My daughter once asked a great question, "Could I get hit by falling space debris?" I told her, "You would probably be more likely to win the lottery, get struck by lightening, and mauled by a polar bear, all on the same day." I'm not sure I answered her question the way she expected, but I think it's true.
To understand if falling debris is dangerous, you have to first put things into perspective. An average of approximately one recorded object falls to Earth each day, and there have been no confirmed cases of serious injury or major property damage to date according to NASA.
The probability of being hit is almost nil when you consider the vast, openness of our planet. If you think about it, humans only inhabit a small percentage of Earth's land surface. For example, only about 3% of Earth's land surface is occupied by urban areas. The rest of the surface tends to be sparsely populated. Also, consider that more than 72% of the Earth is made up of oceans and lakes where people do not live. So the chances are very remote that anyone can get hit by falling space debris.
The highest concentration of orbital debris exists in what is referred to as Low Earth Orbit (LEO). The NASA Orbital Debris Program Office has computer generated images of objects in Earth's orbit that are currently being tracked.
Space debris is an accumulation of discarded man-made objects that currently orbit around the earth. Examples of orbital space debris include old satellites, derelict spacecraft, spacecraft components, rocket pieces, spent rocket stages, and any number of fragments from spacecraft or from the byproducts of propulsion systems. The debris that orbit earth come in many sizes ranging from dust and particle-size microdebris (smaller than 1 cm in diameter), to much larger objects (larger than 10 cm). More than 21,000 orbital debris larger than 10 cm are known to exist. The estimated collection of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles smaller than 1 cm exceeds 100 million. The highest concentration of orbital debris exists in what is referred to as Low Earth Orbit (LEO) 2,000 km from the Earth's surface.
Fun fact: The primary source of information on space debris is the Space Surveillance Network of the United States. SSN actively tracks and records as many as 12,500 space objects larger than 5 to 10 cm in Earth orbits. The SSN has been tracking space objects since the Soviets launched Sputnik I in 1957. The SSN regularly examines the trajectories of orbital debris to identify potential collisions with active spacecraft. For example, the International Space Station ISS would be notified to change course if there was the possibility of incoming debris.
Air drag is a major cause of orbital decay for satellites in Low Earth Orbit (LEO). Newer satellites are designed to burn up completely on re-entry.
After prolonged air drag, the orbit of debris begins to decay. A large amount of debris burns up when it enters the Earth's atmosphere upon reentry. Objects that don't burn up in the atmosphere generally fall into the ocean or onto sparsely populated areas.
Orbital Debris Quarterly News is an excellent source to follow. The newsletter explores current threats of debris to spacecraft, and has loads of other information available to the public. For example, the October 2012 issue reports the failure of a Russian partially-loaded rocket body might explode as two similar stages have done in the past. The new Proton Briz-M stage was left stranded in an elliptical orbit with a perigee in low Earth orbit (LEO), where debris from a future explosion could pose a threat to numerous operational spacecraft there. You can stay current by visiting The Orbital Debris Quarterly News (ODQN) page.
The top 5 countries that have contributed the most cataloged rocket bodies and space debris are shown in the table below as of October 2012.
|Country||Rocket Bodies & Debris|
|Russia / CIS||4,774|