The low LEO protected region: new challenges from ASAT tests, small satellites and mega-constellations

Since the definition of a Low Earth Orbit (LEO) protected region, up to the altitude of 2000 km, at the beginning of the 2000s, most of the attention of the space debris mitigation community was focused on heights greater than 600 km. In such orbital regimes, in fact, the average residual lifetimes of inert satellites and rocket bodies become typically higher than 20 years and the highest concentrations of functional satellites and space debris are found, specifically between 600 and 1000 km, and between 1400 and 1550 km. The low LEO region, below 600 km, even though currently characterized by lower object densities, is however extremely important for space applications. In fact, since the last Apollo mission to the Moon, in December 1972, all human spaceflight was carried out there, the International Space Station (ISS), the planned large Chinese space station and the Indian crewed program are making and will make use of this region of space, and also absolutely critical missions, like the Hubble Space Telescope (HST), are orbiting below 600 km. The recent Indian ASAT test, carried out on 27 March 2019, destroying the Microsat-R spacecraft at the altitude of about 300 km, produced approximately one hundred detectable fragments crossing the altitude of the ISS, and at least several months will be needed to have the situation coming back to the pre-test conditions. Moreover, in recent years there was a dramatic increase in the launch rate of small satellites and cubesats in low LEO, boosting the number of potentially risky objects to be tracked and monitored. But the most really worrying development currently going on is the recent (26 April 2019) decision by the US Federal Communications Commission to grant SpaceX permission to deploy the first segment of the company's Starlink mega-constellation broadband network into much lower orbits than originally planned. The regulatory commission, in fact, approved the SpaceX proposal to fly more than 1500 of its Starlink satellites at an altitude of 550 km, with an inclination of 53 degrees, instead of the 1150 km originally planned, starting in May 2019. Even though any failed satellite of the planned mega-constellation will decay from orbit in less than 25 years, therefore formally complying with international space debris mitigation guidelines, it is realistic to expect a relatively high number of failures, considering the experimental nature of spacecraft tested in space, and in great numbers, for the first time. The short and medium term consequences for the HST, orbiting just 10 km lower and basically non-maneuverable, and for the ISS, just to take a couple of extremely valuable and high priority spacecraft in low LEO as an example, will be probably far from negligible. The aim of this presentation is therefore to introduce the new challenges to be faced by spacecraft and space operations below 600 km, in the coming years, due ASAT tests, small satellites and mega-constellations.