Satellites Falling from the Sky?

We can confirm, as a team of master’s students, satellites fall from space each week. On purpose. Quite the statement, but not one to be too concerned with, especially since there is only one recorded case of someone that has been hit by a piece of satellite falling from the sky. If you’ve tuned in for our blog post: Traffic Jams in Space? Why We Should Be Paying Attention Now, you’ll be aware that there is a growing debris problem and may remember that one of the solutions to reduce debris in orbit is to send it back down to Earth. Wild, right?

Lottie Williams, the Only Recorded Person to be Hit by Orbital Debris pictured with the Debris (Gini,2011)

It has a fair premise however, a while ago NASA and ESA (the European Space Agency) created a new regulation to ensure inactive satellites weren’t simply abandoned as debris when no longer useful. Instead, you are required to detail a plan to relocate your satellite away from populated orbits 25 years after the mission ends. So, what to do with a satellite that doesn’t work anymore? There are two options, the first is difficult. It requires the satellite to be moved to a less populated orbit where it can hang out for the foreseeable, known commonly as a graveyard orbit. Lots of small satellites, popular these days in Low Earth Orbit (LEO, the most popular orbital region), are unable to carry the propulsion needed to get themselves into a graveyard orbit, so are left only with the second option.

This option is to allow the satellite to re-enter Earth’s atmosphere, and ideally be disposed of. Because satellites orbit at very high velocities, it takes awhile during re-entry for the satellite to slow down due to the atmospheric drag.  During this time, the satellite experiences a harsh environment of high loads and heating which in many cases causes the satellite to be torn up and burn completely, disposing of the object.

Schematic of the Typical Process Followed When a Satellite is Disposed of Via Atmospheric Re-entry (Emanuelli,2014)

However, we design our satellites often to survive launch, the very same environment (in reverse mind you) that we now want it to demise in. That causes the tricky situation where the satellite may be designed in a certain way or use certain materials to withstand high loads and heating. We’ve seen many examples in recent years of large satellite fragments surviving re-entry. So, to ensure complete disposal, or often disposal to the point where there is no significant risk posed to Earth’s population, infrastructure and environment (also defined by NASA), a process called Design for Demise (D4D) has been developed. This involves designing the satellite to ensure it is disposed off during re-entry at the end of its life. This process takes place before the satellites manufacture, and models have been developed to simulate the re-entry environment and assess the satellite’s probability of demise and resulting risk.

Part of a Fuel Tank Belonging to a Satellite Launched in the 90’s, Found After Colliding with an Orchard in California (ABCNews,2011)

The difficulty posed, is that the re-entry analysis models are simply tools that approximate the environment we experience upon re-entry. Because, like most things in physics, the complexity of these events, which are specific to each satellite and their initial conditions are difficult to fully define. And therefore, the models have uncertainties. The best way to validate and develop these models requires real world data. However, it is difficult to recreate a re-entry event in a laboratory setting. Although, attempts have been made and models approximated from their results. And it is also difficult to collect re-entry data due to the harsh physical loads and heating environments experienced. Again, though, studies have been conducted, mainly for the validation of heat shield technology essential to the success of Apollo era missions.

However, in recent years, with the advent of small, and more affordable, standardized satellites like CubeSats, it has become more plausible to develop a mission solely with the aim of collecting re-entry data. A great example is the recent QARMAN mission which aims to collect aerothermal re-entry data, which can be translated to collecting the conditions experienced at the most intensive period of heat transfer during re-entry.

QARMAN CubeSat designed for a stable re-entry, with a heat shield to allow it to survive and record data through the most heat intensive periods of re-entry (ESA,2020)

To define an area of focus for the STRATHcube secondary payload objective we first examined the uncertainties in re-entry models. Re-entry models are composed of a number of building blocks like the atmospheric, aerothermal, fragmentation, thermodynamic and aerodynamic components. Each of these building blocks is simply a model and therefore has associated uncertainties. To prevent this post from getting longer than it already is, I’ll summarize it as:

-      Atmospheric uncertainties are almost always small and a very well investigated area anyway (as its easier to carry out).

-      Thermodynamic and aerodynamic models for re-entry are relevant at lower velocities, which have a lower impact on the survivability of a re-entering object.

-      Fragmentation (which defines the break-up of a satellite) uncertainties are typically most significant, with few detailed physical observations made.

-      Aerothermal (the hybrid between aerodynamics and thermodynamics at very high velocities) uncertainties follow closely behind fragmentation, with physical data particularly difficult to obtain.

We considered both fragmentation and aerothermal experiments as possible secondary payloads, as well as the simpler objective to take upper atmosphere measurements as the CubeSats orbit decayed. The simplest option was considered as a baseline, as after all the primary payload must first be prioritized. Tune in to our upcoming blog: STRATHcube’s Secondary Payload: Development Through Semester 1 to hear more about the development of the Secondary Payload.

-      This is STRATHcube signing off, until next time.