How do you circle a moon?

The goal for the Martian Moon eXploration (MMX) mission is clear: we plan to use a sample gathered from the Martian moons to better understand the history of the red planet.

For this first sample return mission from Martian space, the technology demands are heavy! We need to travel and return from Mars, reach the Martian moons and descend, land and leave the surface, sample the soil from this small celestial body and store it safely for the journey home.

But the technology required is not the only challenge for the mission.

The MMX spacecraft is planning to launch in 2024 and spend three years around Mars from 2025 – 2028 before returning to Earth in 2029. The question our “MMX Proximity Operation Team” are tackling is exactly how the spacecraft will spend those three years.

Figure 1(a) Mars-centered inertial coordinate system Figure 1(b) Phobos-centered rotating coordinate system (Mars-Phobos line fixed)

It may initially seem premature to worry about the spacecraft’s path and orbit upon arrival at Mars. But the science and engineering goals for the mission are both precise and challenging, frequently requiring the spacecraft to be in a particular location or orientation with respect to Mars, the moons, the Earth or the Sun. As a result, we need to consider our route through Martian space early so that any structural or operational requirements will be reflected in the spacecraft design.

The MMX spacecraft has a wide variety of scientific instruments for investigating the moons, including telephoto and wide-angle cameras, a near-infrared spectrometer (NIRS), a mass spectrometer, a gamma ray and neutron spectrometer (GRNS), instruments to measure dust and a laser altimeter to measure distances. Each of these instruments have specific requirements for gathering the best data. Our team must therefore formulate a schedule for when and how the spacecraft will observe while in the Mars area.

To plan the scientific observations of the Martian moons, we first need to determine how the spacecraft will move within the moon’s vicinity. Interestingly, there are a number of very different choices that depend on the local environment of the celestial body. We can see examples of these demonstrated by different Japanese spacecraft:

The lunar orbiter SELENE (Kaguya) had a low altitude orbit over the poles of our Moon, with an average altitude of about 100km and an orbital inclination of around 90 degrees. Meanwhile, the Venus Climate Orbiter, Akatsuki, entered into an elliptical orbit for its atmospheric observations. In contrast to these true orbits, the asteroid explorer, Hayabusa2, will approach asteroid Ryugu between 0km (for sample gathering) and some tens of km, but will not actually orbit. This is known as a hovering method consisting of repeating approaches to the asteroid surface.

The path of the MMX spacecraft will be different again, as it follows one of the Martian moons on its orbit around Mars. The dynamics of the exact path is influenced by several parameters such as the mass ratio between Mars and the moon and the moon’s semi-major axis around the planet. The result is the spacecraft will adopt a type of orbit that has not previously been used for scientific observations. If observed from Mars, the spacecraft will appear to follow the moon around its orbit with Mars in the centre (see Figure 1a). However, an observer on the moon would see the spacecraft appear to “look around” the moon’s surface. Such an orbit is called a “Quasi-Satellite Orbit” (QSO) as the spacecraft appears to be orbiting the moon as a satellite from the moon’s location (see Figure 1b).

Figure 2(a). 2D-QSO Figure 2(b). 3D-QSO

Our QSO orbit includes a so-called “2D-QSO”, in which the spacecraft moves in the same orbital plane as the Martian moon, and also a “3D-QSO” where the spacecraft’s path dips outside this plane. In the 2D-QSO, the point on the moon’s surface directly below the spacecraft is always near the moon’s equator. In the 3D case, the spacecraft is sweeping over mid- and high-latitudes on the moon’s surface to ensure thorough coverage (see Figure 2a & b).

At the moment, we have provisionally set three types of 2D-QSO paths at high altitude (between 100 – 200km), medium altitude (50 – 100km) and low altitude (30 – 50km) above the moon surface. This is the baseline for considering the scientific observations.

Each of the scientific instruments have specific requirements that specify details such as spacecraft altitude, orientation and the length of time required for their observation. Conditions such as the distance to the Sun and Earth, the mutual configuration of the Earth, Sun and Mars, the timing of shade and occultation as these bodies pass one another, and what region of the moon will be visible play vital roles in when and how the data is gathered.

Environmental conditions such as the direction of the Sun and Earth are major limitations. For example, if the incident angle of sunlight is specified for a particular instrument and we miss the period that satisfies this condition, then it may become impossible to gather this data for the entire moon’s surface during the mission! In addition, the spacecraft itself has limitations as the trajectory cannot be changed endlessly due to carrying a finite amount of fuel.

The spacecraft’s safety is also of paramount importance. For the probe to successfully operate in the immediate vicinity of Martian moons and land, a precise physical model of the moon is needed to map the gravity, shape and unevenness of the surface. This is currently not available, so information about the moon will be updated once the spacecraft arrives and necessary adjustments made.

The biggest event during the mission will be the landing and sampling from the moon’s surface. When the landing site selection (LSS) is held, all the observational data acquired until that point in the mission will be consolidated to help narrow down the candidates for the sampling point. Before this occurs, the MMX spacecraft must therefore have had an opportunity to take all the necessary observations!

Our final scenario must achieve the necessary operation requirements and safety for our instruments and spacecraft described above, but also ensure the best science results are achieved. While we have considered the spacecraft route from the operations and engineering perspective, the “Science Operation Working Team” (SOWT) will consider our path from the view of collecting the best scientific results. We hear from them in another post!

(Based on the article in Japanese by Hitoshi Ikeda from Research and Development Directorate, Research Unit Ⅰ)