Ulysses: The Explorer That Unveiled the Secrets of the Sun's Poles

Imagine trying to study Earth without ever leaving the equator. Our understanding of the climate, the oceans, or the poles would be incomplete. Until the 1990s, the same was true for the Sun. All our spacecraft observed our star from the plane where the planets orbit (the ecliptic). But a daring mission, the Ulysses probe, broke that mold to show us, for the first time, the unknown face of the Sun: its poles.

A Feat of Cosmic Engineering

Launched on October 6, 1990, from the Space Shuttle Discovery (STS-41 mission), Ulysses was a pioneering collaboration between the ESA and NASA. Its objective was a monumental physical challenge: to escape the plane of the ecliptic to fly over the solar poles, something that would require propulsion energy unattainable for any rocket of the time.

The solution was a maneuver as elegant as it was ambitious.

In February 1992, the spacecraft reached Jupiter and used the gas giant's gravity as a cosmic slingshot to deflect itself southward, out of the planetary plane. The result was a highly inclined elliptical orbit—80.2°—that allowed it to fly over the Sun's north and south poles approximately every 6.2 years.

It was the first time in history that a spacecraft had achieved such a perspective.

The Heart of the Spacecraft: Technology for the Abyss

With a mass of only 371 kg and the size of a small desk, Ulysses was designed to withstand an extreme environment. Since its orbit took it as far as Jupiter—where sunlight is 25 times weaker than on Earth—solar panels were useless. Instead, it relied on a Radioisotope Thermoelectric Generator (RTG) powered by plutonium-238. On board, it carried 10 main instruments (plus two additional experiments), which functioned as its senses:

- Magnetometer (VHM/FGM): Measured the strength and direction of the solar magnetic field.

- SWOOPS and SWICS: Plasma analyzers to study the composition, velocity, and temperature of the solar wind.

- DUST: Sensor to detect impacts of interplanetary and interstellar dust particles.

- GRB: Gamma-ray burst detector, useful for high-energy astronomy.

- COSPIN: A set of detectors to study cosmic rays and energetic particles.

- URAP: Instrument to measure radio and plasma waves in the solar environment.

Each contributed essential pieces to reconstruct the three-dimensional dynamics of the solar environment.

Discoveries that Rewrote the Textbooks

During its nearly 19 years of operation, Ulysses made three complete passes over the solar poles, observing the Sun during both periods of calm and at its peaks of activity. Its findings were revolutionary:

The Solar Wind Has Two Speeds

Ulysses confirmed that the solar wind is a “two-speed machine”:

- From the poles flows a fast and stable wind (750–800 km/s).

- From the equator emerges a slower, denser, and more turbulent wind (~400 km/s).

This discovery completely changed models of the heliosphere.

The Solar “Bubble” is Not Spherical

The mission revealed that the heliosphere is asymmetrical and changes drastically with the 11-year solar cycle.

- At solar minimum, it is more stable and uniform.

- At solar maximum, it becomes a chaos of coronal mass ejections and magnetic distortions.

A More “Open” Magnetic Field

Ulysses measured how the solar magnetic field extends into space more uniformly than expected at high latitudes, creating veritable highways for charged particles. Interstellar Invaders

It was the first mission to directly detect neutral helium atoms and dust from the Local Interstellar Cloud, allowing scientists to study the galactic environment without leaving the Solar System.

Comet Tail Record

In 1996, it crossed the tail of Comet Hyakutake at a record distance of 500 million kilometers from its nucleus, demonstrating that ion tails can extend much farther than previously imagined.

A Silent and Lasting Legacy

The mission came to an end on June 30, 2009. This was not due to technical failures, but rather a natural decline: the RTG (radioisotope thermoelectric generator) no longer generated enough heat to prevent the hydrazine—the maneuvering fuel—from freezing in the pipes. In a final act of service, the spacecraft oriented its antenna toward Earth before falling silent.

Update

Today, its legacy lives on in missions like Solar Orbiter, which will take the first real images of the Sun's poles, and Parker Solar Probe, which is plunging into the corona to study the origin of the solar wind.

Ulysses taught us that the Sun is a complex and dynamic three-dimensional structure, and that to understand how it influences Earth, we must dare to observe it from all angles... even from "above."