been excellent, being ~97% on average over the mission. This data base represents the most complete set of continuous interplanetary measurements ever recorded. Further details regarding the spacecraft and its scientific investigations can be found in /1/.

The Ulysses Polar Passes

The Ulysses polar passes are defined to be those periods during which the spacecraft is above 70 degrees heliographic latitude in either hemisphere. The mission was designed to maximise the total duration of the polar passes, with a minimum requirement of 150 days. In fact, the actual mission performance is significantly better than this and the spacecraft spent a total of 468 days above 70 degrees during the first four polar passes. Because of Jupiter's position with respect to the solar equator at the time of the fly-by, the first polar pass took place in the southern hemisphere. Starting on 26 June 1994, Ulysses spent 132 days at southern heliographic latitudes greater than 70 degrees, reaching a maximum latitude of 80.2 degrees in mid-September. The first polar pass ended on 5 November 1994. (It should be noted that the out-of-ecliptic trajectory enables a survey to be made of all magnetic latitudes, since the inclination of the Sun's magnetic dipole axis with respect to its rotation axis is generally greater than 10 degrees). 

In contrast to the initial climb from low to high latitudes, which took more than 2 years and covered a range of radial distances (5.4 to 2.3 AU), the south pole-to-equator segment of the trajectory was completed in only 6 months and at a more constant heliocentric radius. The second (northern) polar pass took place almost exactly one year after the first, and was slightly shorter in duration (102 days). The third and fourth polar passes occurred 6.2 years after the first and second, respectively. Maximum southern latitude was reached on 27 November 2000, and maximum northern latitude on 13 October 2001. The end of the second northern pass occurred on 10 December 2001, marking the completion of the so-called "Solar Maximum" phase of the mission. Ulysses' exploratory journey is far from over, however. Mission operations, with full coverage, will continue until at least September 2004, when Ulysses will have just passed aphelion for the third time. Given the key role that Ulysses plays in the network of heliospheric spacecraft that includes Wind, ACE and Voyager, there are tentative plans (not yet approved) to operate the spacecraft until early 2008, permitting a third set of polar passes in 2006-2008.

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Ulysses and the Solar Cycle

The phenomena being studied by the Ulysses mission are strongly influenced by both the 11-year solar activity cycle, and the 22-yr Hale solar magnetic cycle. The first and second polar passes occurred during the descending phase of solar activity cycle 22, close to solar minimum. The third and fourth polar passes, on the other hand, took place in 2000 and 2001, close to solar maximum. The polarity reversal of the Sun's polar cap magnetic fields occurred in 2000-2001, leading to a global reconfiguration of the heliospheric magnetic field. If approved, the continuation of the mission until 2008 would allow high-latitude measurements to be taken under near-minimum solar activity conditions, but with the opposite magnetic field polarity compared with 1994/1995. Such measurements are crucial to understand in detail the propagation of cosmic rays and solar energetic particles in the complex heliospheric magnetic field, for example. An overview of  Ulysses Mission Milestones is shown in Table 2.

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Scientific Goals of the Ulysses Mission during the period 1995 to 2004

During solar maximum, the conditions encountered by Ulysses were expected to be dramatically different, especially in the polar regions, from those during the "Solar Minimum Mission". This was indeed the case. The polar cap magnetic fields were in the process of vanishing and then reversing polarity. The Heliospheric Current Sheet (HCS), which had a low inclination at solar minimum, was highly inclined.

This complex and dynamic field topology had important consequences for the solar wind, solar energetic particles and cosmic rays. Some of the basic questions to be answered in this phase of the mission included:

Where do the heliospheric magnetic fields and solar wind originate? Are the large-scale properties of the solar wind, particularly the speed and density, still correlated with distance to the current sheet? What will be the magnetic topology of CMEs encountered at high latitude? The solar wind, being closely coupled to coronal magnetic fields, is also expected to undergo drastic changes. CMEs are expected to dominate corotating structure even at high latitudes. Will the flow from the polar caps be correspondingly irregular? Will shocks be present? Will high speed streams still originate from coronal holes?

Galactic cosmic rays will be strongly modulated. How will their properties (e.g. intensity) differ from the poles to the equator? Will it be possible to determine the relative importance of drifts and merged interaction regions in the modulation process? What role does the HCS (or sheets) play? Can the anomalous cosmic ray component penetrate into the polar regions? At lower energies, the properties of solar energetic particles will probably be very different. The increased solar activity will ensure a large number of flares, including some that are very intense. Will particles accelerated in flares or CME shocks be detected at high latitudes? Will evidence for local acceleration at transient shocks be found in the polar regions?

The above list of questions is clearly not exhaustive. Answers to many of them were obtained, at least partially. And, as might be expected, new puzzles were found. Nevertheless, the Ulysses "Solar Maximum Mission" constituted an effectively new mission with unique scientific goals that could not be addressed by any other project. Furthermore, the possibility of continuous measurements of all the key properties of the solar wind and other heliospheric phenomena over an extended period using the same set of instruments is an added bonus of considerable value. The current phase of the mission, leading to a second "encounter" with Jupiter in February, 2004, has its own unique aspects, related in particular to the reversed polarity of the global heliospheric magnetic field.

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References

1. K.-P. Wenzel, R.G. Marsden, D.E. Page and E.J. Smith , The Ulysses mission, Astron. Astrophys. Suppl. Ser. 92, 207 (1992).

2. K.-P. Wenzel and E.J. Smith, The Ulysses mission: in-ecliptic phase, Geophys. Res. Lett. 19, 1235 (1992).

3. E.J. Smith and K.-P. Wenzel, Introduction to the Ulysses encounter with Jupiter, J. Geophys. Res. 98, 21, 111 (1993).

4. R.G. Marsden (Ed.), The High Latitude Heliosphere, Proc. of the 28th ESLAB Symposium, Space Sci. Rev. 72 (1995).

5. E.J. Smith, R.G. Marsden and D.E. Page, Ulysses Above the Sun's South Pole: An Introduction , Science, 268, 19 May (1995).

6. E.J. Smith and R.G. Marsden, Ulysses observations pole-to-pole: an introduction, Geophys. Res. Lett. 22, 3297 (1995).

7. R.G. Marsden and E.J. Smith, Sky & Telescope, March 1996, 24 (1996).

8. R.G. Marsden, E.J. Smith, J.F. Cooper, C. Tranquille, Ulysses at high heliographic latitudes: an introduction, Astron. Astrophys. 316, 279 (1996).

9. R.G. Marsden, K.-P. Wenzel, E.J. Smith, The Heliosphere in Perspective – Key Results from the Ulysses Mission at Solar Minimum, ESA Bulletin 92, 75-81, (1997).

10. E.J. Smith and R.G. Marsden, The Ulysses Mission, Scientific American 278, 74 (1998).

11. R.G. Marsden, Ulysses at Solar Maximum and Beyond, ESA Bulletin 103, 41-47 (2000).

12. R.G. Marsden (Ed.), The 3-D Heliosphere at Solar Maximum, Space Sci. Rev., 97 (2001).

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