You might want to look up. Or maybe not.
At some point between now and Saturday, a 6.5 tonne, bus-sized NASA satellite will burst through Earth’s atmosphere, breaking into fiery chunks that could land on you, your family, your house, your pet.
But really what are the chances? Well, let’s see …
According to NASA, there’s a one in 3,200 chance that one of the 26 fragments expected to survive entry to Earth’s atmosphere will injure someone.
So should we be worried?
Duck and cover
The much-anticipated crash-landing of “UARS” (Upper Atmosphere Research Satellite) is causing concern not seen since Skylab in 1979, parts of which rained down on Western Australia, killing or injuring no-one (that we know of).
But why do we even expose ourselves to this risk from space? A great deal of our personal communication, the political stability of governments and entire economies depend on the immediate, global availability of information, and satellites play a key role in this.
And yet satellites rarely make it into the mass media – unless something (supposedly) spectacular happens.
So what’s so special about UARS?
When it comes to satellites, UARS is in a class of its own.
It is a truly massive structure, deployed from Space Shuttle Discovery in 1996 and, after a highly productive life as a research satellite, finally decommissioned six years ago.
As with Skylab, it’s likely a few pieces of UARS will survive the rapid re-entry and make it to Earth.
Although the manoeuvre has been meticulously planned and risk-assessed since 2002, NASA’s top minds find it very difficult to calculate the precise location of impact well ahead of time.
The strike zone, in this case, will not be known until hours before re-entry, and even then it’s impossible to tell whether anything within this area will actually get hit or not.
The precise amount of deceleration caused by air friction and the resulting flight trajectory of the tumbling structure are equally difficult to predict.
Because the satellite will disintegrate, there are several independent scenarios involving its fragments.
So how does this happen, and why is NASA ostensibly putting us in this kind of danger?
To answer this, we need to take a look at the lifecycle of a satellite and understand why its demise is actually a good thing.
Going around in circles
All satellites revolve around another object, in this case Earth.
The moon is one example of a natural satellite, although it orbits at fairly great distance from Earth.
A man-made satellite is launched into space by rockets, and depending on the purpose of the satellite, its orbit is much closer to the Earth’s surface.
The most important point is that this orbit, near or far, needs to be quite steady for a satellite to function properly, and the simplest geometric example of a stable orbit is a circle.
A stable orbit is a very delicate equilibrium between two opposing forces, but luckily we don’t really need to know much about Kepler’s Laws of Planetary Motion to understand this.
We only need to remember the closer to Earth a satellite travels, the faster it needs to move in order to stay in its orbit.
But Earth isn’t perfect, and neither is its gravity.
Minute variations in the delicate balance between the two opposing forces cause the orbit to change slowly. Since we don’t want to re-adjust our satellite dishes every few days, larger satellites carry fuel and use rocket thrusters to correct these deviations and to bring the satellite back into its designated orbit.
It’s a hard life
Space is an inhospitable place, and not just for astronauts.
Once a satellite has reached its stable orbit, it’s constantly subjected to attacks. It suffers vastly changing temperatures caused by exposure to the sun alternating with the Earth’s shadow, bombardment with space radiation and energy particles (“solar wind”) that gradually destroy its electronic components. Finally, and most prosaically, it can simply run out of batteries.
And then there’s always the risk of disastrous collisions with space junk – anything from barely visible fragments to massive parts of old satellites and rockets that surround Earth.
All satellites therefore have a design lifespan after which they are more likely to malfunction or become technically obsolete. The fuel supply is limited, too.
Unless it’s unfortunate enough to be completely disabled by space junk or by a massive solar storm, the end of a satellite is actually desirable and pretty well planned.
Due to high risk of damage to very costly satellites, it’s no longer acceptable to simply leave inoperative satellites to become uncontrollable space junk in random orbits.
Instead, most satellites are given quite a spectacular funeral.
The structure’s last remaining fuel is used to manoeuvre it into a “disposal orbit” below 400 kilometres altitude, where sparse air particles create enough friction to gradually slow down the satellite over a period of years.
But slower speed and low satellite orbit don’t work well together, so gravity takes over and gradually pulls the satellite even closer to Earth until the air friction in the atmosphere is great enough to incinerate the satellite in less than a minute.
In most cases, nothing will survive.
With more than 22,000 satellites and other objects larger than an apple orbiting Earth at any one time, as well as millions of smaller fragments, this scenario happens all the time, intentionally and accidentally – but few people would ever notice it.
Given that, on average, one piece of space junk makes it back to Earth each day and nobody ever gets hurt, you can probably be quite unconcerned about UARS.
Unless you’re the one beating the odds of the lottery jackpot …