Why It’s So Hard to Predict Where Chinese Space Station Will Crash

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The now defunct Chinese space station Tiangong-1 is en route to crash into Earth – completing its “atmospheric reentry phase”.

While experts have been aware that this would happen for more than a year, there has been huge uncertainty around the exact timing.

As the station’s orbital altitude has decreased, however, this uncertainty has gradually reduced and it is now possible to determine that it will deorbit within a few days.

Most of the 8.5-tonne station will burn up and disintegrate as it passes through the atmosphere, though some debris may hit Earth.

Advancing China’s space programme is a priority for President Xi Jinping, who has called for China to become a global space power with both advanced civilian space flight and capabilities that strengthen national security.

Beijing insists that its space programme is for peaceful purposes, but the US Defense Department has said China’s programme could be aimed at blocking adversaries from using space-based assets during a crisis.

The Chinese space agency has been tracking the space station (pictured before it was launched in 2011), and vowed to issue warnings if there are any potential collisions imminent. But not everyone is convinced by this

The Chinese space agency has been tracking the space station (pictured before it was launched in 2011), and vowed to issue warnings if there are any potential collisions imminent. But not everyone is convinced by this

The vehicle is 10.4 metres long and has a main diameter of 3.35 metres. It has a liftoff mass of 8,506 kilograms and provides 15 cubic metres of pressurised volume. The space shuttle contains hydrazine which is extremely harmful to human health

The vehicle is 10.4 metres long and has a main diameter of 3.35 metres. It has a liftoff mass of 8,506 kilograms and provides 15 cubic metres of pressurised volume. The space shuttle contains hydrazine which is extremely harmful to human health

And although we have the capability to precisely control a spacecraft such as Rosetta – which orbited a few km away from comet 67P while being 405m km away from Earth and travelling at 55,000km per hour – we cannot actually predict the time and place of Tiangong-1’s potential impact on Earth, despite it being only 200km above us.

But why is it so difficult, and will science one day help us nail such predictions?

This graphic predicts details about the atmospheric re-entry and breakup of Tiangong-1.

Credit: The Aerospace Corporation/CORDS

Newton’s laws tell us that satellites orbit the Earth in perfectly circular or elliptical orbits, repeating their path again and again (assuming that gravity is the only force acting on them).

However, this is not true at low altitudes, say below 1,000km, because the satellite is then moving through the Earth’s atmosphere.

This causes “aerodynamic drag” (air resistance) – a force that opposes the satellite’s velocity, which effectively turns the orbit into a downward spiral towards the Earth’s surface.

In theory, we can calculate drag perfectly to predict the path of a satellite.

This can be done using an equation that depends on the velocity of the satellite (), the density of the atmosphere (ρ), a numerical coefficient that depends on the shape of the satellite and its orientation with respect to the airflow (C), and the object’s area (A). For those who are interested, the equation is: D = ½ × C × ρ × A × .

But you don’t have to understand the equation to grasp why it is so hard to calculate drag.

The spacecraft’s velocity is easy to measure fairly accurately using observations.

However, the other parameters are highly uncertain – making it difficult to determine Tiangong-1’s path.

For vehicles such as cars and aircraft, C can be estimated theoretically or with computational fluid dynamics and measured experimentally in a wind tunnel.

The main problem here is that Tiangong-1’s shape is complex, and the object is uncontrolled and tumbling chaotically, resulting in a constantly changing C.

The other unknown is the density of the atmosphere, which decreases with altitude. However, particularly at high altitudes, this varies due to a number of unpredictable factors – the most important of which is solar activity.

Tiangong-1

Credit: TRMO/youtube, CC BY

The solar magnetic activity follows an 11-year cycle, which results in a periodic increase and decrease of the amount of radiation and charged particles emitted.

These interact with a part of the Earth’s atmosphere called the ionosphere, changing its density.

A good indicator of solar activity is the number of observed sunspots.

But while the solar cycle can be monitored, the level of activity also changes unpredictably, leading to unpredictable changes in the density of the atmosphere.

Another important factor is that the satellite will disintegrate and burn during the final phases of reentry, adding further uncertainty to all terms of the drag formula.

An image from ESA shows the region when Tiangong-1 is expected to re-enter.
An image from ESA shows the region when Tiangong-1 is expected to re-enter.

Credit: ESA

This explains why it is near impossible to predict an impact point (or region) along the satellite path.

That said, you can get a rough idea of the area of probable impact, based on the inclination of the spacecraft’s orbit.

We know that Tiangong-1’s orbit only enables it to reenter between the latitudes of -43 (north) and +43 (south) degrees around the equator. As you can see in the map above, this leads to an extended band of probable impact, mainly south of the equator.

To prevent accumulation of debris in orbit around the Earth, which can pose a threat to spacecraft and satellites, it is now recommended that satellites in low Earth orbit are commanded to reenter Earth’s atmosphere within 25 years of mission completion.

China's school-bus-size prototype space station Tiangong-1 is nearing its plunge to Earth.

China’s school-bus-size prototype space station Tiangong-1 is nearing its plunge to Earth.

Credit: The Aerospace Corporation/CORDS

It is therefore of growing importance to be able to avoid threats to population and objects on Earth as these spacecraft crash.

Models and experimental data for atmospheric drag are continuously being improved, but it is unlikely that they will ever reach the required accuracy to allow us to predict exact impact points.

Instead, future satellites need to be designed with reentry as a crucial part of the mission. Active and controlled reentry – for example, by using drag sails or thrusters – could reduce uncertainties and ensure that the satellite burns completely in the atmosphere while following a trajectory carefully calculated in advance.

Satellites should also be designed and tested such that, during reentry, they fragment in a desired way and not cause a threat to Earth.

This concept, analogous to controlled deformations in cars to protect the passengers in an accident, is known as “design for demise”.

This is not something that is enforced today.

There can always be improvements in safety.

But even though the spacecraft’s reentry isn’t controlled or predictable, we needn’t worry about being struck by it.

The odds of you being hit are next to zero, while the chances of it striking anyone at all are about one in 3,200.

Damages and compensation

No guidance is given under the Treaty regarding how liability for the damage is to be calculated. But a further treaty, called the Liability Convention, provides some further guidance.

The Liability Convention provides in Article II that a “launching state” will be:

(…) absolutely liable to pay compensation for damage caused by its space object on the surface of the Earth or to aircraft in flight.

This high standard of absolute liability reflects what were perceived by the drafters of the treaty as particularly vulnerable parties. People and property on Earth and aircraft in flight cannot avoid or reduce their potential harm from space from catastrophic launch failure or space debris.

But where damage is caused other than on the surface of the Earth or aircraft in flight the principle of fault is applied. The Convention does not elaborate on how fault is to be determined.

On Earth we are used to applying principles of negligence to accidents involving people or property. Negligence considers issues such as the potential of harm occurring, the foreseeability of that harm and whether sufficient measures were taken to reduce or avoid that harm.

It is not clear if these sorts of calculations exist with respect to space.

What is clear is that the Convention is intended to be “victim-oriented”. Claims for damage may relate to any harm caused by the space object including direct and indirect harm.

Article XII says compensation is to be determined on the basis that it should restore the person:

(…) to the condition which would have existed if the damage had not occurred.

But it wasn’t our fault

What about where objects collide in space causing harm to a third party? In this case liability may be shared by the launching States of the colliding space objects, again in accordance with their respective fault, where the damage is in space, absolutely if the damage is on Earth or an aircraft in flight.
Some exceptions exist where the State making the claim with respect to harm is actually responsible for that harm, through its own gross negligence or an intention to cause harm.
There has only been one claim made under the Liability Convention. The Government of Canada made a CA$6 million claim for compensation to the Soviet Union after its Cosmos 954, a nuclear powered satellite, crashed in Northern Canada on January 24, 1978.
While the final, diplomatically negotiated settlement of CA$3 million, did not specifically mention the Liability Convention, it is generally considered that the settlement was negotiated in the context of the Convention. Those costs related to clean up of the contaminated site in such a remote area.
In 1979 debris from NASA’s Skylab fell to Earth in Western Australia. NASA advertised for claims with respect to damage caused by the debris, but no State-based claims were formally made under the Liability Convention.
There were some claims regarding illegal dumping: the local Shire of Esperance issued NASA with a A$400 littering fine. It was eventually paid in 2003 by a US radio presenter and his listeners who raised the funds.

The $400 cheque paid by California radio breakfast show host Scott Barley and his listeners in 2009 in a belated response to a fine levied against NASA for AAP Image/James Shrimpton
Tiangong-1 was China’s first attempt at a space station. It was launched aboard a Long March 2F/G rocket from the Jiuquan Satellite Launch Center on September 30, 2011, so it is China’s responsibility.
In the unlikely event that a piece of the Tiangong-1 falls on an Australian, the Australian government would need to pursue a claim with respect to any injury the person suffered against the Chinese government. Such a claim could take many years through diplomatic channels.
Unfortunately, an individual cannot make a claim on his or her own behalf.
I therefore suggest that before you get hit by a piece of space junk, you check your health insurance!

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