ULYSSES: 15 YEARS IN ORBIT

 

The joint ESA-NASA Ulysses mission, a grand space adventure that was first discussed nearly half a century ago, celebrated its 15th launch anniversary this week. Carried into space on 6 October 1990 by the space shuttle Discovery, the European-built Ulysses spacecraft has already travelled an amazing 7 billion km and is still going strong! During this exploratory voyage, Ulysses has literally opened new windows on the heliosphere, that vast region of space carved out by the Sun's influence. What makes Ulysses so special? Firstly, its orbit: Ulysses is the first - and probably the only - space probe to be placed in a polar orbit around the Sun. From this unique vantage point, Ulysses is able to study in situ the previously unexplored regions of space above the Sun's poles. Secondly, Ulysses carries a comprehensive suite of sophisticated scientific instruments, several of which are of a kind never flown in space before. In addition to enabling the mission's "core business" - providing the first survey of the solar wind in four dimensions (three spatial dimensions and time) - this combination has enabled scientists to make many groundbreaking discoveries, some in areas that were not even imagined when the mission was first planned. Ulysses "firsts" include:

 

First direct measurements of interstellar dust and neutral helium gas

Astronomical observations suggest that the Sun is presently moving through a warm, tenuous interstellar cloud made of dust and gas, one of several that make up our local galactic neighbourhood. Scientists are eager to learn as much as possible about this local interstellar environment and its interaction with the heliosphere. Using instruments on board Ulysses, we are able, for the first time, to make direct measurements of dust grains and neutral helium atoms from the local cloud that penetrate deep into the heliosphere. These measurements have allowed scientists to determine the flow direction of the dust and gas, as well as the density and temperature of the neutral helium and the mass distribution of the dust particles. This in turn is helping us to understand the properties of the local cloud as a whole. (See also the section on pickup ions).

 

First measurements of rare cosmic-ray isotopes

Together with the interstellar neutral gas and dust, cosmic-ray particles are the only sample of material from outside the heliosphere that is available for direct in-situ study. By measuring the composition of cosmic-ray nuclei, scientists are able to distinguish between different theories of their origin. This is particularly true of the so-called isotopic composition (isotopes are nuclei having the same atomic number, or charge, but different atomic weights, e.g. 20Ne and 22Ne). Ulysses carries an instrument that is able, for the first time, to make the precise measurements of rare cosmic-ray isotopes needed to test current theories of cosmic ray origin.

 

First measurements of so-called "pickup" ions of both interstellar and near-Sun origin

Pickup ions are created in the heliosphere when neutral atoms become ionized by charge-exchange with solar wind ions or by photo-ionization. Thanks to Ulysses, an entire branch of science has been built around pickup ions. New sources have been discovered. Solar wind particles appear to become embedded in dust grains near the Sun, and are subsequently released to form a pick-up ion population known as the "inner source". Comets emit neutrals that form pickup ions, and have an extended tail that can be observed, from which the composition of the comet can be determined. Interstellar neutral gas is a sample of the interstellar medium and thus the composition of the Galaxy in the present epoch, as opposed to when the solar system was formed 4.5 billion years ago. The isotope 3He was measured in the interstellar pickup ions by Ulysses, and provides an important constraint on the evolution of matter in the universe. All of this groundbreaking science comes from the pioneering measurements of a single instrument (SWICS) on Ulysses!

Using pickup ion measurements from Ulysses, comet tails have been detected as far away as 3.5 AU (more than 500 million km) from the nucleus.

 

First in-situ observations of comet tails at large distances from the Sun

See this article.

First observations of particles from solar storms over the solar poles

A fundamental Ulysses discovery is that energetic charged particles are able to move much more easily in latitude than was imagined prior to launch. Electromagnetic forces tie charged particles to the magnetic field lines in space, and large excursions in the direction of the field were not anticipated. If the Sun did not rotate, the solar wind would drag open magnetic field from the Sun's surface radially outward in all directions. Because of the rotation, however, the field is wound into an Archimedes spiral pattern in which the field lines lie on cones of latitude. Under these conditions, charged particles would not be able to move easily in latitude. The ideal situation described above apparently does not apply in reality. Ulysses observed large numbers of energetic particles over the solar poles, far away from the location of the solar storms that created them. Either the particles were able to jump across the magnetic field, or the field lines themselves undergo large excursions, enabling low-latitude sources to be connected to high latitudes. Scientists are still debating which answer is the correct one. In any case, Ulysses has revealed that previous ideas as to how particles are transported in the heliosphere need a thorough revision. This is not only of academic interest. Astronauts in deep space could be exposed to radiation from sources that were previously considered to be at a safe location.

 

 

Other areas where Ulysses data are providing new and exciting insights range from the origin of the solar wind itself, and the way the Sun's magnetic field reverses polarity, to the nature of the boundary of the heliosphere and the interstellar medium. Ulysses data have even provided important constraints on fundamental cosmological concepts like the evolution of matter in the Universe.

 

An important aspect of Ulysses' scientific investigations has been the effect of varying solar activity during the mission. Ulysses circles the Sun in just over 6 years, which is roughly the time it takes the Sun to go from the minimum to the maximum of its activity cycle. When Ulysses first flew over the Sun's polar regions in 1994 and 1995, solar activity was close to minimum, providing a view of the 3-dimensional heliosphere at its most simple. Fast solar wind from the polar regions flowed uniformly to fill a large fraction of the heliosphere; variability was confined to a narrow region around the solar equator. When Ulysses returned to high latitudes in 2000 and 2001, things were very different. Active regions on the solar surface abounded, and solar storms were the order of the day. Solar wind streams from the poles appeared indistinguishable from streams at low latitudes. Amid all this apparent chaos, Ulysses found that the reversal of the Sun's magnetic field polarity, which occurs every 11 years, happens in an unexpectedly simple fashion. The main component of the field is a dipole (equivalent to a bar magnet), and this appears to simply rotate through 180 degrees to accomplish the reversal. Given the complexity of the field at the solar surface, as evidenced by the behaviour of sunspots, this is surprising.

 

With all these successes already under its belt, what lies ahead for Ulysses? ESA's Science Programme Committee has approved funding to continue spacecraft operations until the end of March 2008. It is hoped that NASA will follow suit when it conducts a review of currently operating Sun-Solar System Connections missions, including its participation in Ulysses, in November. This will enable scientists to study the 3-D heliosphere over a large fraction of the Sun's 22-year magnetic cycle. In February 2007 Ulysses will visit the Sun's south pole for the third time. Conditions are expected to be similar to those encountered in 1994, with one major difference: even though the Sun will once again be close to its activity minimum, its magnetic field polarity will be opposite to that during the first polar pass. This will provide a unique opportunity to obtain closure on questions concerning the movement of cosmic rays and interstellar dust particles in the heliosphere, as well as a puzzling asymmetry in the Sun's magnetism that was discovered during the first polar passes.

 

As well as being an important scientific mission in its own right, Ulysses is also a key member of the fleet of spacecraft collectively referred to "The Sun-Solar System Connections Great Observatory" that includes SOHO, NASA's ACE, Wind and Voyager satellites, and Cluster. The twin STEREO probes and Japan's Solar-B satellite will soon join this impressive fleet, forming an unprecedented tool to study the Sun and heliosphere as an integrated system. Campaigns using the Great Observatory, together with ground-based facilities, are being planned in the framework of the upcoming International Heliophysical Year (IHY) in 2007. In short, important work lies ahead for Ulysses before it finally rests on its laurels One thing is sure: the dream of those visionary scientists who first discussed an out-of-ecliptic space probe back in 1959, has become an amazingly successful reality!

 

The Future

Even though the spacecraft and instruments are still in excellent health, Ulysses cannot remain operational indefinitely. This is mainly because a Radioisotope Thermoelectric Generator (RTG) provides the onboard electrical power for the spacecraft. An RTG generates this power by converting the heat from the decay of its radioactive fuel into electricity. As a consequence, the RTG output is diminishing by about 4 Watts each year, and will eventually reach a point where insufficient power is available. The situation is made even more difficult by the fact that Ulysses spends a significant fraction of each orbit at large distances from the Sun, requiring power-hungry heaters to keep the onboard hydrazine fuel (needed to maintain the High Gain Antenna pointing to the Earth) from freezing. This has already necessitated the introduction of a payload power-sharing scheme whereby, starting in October 2004, several instruments have had to be switched off temporarily. Full payload operation will be again be possible in 2007 when Ulysses comes close enough to the Sun to benefit from solar heating. This period covers a major part of the third and fourth polar passes, as well as the rapid transit from southern to northern hemisphere. In early 2008, however, Ulysses will head back to the cold. At this point, the challenges faced by the spacecraft operations team will increase significantly.