Xihe-2: the Sun and the Earth in a single glance

Imagine being able to observe the Sun and the Earth from a privileged point of view, space, and from such a distance that no other celestial body can interfere. Here, to do something similar and be able to study the interactions of solar activity on our planet, we need to make a journey of 149 million km from Earth and arrive at the Lagrangian Point L5. This will be Xihe-2 mission.

Until now, no space mission has ever been so ambitious but, recently, China announced that, in 2026, it will launch the Xihe-2 observatory precisely in this distant equilibrium point of the Earth-Sun system.

What are Lagrangian Points

In 1772, the French mathematician Joseph-Louis de Lagrange demonstrated that, in a system composed of two celestial bodies with a large mass, there are five points in which the interaction of the respective gravitational forces allows a third, less massive body to maintain a stable position relative to the two larger bodies.

These points were called oscillation points and, after Lagrange’s death, called Lagrangian points.

Of these 5 points, three are considered unstable (L1, L2 and L3) since an object stationed there would still need periodic corrections to maintain its position. However, the stability of the object in one of these points can be obtained by placing it in a particular orbit known as a Halo orbit, Lissajous orbit or Lyapunov orbit.

The situation is different for the two points L4 and L5 in which, due to the almost perfect stability of the gravitational forces in the field, an object can station itself almost without the need for corrections. In science fiction literature the L4 and L5 points are associated with space colonies.

They are also called Trojan points because they identify the position of those swarms of asteroids found in them and which keep their distance from the planet with which they share the orbit unchanged and constant.The Lagrangian point L3, in addition to being very distant (the maximum distance is approximately 300 million km) does not have great importance for observing the Earth-Sun System. Moreover, it is also affected by the gravitational influence of the other nearby planets, the same cannot be said for L1, L2, L4 and L5.

Missions to L1 and L2

So far, the two points L1 and L2 have been exploited by various observation missions precisely because of their characteristics. In fact, both are located at a relatively close distance to our planet (1,5 million km) and offer an interference-free view of the Sun, as in the case of L1, and of deep space, as for L2.

In fact, the solar observatories SOHO (Solar and Heliospheric Observatory) and ACE (Advanced Composition Explorer) operate in L1. This point will soon be joined by the Indian solar observatory Aditya-L1, successfully launched on Sep. 2, 2023.

In L2, however, Planck Surveyor, the Wilkinson Microwave Anisotropy Probe, the Herschel Space Observatory, the GAIA probe, the James Webb Space Telescope, and the Euclid Space Telescope operate.

Xihe-2 and the legacy of the Xihe observatory

The Chinese space agency has already sent a mission to observe the Sun: Xihe.

Officially called CHASE (Chinese H-Alpha Solar Explorer), it was named Xihe in honor of the Chinese deity of the Sun. It was launched on Oct. 14, 2021, by a 2D Long March in a sun-synchronous orbit with apogee at 517 km altitude. It is equipped with an H-alpha imaging spectrograph (HIS) that was able to acquire spectroscopic images of the sun in the H-Alpha band.

With the new Xihe-2 mission, the scientific community intends to take a big step forward in the knowledge of our star by bringing, for the first time in history, an object built by humanity to the L-5 Lagrangian Point. This observation point will allow an overall view of the Earth-Sun system that would otherwise be impossible and will make it possible to acquire information on the interactions of solar activity with the atmosphere and the Earth’s magnetic field.

Its on-board instrumentation, in synergy with Xihe, will be able to produce three-dimensional models of these interactions.

For the future, the Chinese Space Agency (CNSA) intends to complete the observation network of our Sun, with two further probes: Xihe-3 and Xihe-4. Xihe-3 will have a polar solar orbit like that of the Ulysses Probe but with a higher inclination, Xihe-4, still at the proposal stage, which will study our star from a close orbit.