INTRODUCTION
For a planet with no atmosphere, any object that is on a collision course will hit the planet with hypervelocity (>3 km/sec) and cause an impact crater. The subsequent events will depend upon the size of the impacting object.
However, in the case of a planet with an atmosphere, such as the Earth, there are several different scenarios that might occur, dependent upon the size and mass of the object. There are also two fundamental cases to consider - objects that have been orbiting the planet and reenter the atmosphere (due to atmospheric drag), and objects that have not before been associated with the planet (ie an object in heliocentric orbit) and thus enter the atmosphere from any angle and with a much wider range of speeds.
REENTRY
There are currently tens of thousands of pieces of space debris over 10 cm in size that are orbiting the Earth. These are causing and will continue to pose significant hazards to space operations. The vast majority however, pose no hazard to the Earth's surface, as they will totally ablate (burn up) during atmospheric reentry, if and when this occurs.
In general an orbital space object has to be over 1000 kg in mass to be likely to partially survive the atmospheric reentry process. This includes derelict satellites, space stations and rocket bodies. Only small pieces of these will hit the ground.
Prior to 2020 we could expect on average one or two pieces of debris (size greater then 100mm) to reenter each day.
It has been estimated that of the 200 to 600 orbital reentries each year, about 20% may be large enough to have partially survived reentry and dropped at least some fragments on the Earth's surface. This equates to an average of about 80 per year. Most of these will be small pieces, and most will drop into the ocean. On the average Western Australia might be hit once in about 4 years by some type of artificial space debris. This is less than the meteorite hit rate.
These statistics have changed after about 2020 because of the launch of megaconstellations (eg Starlink) of satellites into low Earth orbit.
So far there have been no reports or deaths from reentering space debris, although there have been a few incidents of property damage and possibly some minor injuries. One woman in the southern US was hit by a piece of insulation from a US military satellite.
The table below shows estimated relative risks of casualties.
This means that once in 100 years someone somewhere in the world may be injured or killed by a piece of reentering space debris. We should also note that currently the risk of meteorite entries exceeds that of reentry events.
REENTRY DEBRIS
Reentry debris may be divided into large and small objects as shown below.
There are generally two types of objects that survive reentry. Refractory materials such as titanium, steel and glass that can survive the heat of reentry, and interior or otherwise protected components such as insulation and compact components.
The Aerospace Corporation in Los Angeles maintains a catalog of all known recovered reentry debris. At the end of 2013 the database contained 69 items. Of these 8 events occurred in Australasia (about one every five years). The table below is an extract of that catalog for reentries over Australia that produced some debris on the ground.
| In April and June 1963, two spherical pressure vessels were found near Broken Hill, New South Wales, Australia | Believed to be from U.S. Agena rocket stage used to launch U.S. Air Force test satellites on Dec. 14, 1962 and Jan. 7, 1963. Both rocket stages reentered from orbit in January 1963. |
| In September 1965, a titanium sphere (diameter 0.5 m), called the Merkanooka ball, was found in Australia. | Identified as a tank used for drinking water in Gemini V spacecraft launched 21 August 1965, components of which reentered in late August 1965. |
| In April 1972, four titanium pressure spheres (diameter 0.33m, mass 13.6 kg each)were found near Ashburton, New Zealand. | Probably from Soviet Cosmos 482, launched March 31, 1972, part of which reentered April 2, 1972 |
| In July 1979, numerous tanks, spheres, heat exchangers and other debris fell over south-western Australia, including the towns of Esperance, Balladonia, and Rawlinna, and extending into central Australia. | Identified as debris from Skylab, launched May 14, 1973, which reentered July 11, 1979. |
| In June 1988, a titanium pressure sphere (diameter 0.37m) was found in the region of Marble Bar, Australia. | Probably from Soviet Foton 4, launched April 14, 1988, reentered April 28, 1988. |
| In July 2008, a metal rocket-motor casing was found in Australia | Identified as debris from 3rd stage of Delta II booster used to launch INSAT-1D on June 12, 1990. Stage reentered Sep 5, 1990. |
TWO WESTERN AUSTRALIAN REENTRIES
SKYLAB - 1979
| In July 1979 the US Skylab space station reentered over SE Western Australia with a large number of objects impacting the ground. The Esperance museum contains the greatest collection of these items, but several museums in Australia have some Skylab debris items on display. |  |
MARBLE BAR TITANIUM SPHERE - 1988
 |
This was found by an employee of the Western Australian Water Corporation. We analysed it using an X-ray microprobe. Object was about 380mm in diameter, was almost pure titanium and showed melt flow around the attachment point. It had a CO2 laser weld around the equator and Cyrillic writing was visible on the surface. It was eventually sold in 2005 in New York for $100K to a collector. |
THE REENTRY PROCESS
Reentries always occur at very low grazing angles - less than one degree.
Breakup starts at around 78 km altitude. The time from breakup to impact is from 6 to 30 minutes.
The footprint over which debris from a large reentry may land is up to 70 km wide and 2000 km long as shown above. The length of the reentry process footprint mostly depends on the reentry process above an altitude of 50km, whereas the width is affected by tropospheric winds below 10km altitude.
The above map shows the debris field for the tragic uncontrolled reentry of the US Space Shuttle Columbia. This is the largest mass (60 tons) to experience an uncontrolled reentry. There were 85,000 pieces found scattered over an area of 1000km x 20 km. These constituted 38% of the total mass of the shuttle before reentry. The search cost was three hundred million dollars.
REENTRY PREDICTIONS
The US Strategic Command usually issues reentry predictions at the web site
The Aerospace Corporation CORDS (Center for Orbital Reentry Debris Studies), located in Los Angeles, issues predictions for large object reentries at least 5 days in advance at
Some private groups or individuals publish reentry predictions. One group specialises in military satellite predictions (which are generally not provided by US government agencies).
None of the above prediction sources provide 100% coverage of reentries.
The NASA Orbital Debris Program Office (ODPO) now routinely conducts a survivability analysis on all large satellites.
They use two models in this process. The simplified DAS (Debris Assessment Software) which is freely available, and the higher fidelity ORSAT (Object Reentry Survival Analysis Tool). Both of these are highly theoretical with very limited calibration data. Fortunately, most objects reenter over the ocean and tests of the predictions are few and far between.
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The UARS (Upper Atmosphere Research Satellite) had an initial mass of 5668 kg and reentered in 2002.
Note that:
Timing predictions for reentries have a minimal error of 10 % (and sometimes up to 25%). This means that a week before the event we can only know the day of reentry. As a satellite nearing reentry makes over 16 orbits of the Earth during this time, a knowledge of where the reentry will occur is impossible.
A day before the event, we can confine the reentry path to a little over a single orbit. This can be used large areas of the Earth, but anywhere under that single orbital path is a possible target.
Even two hours before the event the error is 12 minutes. For the satellite travelling at 8 km/sec this equates to a positional error in the reentry interface point of 5760 km ! And this does not include the 1000 km or so interface footprint over which the debris may be scattered.
Long term (one week to one year) can provide a statistical likelihood of the impact area (as shown below) above based purely on the inclination of the satellite orbit (which does not change over these time periods.
A computer survival analysis for a particular satellite can also estimate the number of objects expected to survive and their total mass.
Predictions of a week or so can provide the day of reentry and a plot of the 16 or so orbits that the space object will travel over during that time (shown below). This can be used to exclude some areas from concern.
Predictions a day or less before the reentry event can generally indicate the orbit over which reentry will occur. This can be used to exclude large areas of the world, and a possible warning for the locations under the final orbit.
A reentry event may pose a hazard in the following conditions.
The probability of injury or death from a space object reentry and the number of injuries or deaths that can occur are both so low that in most cases local emergency responses are the only necessary response to cope with a reentry event, and then only after it has occurred.
There is only one exception to this, and that is if the reentering object is known, suspected or found to contain radioactive (or very possibly other) hazardous materials, such that it presents no increased impact hazard, but a hazard if an unsuspecting person were to discover the reentered objects.
There are two types of radioactive components that have been carried by satellites or spacecraft in the past:
(1) RTGs (Radioisotope Thermoelectric Generators). These are used on deep-space craft that venture to parts of the solar system where solar power is inadequate to supply the operating power needed.
(2) Nuclear Reactors. These have been used mainly by the Soviets to supply the large power requirements of radar reconnaissance satellites in low Earth orbit.
RADIOISOTOPE THERMOELECTRIC GENERATORS
An RTG converts the heat from radioactive decay into electrical energy. RTGs are frequently used on deep-space missions to the outer planets where solar energy does not provide adequate power (too far from Sun).
The radioactive isotope of choice is plutonium-238 (Pu-238). RTGs are normally only a reentry hazard if the launching rocket fails to reach its nominal orbit. Pu-238 is non-fissionable and US RTGs are very robust. The maximum amount of Pu-238 is about 10 kg (for 200W of power). Plutonium is biologically toxic and is mainly an alpha particle emitter with some gamma radiation.
NUCLEAR REACTORS
Nuclear reactors have been used in space applications where high power is required. The US has placed one reactor in space and the Soviets ~33.
The Soviet reactors were used to power Radar Ocean Reconnaissance satellites used to track movements of US Naval vessels. They had very low orbits and solar panels could not be used because the drag would have caused reentry in a few days.
All remaining Soviet space nuclear reactors have been boosted into high orbits where they will not reenter for around a thousand years. The last RORsat was launched in 1988. Russia is planning a 1 MW space reactor for 2020.
The image below is a scale model of a Soviet space nuclear reactor (Topaz). The actual reactor was 4.6 m in length, weighed 980 kg, and had an electrical output of 5 kW.
All past reactors have used highly enriched uranium-235 oxide (90%), with a maximum fuel mass of < 50 kg.
CASE STUDY - COSMOS 954
This was a Soviet military radar ocean reconnaissance satellite that carried a nuclear reactor. The reentry footprint occurred over Canada. The main part of the reactor was deposited on a frozen lake, but the containment vessel of the reactor was breached and other radioactive contamination occurred across a large area. The cleanup cost to the Canadian government was $6M of which the Soviets paid $3M. No one was injured.
Reentry occurred 24 January 1978 over the Canadian North-West territories in a footprint covering 100,000 square kilometres.
The highest radiation levels encountered (~ 200 R/hr) could cause severe radiation sickness and death from only a few hours exposure. The cleanup operation (Operation Morning Light) lasted about 3 months.
CASE STUDY - RUSSIAN MARS 96
This was a Russian mission to Mars that failed to reach the correct Earth orbit. It carried an RTG. USAF Space Command determined that the reentry footprint could occur over Australia, but did not issue a warning to the Australian government, although a warning was issued to some individuals (mostly US government employees) in Australia via the US Embassy in Canberra.
The reentry actually placed debris not over Australia but over Bolivia and Chile. All evidence points to the fact that the RTG probably landed in the Andean mountains, but no-one has yet recovered it. And no directed search has ever been made!
This incident upset the then Australian Prime Minister and resulted in the negotiation of a warning agreement between the US and Australia, as well as the generation of an Emergency Management Plan mainly involving ARPANSA (the Australian Radiation Protection and Nuclear Safety Agency).
A DEBRIS REENTRY HAZARD RESPONSE
A suggested debris reentry hazard response is shown below.
The Desert Fireball Network is a research project based at Curtin University under the direction of Dr Phil Bland. They have over 50 sky sensors across Western and Central Australia. This network has the potential of locating reentered space objects to within a hundred metres. It also encourages public reports of fireballs to augment their sensors, and may be accessed at the web site shown below.
A satellite in low Earth orbit (below an altitude of 1500 km) will lose energy due to air resistance, and drop slowly back toward the Earth. The upper atmosphere is very tenuous and its density decreases dramatically with altitude. It thus can take a long time to decay back into the Earth's lower atmosphere. The table below indicates the approximate lifetime of a satellite versus its initial altitude.
So far we have only considered reentry of artificial space debris. However, a lot more pieces of natural space debris enter the Earth’s atmosphere every day.
Most meteoroids burn up completely in the atmospheric entry process, but as in the case of artificial debris, some the larger meteoroids will make it to ground. These remnants are termed meteorites, and generally have a mass of only one percent of the meteoroid mass. That is, a 100kg meteoroid will drop a 1 kg meteorite. Meteoroids with a mass of 10 kg will usually burn up completely leaving no meteorite.
There are many differences between reentering artificial (orbital) space debris:
The basic physics of meteoroid entry through the atmosphere has been known for almost 100 years. Specific equations can be coded to produce graphs of the reentry process.
The major unknowns are the strength of the body. Meteoroids often explode during entry due the extreme deceleration they experience. This fragmentation produces a number of smaller bodies, which then must be each considered separately.
The light curves for non-fragmenting and fragmenting bodies are shown below. The two top curves are for meteoroids with minor fragmentation. The bottom curve shows a meteor where the body was ‘pulverised’ at 0.5 seconds with no visible light beyond this point.
For more detail see meteor flight.
The graph below is the output of an ASA program modelling the
atmospheric entry of meteoroids>
Curtin University in Western Australia runs the Desert Fireball Network of many tens of stations across mostly the south-western part of Australia.
One station of the DFN shown below.
A meteorite (bottom right) recovered from DFN observations and triangulation (top), and being examined by the DFN recoverers, Anthony and Hadrien (bottom left).
It is obvious that the impact speed was very low, as no crater was formed. This is typical of meteorites. Only for masses
greater than 10 tons will the impact speed be sufficient to form a crater with the rock then vapourised.
The following table may help you decide whether you have seen an orbiting space object, a meteor/fireball or some reentering debris.
Output from the NASA ODPO ORSAT reentry model
Text output from the NASA ORSAT model for the UARS reentry
REENTRY HAZARDS
Altitude Lifetime 200 km 1-5 days 300 km 1 month 400 km 1 year days 500 km 10 years 700 km 100 years days 900 km 1000 years
ATMOSPHERIC ENTRY
Orbital space debris (OSD) will all reenter with an initial speed of ~8 km/sec
OSD can normally be tracked for at least several days before reentry
OSB is mostly aluminium (2700 kg/m3 ) but with bulk densities much lower
The OSD reentry interface is around 80 km
Curtin University - Desert Fireball Network
SATELLITE, METEOR OR REENTRY??
Orbiting Satellite or Large Debris
Meteor or Fireball
Reentering Debris
Australian Space Academy