There are two ends to the spectrum of hazardous asteroids or comets that could hit Earth, the really big objects like the one thought to have been a major factor in wiping out the dinosaurs (and much more) around 66 million years ago, and the small stuff that injured thousands of people in Chelyabinsk in Russia in 2013.
This October 12 (US time) another chunk of the small stuff, micro-asteroid 2012 TC4 will hurtle past Earth, possibly as close as 6800 kilometres from its surface, although more likely at a greater minimum distance, perhaps as far out as as 170,000 kilometres.
Being so comparatively small that it has proven exceptionally hard to see or track since it was discovered in 2012, and thus difficult to predict in terms of how close it will pass, 2012 TC4 is seen by NASA as a critical test of its space guard capabilities, which despite various funding problems, attempt to detect potentially dangerous objects that might one very bad hair day, collide with Earth.
This will be at the latest of three similar hazardous natural cosmic objects to demonstrate the risks of such collisions since the Tunguska meteor event of June 30, 1908, and Chelyabinsk, on February 15, 2013. However unlike those events, this one has been seen coming.
2012 TC4 is particularly interesting because it is estimated to be in the same size range, of 10-30 metres in cross section as Tunguska and Chelyabinsk.
It is also interesting because depending on just how close 20212 TC4 gets to Earth, its future regular orbits through the inner solar system may be modified by our planet’s gravitational field to either expel it further away from our domain, or drag it into a future collision with Earth, or its Moon. (Which is also a less than desirable outcome, especially if the Moon is being industrialised in coming centuries.)
There is no doubting the capacity of the Tunguska and Chelyabinsk events to have killed millions of people if they had ‘exploded’ low over settled areas. The damage they can do depends on the trajectory and the altitude at which an object traveling at many kilometres per second has its kinetic energy released explosively as its speed is abruptly reduced by atmospheric drag or even impact with the planet’s surface. Tunguska was by chance, on a course where it would have incinerated St Petersburg instead of flattening a huge area of uninhabited Siberian forests when it disintegrated, had it arrived only slightly sooner in 1908 than it did.
The Tunguska object has since been argued to have been a shard of cometary rocks or ices off Comet Encke, a usually dim and unspectacular inner solar system periodic comet that gets nearish to Earth around every 33 years. Shorter period comets like Encke throw off dusty debris trails that contribute to known meteor showers at regular intervals.
Chelyabinsk may have been larger than Tunguska, but came undone at a much higher altitude and released according to some estimates around 30 times as much energy as the atomic bombs that were dropped on Hiroshima and Nagasaki prior to the end of Japan’s participation in World War II.
Like many other ‘cosmic’ events involving satellite re-entries or bright meteorites Chelyabinsk was caught from multiple angles and distances by numerous dash cams and security cams, allowing very accurate measurements through triangulation to be arrived at in terms of speed, direction, altitude and even likely nucleus shapes. (Dash cams and CCTV security cams also appear to have ended unsubstantiated reports of alien visitations and the like. It’s amazing how eyewitness accounts of such events evaporated once the age of perpetual surveillance of the skies began.)
The dilemma for projects that look for potentially dangerous comets and asteroids is that the really big menaces are likely to be found decades, even centuries, before they pose a risk to Earth because they are more readily visible. These are objects that might be thousands of metres across. Because of their much higher mass they are also comparatively less subject to disruption by encountering the gravitational effects of other cosmic bodies. But small objects like 2012 TC4 are hard to see, and can be thrown into very different trajectories if they happen to enter the Earth-Moon system at particular distances sometimes referred to as gravitational keyholes from the surface of our planet.
Those challenges of detection and future orbital calculations have not been fully resolved. 2012 TC4 could provide valuable learnings in advance of that fateful day when something deadly this way comes.