To measure these flows in real time requires a fleet of 34 satellites revolving around the Earth in two different orbits to capture plasma movement at different spatial and temporal scales.
Advanced Space's role was to help design the orbits for the 34 spacecraft as they performed what Dr. David Malaspina, the principal investigator from Colorado's Laboratory for Atmospheric and Space Physics (LASP), described as "a CT scan for the magnetosphere." This fleet of spacecraft would consist of 30 vehicles approximately the size of a small refrigerator and another four, each the size of a motorcycle.
To achieve the mission's science goals, the 34 satellites are distributed between two coplanar, near-equatorial orbits that overlap to form a radio tomographic (RT) imaging plane on the apogee side.
In each of the orbits, the satellites are evenly spaced in mean anomaly (the fraction of the satellite's orbit that has elapsed since it passed its closest point to Earth) such that they bunch up near apogee to enhance RT resolution.
The orbits were designed to optimize RT imaging of the inner magnetosphere, in-situ observations near the plasmasphere, and large-scale photon imaging of extreme ultraviolet emissions. Both the inner and outer orbits were designed to have the same orbital precession rates to ensure that the area between the orbits remains fixed.
We performed a trade study to determine an optimal mission design. The following contour plot conveys the trade space that was considered. The goal was to weigh orbital altitude and the amount of science likely to be performed over the mission duration (see below).
The criteria used to identify desirable mission options within this trade space included:
+ De-orbit delta-v less than 500 m/s
+ More than 1.5 RE between the apogees of orbits
+ Inner orbit apogee < 4.5 RE
+ More than 40% of time above 5 RE for Outer Orbit
+ Perigee altitude above 400 km
The graphic above illustrates how these criteria meld into the final design, and one can see how a shift of the Inner orbit affects the Outer orbit, and vice versa. The orbits selected meet all mission objectives.
The precession rate for the orbit that was ultimately chosen is approximately 0.81 degrees per day for argument of periapsis (AoP) and -0.41 degrees per day in right ascension of the ascending node (RAAN), resulting in an overall orbital precession of 0.41 degrees per day. This rate results in a full orbital precession through all local times in about 1.7 years, allowing the mission to precess through all local times almost twice in 3 years.
Setting the radius of the outer orbit's apogee to 6.25 Earth radii (RE) ensured that each satellite in the outer orbit would spend over 50% of the mission above 5 RE. Based on the relationship between the AoP precession rate and the orbit geometry, the radius of perigee can be extracted once the AoP precession rate and apogee radius are set. Therefore, the corresponding radius of perigee is about 1.1 RE.
The delta-v required to deorbit these satellites to a 100 km altitude is ~57 meters per second (m/s). To maximize the RT imaging area, the distance between the apogees of the orbits was to be kept near or above 2 RE. The radius of apogee chosen for the inner orbit is 4.25 RE. This results in a difference of 2 RE between the inner and outer orbit apogees. The corresponding perigee radius is 1.52 RE. This orbit ensures that each satellite in the inner orbit spends more than 60% of its time above 3 RE.
The delta-v required to deorbit the inner satellites is significantly higher than that required for the outer orbit. Thus, the inner orbit satellites will instead be disposed of in a graveyard orbit between low Earth orbit and medium Earth orbit. The delta-v required to achieve this is significantly lower, around 40 m/s.
The Science
The magnetosphere is a dynamic system, with regions of cold, dense plasma changing size and shape as they flow around the planet. Previous space measurements captured data from single locations at specific times along their orbit. What the team led by Malaspina wants to achieve with this fleet of 34 satellites is to create spatially resolved images of the magnetospheric cold plasma, making real-time movies of the plasma flows. The mission would "follow the mass," measuring how much atmospheric mass is lost to the solar wind, and how much is returned to Earth via the Earth's magnetic field. "The whole system breathes," Malaspina explained.
By broadcasting radio waves to each other, the combined fleet of spacecraft create a mesh of plasma density observations that can be used to create a picture of the whole area within the orbit once every 10 seconds. The larger spacecraft make point measurements inside the region that the other 30 are imaging. They also carry ultraviolet cameras to measure the flow of cold dense plasma into and out of the Earth's equatorial plane.
The orbits have been designed so that the spacecraft collect most of their data at 3 RE above Earth to capture plasma flows in different regions of the magnetosphere. Advanced Space has supported several constellation / formation designs. PILOT is very exciting, balancing the science requirements of a 34-satellite constellation in two orbits.
Advanced Space has also designed the ESCAPADE mission, which will place two spacecraft into formation about Mars. Further, Advanced Space has worked with commercial partners to support mega-constellation formation, orbit transfer, and maintenance. Constellation work is challenging in many ways but offers results that cannot be achieved with single platforms.
The Future
PILOT was funded as a mission concept study to be reviewed for development within the National Academies of Sciences, Engineering and Medicine (NASEM) Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. We expect that the NASEM will recommend PILOT as a high-priority mission for NASA to implement in the future, and Advanced Space looks forward to bringing these orbits to life so humanity can learn more about how interaction between the Sun and Earth's magnetic field affects our home planet.
Related Links
Advanced Space
Solar Science News at SpaceDaily
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