Debris Impacts in Orbit

Debris Impacts in Orbit

Space is widely viewed by the general public as vast, empty, and unknown. Looking at space on a grand scale, this popular opinion is fairly accurate. Take the solar system as an example, it is a body of space that we accept as being large, and yet in comparison to the known universe is incredibly small. The solar system has one star known as the Sun, eight planets, several moons revolving around these planets, the asteroid belt separating the inner and outer planets, another asteroid belt known as the Kuiper Belt that surrounds our eight planets and Sun, and many more bodies like asteroids and comets. Even with all these objects in the Solar System, space is still considered to be mostly empty. When considering only a small area of space encompassing only the Earth and the surrounding area (to about 1000 km altitude – a little more than the distance between Washington D.C. and Chicago, IL), there are hundreds of satellites orbiting the planet, in a wide variety of orbits, performing a number of different tasks every second. In addition to all those useful satellites, there are thousands upon thousands of pieces of debris. The accepted definition for the term debris, among the aerospace community, is essentially an object that serves no useful purpose and is uncontrollable in terms of its motion. Objects that can be labelled as debris can span from something as small as a bread crumb to something as large as a dead, school-bus sized satellite that is no longer functioning and has stopped responding to commands from controllers on the ground. And all these objects pose a problem to satellites in space on a constant basis. Objects that are about 10 cm or more in diameter can be tracked from either ground or space based assets. The big problem, however, is that the number of smaller objects (that are un-trackable) greatly outnumber the larger objects, and if the smaller objects can’t be detected, nothing can be done to avoid hitting them. The impact on Sentinel-1A is a perfect example.

Sentinel-1A Impact

On August 23, 2016, European Space Agency’s (ESA) Sentinel-1A satellite was struck by a piece of space debris on one of its solar arrays. Many satellites have daily encounters with space debris of various sizes, but what sets this impact apart from the rest is that the resulting damage is clearly visible in a before/after image provided by the ESA (see image below). Space debris impacts can have effects ranging from negligible and undetectable to catastrophic and mission-ending.

Before/after image of a space debris strike on the solar panel of the Sentinel-1A satellite. Image credit: ESA.

The Copernicus Sentinel-1A spacecraft is one of several satellites commissioned by the European Space Agency to provide services that are related to land, marine, atmosphere, climate change, emergency management, and security. While monitoring the health and status of the satellite, the operations team in Darmstadt, Germany noticed that there was a sudden, small power reduction coming from one of the satellite’s solar arrays. They also noticed a small but simultaneous change in the orientation and orbit of the satellite. Following a preliminary investigation of these unusual readings, the team suspected that a possible root cause may be a debris or micrometeorite impact to the solar array. In order to try and understand the reduction in power, the satellite operators performed numerous, detailed analyses, and also decided to turn on the satellite’s on-board cameras to take a look at the solar arrays. After the cameras were turned on and pictures were transmitted to the ground, the cause was clear. One camera had taken a picture of the solar array that clearly showed a small impact “crater” on one of the satellite’s solar arrays. The size of the impacting object can be estimated based on the size of the “crater”, the change in attitude, the change in orbit, the typical speed of debris fragments, as well as other parameters. The resulting estimate was a debris object a few millimeters in diameter.

How Significant is an Impact Event?

Debris impacts can have a wide range of effects depending on the debris size and impact speed. The table below summarizes various sizes of debris and their potential effects if impacted.

Debris at various sizes and their subsequent effects from a 10 km/sec impact speed

From the pictures sent to the ground from Sentinel-1A, the affected area on the solar array spans a diameter of about 40 centimeters, which is not surprising given the analogies provided above. Fortunately, the reduction in the power is relatively small in comparison to the overall power provided by the solar arrays. Since the total power output from the solar arrays is more than what is necessary to operate the spacecraft, this impact event has not impaired the satellite’s routine operations and it continues to conduct business as usual.

Debris Effects on Other Live Spacecraft

This photo shows the result of an impact on a Hubble Space Telescope antenna dish. Photo Credit: NASA

 

A piece of space debris or micrometeoroid struck a window on Space Shuttle Challenger during mission STS-7 and left visible damage. Photo Credit: NASA

 

Above, a “bullet hole” can be seen on an ISS solar array where a piece of space debris or micrometeoroid impacted. Photo Credit: Chris Hadfield / Wikimedia Commons

 

 

Approved for public release. OTR 2017-00313