Look out below – Design for Demise study aims to cut satellite reentry risk

Below about 600 km the vestigial atmosphere drags the sky clear of derelict satellites over a period of months or years. Friction between a fast-moving orbiting item and the air starts to slow it and pull it lower, at the same time generating heat.

By the time an object reaches an altitude below 80 km, aerodynamic stresses should cause the item to break into smaller pieces, which in turn melt then vapourise, in the same way meteors burn up to become shooting stars.

But a surprising quantity of material actually survives its scorching plunge through the atmosphere. Dozens of items have been recovered over the years: while low-melt-rate materials such as aluminium do not generally make it, components made of titanium, steel and glass are more resilient.

The majority of such fragments fall unwitnessed into the oceans. Of the remaining land, only about a quarter is inhabited, so the odds of a human being struck per single fragment should be extremely low – the chance of any injury is truly minuscule compared to, say, a car accident. That has not stopped it happening, at least once: in 1997 lightweight mesh from a Delta II stage harmlessly hit the shoulder of Lottie Williams in Turley, Oklahoma, USA.

The hardiest spacecraft components are more prone to making it back. Fuel tanks, for instance, have variously landed in Thailand, Argentina, Saudi Arabia and the US state of Texas – the woman who found a 260 kg launcher tank on her farm just 50 metres from her home described it as looking like an ‘upside down rhinoceros’.

Since April 2008 all ESA satellites and launcher upper stages intended to be disposed of by atmospheric reentry at the end of their working lives must demonstrate that the risk from fragments surviving reentry to cause casualties on the ground is less than one in 10 000.

Ideally a mission would retain enough propellant to direct itself downwards in a controlled manner, increasing the likelihood of either total incineration or else steering down to an uninhabited area, but this is not always possible within the limited design margin of many spacecraft.

As an alternative, the multidisciplinary ‘Design for Demise’, D4D, approach is the intentional design of spacecraft systems and/or hardware to reduce the casualty risk on the ground, especially in the case of uncontrolled reentry.

So ESA’s Clean Space initiative has issued an invitation to tender for a new study to better quantify the satellite re-entry risk using various re-entry analysis tools, and consider how D4D can best reduce it.

The study includes identifying which parts of a satellite pose the greatest risk and identify ways D4D can be applied to reduce it, from system to subsystem down to equipment level. These D4D techniques will also be applied to a pair of actual ESA missions, to assess their feasibility in practice.

Guidelines will be drawn up for applying D4D in future, and the technology developments needed for its use in future mission platforms will be outlined in a roadmap.

For more information check the invitation to tender package, accessible via ESA’s Electronic Mailing Invitation to Tender System (EMITS).