The Sun’s hidden poles could finally reveal its greatest secrets

Why the Poles Matter

At first glance, the Sun's poles seem calm compared to the active mid-latitudes around ±35°, where sunspots, solar flares, and coronal mass ejections (CMEs) dominate. But appearances are deceiving. The magnetic fields at the poles are vital to the Sun's global dynamo process and may act as "seed fields" that shape the next solar cycle, defining the overall solar magnetic structure. Data from the Ulysses spacecraft showed that the fast solar wind originates mainly from vast coronal holes near the poles. Understanding these regions is therefore key to answering three of the most important questions in solar physics:

1. How does the solar dynamo operate and drive the magnetic cycle?

The Sun's magnetic cycle is a repeating pattern that lasts about 11 years, marked by fluctuations in sunspot numbers and a complete reversal of the Sun's magnetic poles. This process is driven by a complex dynamo mechanism powered by the Sun's internal motion. Differential rotation produces magnetic activity, while meridional circulation carries magnetic flux toward the poles. However, decades of helioseismic studies have revealed conflicting information about how these flows behave deep inside the convection zone. Some evidence even points to poleward flows at the base of the zone, challenging traditional dynamo theories. Observations from high latitudes are needed to clarify these internal flow patterns and refine existing models.

2. What powers the fast solar wind?

The fast solar wind -- a supersonic stream of charged particles -- originates mainly in the Sun's polar coronal holes and fills most of the heliosphere, shaping conditions in interplanetary space. Yet scientists still do not fully understand how it begins. Does it emerge from dense plumes inside the coronal holes, or from the more diffuse regions between them? Are magnetic reconnection events, wave interactions, or both responsible for accelerating the flow? Only direct imaging of the poles and in-situ measurements can resolve these long-standing questions.

3. How do space weather events spread through the solar system?

Space weather refers to changes in the solar wind and solar eruptions that disturb the space environment. Extreme events such as powerful flares and CMEs can trigger geomagnetic and ionospheric storms on Earth, creating dazzling auroras but also threatening satellites, communication systems, and power grids. To improve forecasts, researchers must follow how solar material and magnetic structures evolve across the Sun and through space, not just from the limited perspective of Earth's orbital plane. Observing from outside the ecliptic would provide a crucial top-down view, helping scientists trace how CMEs and other disturbances travel through the solar system.

Past Efforts

Scientists have long recognized the importance of solar polar observations. The Ulysses mission, launched in 1990, was the first spacecraft to leave the ecliptic plane and sample the solar wind over the poles. Its in-situ instruments confirmed key properties of the fast solar wind but lacked imaging capability. More recently, the European Space Agency's Solar Orbiter has been gradually moving out of the ecliptic plane and is expected to reach latitudes of around 34° in a few years. While this represents a remarkable progress, it still falls far short of the vantage needed for a true polar view.

A number of ambitious mission concepts have been proposed over the past decades, including the Solar Polar Imager (SPI), the POLAR Investigation of the Sun (POLARIS), the Solar Polar ORbit Telescope (SPORT), the Solaris mission, and the High Inclination Solar Mission (HISM). Some envisioned using advanced propulsion such as solar sails to reach high inclinations. Others relied on gravity assists to incrementally tilt their orbits. Each of these missions would carry both remote-sensing and in-situ instruments to image the Sun's poles and measure key physical parameters above the poles.

The SPO Mission

The Solar Polar-orbit Observatory (SPO) is designed specifically to overcome the limitations of past and current missions. Scheduled for launch in January 2029, SPO will use a Jupiter gravity assist (JGA) to bend its trajectory out of the ecliptic plane. After several Earth flybys and a carefully planned encounter with Jupiter, the spacecraft will settle into a 1.5-year orbit with a perihelion of about 1 AU and an inclination of up to 75°. In its extended mission, SPO could climb to 80°, offering the most direct view of the poles ever achieved.

The 15-year lifetime of the mission (including an 7-year extended mission period) will allow it to cover both solar minimum and maximum, including the crucial period around 2035 when the next solar maximum and expected polar magnetic field reversal will occur. During the whole lifetime, SPO will repeatedly pass over both poles, with extended high-latitude observation windows lasting more than 1000 days.

The SPO mission aims at breakthroughs on the three scientific questions mentioned above. To meet its ambitious objectives, SPO will carry a suite of several remote-sensing and in-situ instruments. Together, they will provide a comprehensive view of the Sun's poles. The remote-sensing instruments include the Magnetic and Helioseismic Imager (MHI) to measure magnetic fields and plasma flows at the surface, the Extreme Ultraviolet Telescope (EUT) and the X-ray Imaging Telescope (XIT) to capture dynamic events in the solar upper atmosphere, the VISible-light CORonagraph (VISCOR) and the Very Large Angle CORonagraph (VLACOR) to track the solar corona and solar wind streams out to 45 solar radii (at 1 AU). The in-situ package includes a magnetometer and particle detectors to sample the solar wind and interplanetary magnetic field directly. By combining these observations, SPO will not only capture images of the poles for the first time but also connect them to the flows of plasma and magnetic energy that shape the heliosphere.

SPO will not operate in isolation. It is expected to work in concert with a growing fleet of solar missions. These include the STEREO Mission, the Hinode satellite, the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS), the Advanced Space-based Solar Observatory (ASO-S), the Solar Orbiter, the Aditya-L1 mission, the PUNCH mission, as well as the upcoming L5 missions (e.g., ESA's Vigil mission and China's LAVSO mission). Together, these assets will form an unprecedented observational network. SPO's polar vantage will provide the missing piece, enabling nearly global 4π coverage of the Sun for the first time in human history.

Looking Ahead

The Sun is our nearest star, yet much about it remains unknown. The upcoming Solar Polar-orbit Observatory (SPO) mission is expected to change that by giving scientists an unprecedented look at the Sun's polar regions. These areas, which have long been hidden from direct view, will soon be observed in detail, offering new insight into the forces that shape our star and sustain life on Earth.

The importance of SPO goes far beyond pure scientific curiosity. By improving knowledge of the solar dynamo, the mission could lead to more accurate predictions of the solar cycle and, in turn, more reliable space weather forecasts. Understanding how the fast solar wind forms and behaves will also refine models of the heliosphere, which is vital for spacecraft engineering and astronaut safety. Most significantly, advances in tracking solar activity could strengthen our ability to safeguard critical technologies, including navigation and communication satellites, aviation systems, and power grids on Earth.