Moons of Anterra

The Moons of Anterra are the of Anterra. Anterra has a binary moon system comprised of the moons Selene and Eos. Selene is the larger of the two with a radius of 1,950 km and a mass of 1.038 x 1023 kg. Eos is the smaller satellite of Anterra's binary moon system with a mass of 1.153 x 1022 kg and radius of 970 km (approximately 1/11th the size of Selene). Selene itself is one of the largest natural satellites within the Solar System relative to a major planet. Both Selene and Eos lack any significant atmosphere, hydrosphere, or magnetic field.

Selene
Selene is the larger of the two with a radius of 1,950 km and a mass of 1.038 x 1023 kg. Selene has a mean orbital distance of 258,690 km from the center of Anterra, completing one full revolution in 29.5 Anterra days. Selene is in a synchronous orbit with Anterra, meaning its rotational velocity matches its orbital velocity. Because of this, the side of Selene facing Anterra remains unchanged. Humanity did not view the far side of Selene until competing space powers photographed it during initial missions to the moon system in the 1970s.

Eos
Eos is the smaller satellite of Anterra's binary moon system with a mass of 1.153 x 1022 kg and radius of 970 km (approximately 1/11th the size of Selene). It is a sub-satellite, orbiting Selene from a distance of 15,829 km. Unlike Selene, which formed as the result of an early-life collision between Anterra and another, smaller planet, Eos is believed to be a captured satellite that had formed around one of the solar system's gas giants but had an unstable orbit. Eos's orbit around Selene is inherently unstable due to the gravity of Anterra. Researchers predict that in 35 million years, Eos's orbit will degrade to the point that it will leave Selene's orbit and be captured by Anterra - likely leading to a catastrophic outcome if the two bodies were to collide - or Eos will be flung out of the Anterra-Luna system entirely.