Image of the Chelyabinsk asteroid's exploded trail over Chelyabinsk in 2013.
Who could forget the passage of the Chelyabinsk small asteroid as it passed over the city by that name in central Russia in February, 2013? The event was energetic enough to injure over 1,000 people by flying debris as the shock wave from the explosion swept across the Russian city, shattering windows and leaving a trail of damage. The explosion was estimated to have had a force greater than 30 Hiroshima atomic bombs, according to NASA scientists, and the shock wave was so powerful it travelled twice around the world.
As reported in Physics Today (Sept., 2014, p. 32) the object has now gone down in infamy as being 20 m (66 feet) diameter, entering the Earth's atmosphere at 19 km/s. Based on a 3-dimensional simulation using a shock code developed at Sandia National Laboratories (originally intended to model nuclear explosions). the Lab's supercomputer showed that the Chelyabinsk blast was at least a half megaton, or comparable to the yield of many U.S. warheads on ICBMs, like the 'Minuteman'.
Scandia Lab simulation panel for Chelyabinsk event.(From Physics Today, Sept., 2014)
Some of the ancillary information and insights that the Sandia team has assembled with other data:
- The entry at 19 km/s meant that it originated from the asteroid belt between Mars and Jupiter - not from a ballistically launched missile whose velocity would only be about 11.2 km/s or a short period comet with a mean speed of 35 km/s.
- The altitude of the blast indicated the object was small and weak. The diameter of 20 m (66 feet) was estimated base on the observed velocity factored together with the assumed density of the material.
- The asteroid first felt the presence of Earth's atmosphere while it was thousands of miles above the Pacific Ocean and for a dozen minutes the 10,000 ton rock fell swiftly and unobserved passing at shallow angle through the atmosphere where the molecular mean free path was much greater than the 20 m diameter.
- When it crossed over the border into Russia at 3:20:20 UT and was 100 km in altitude 99.99997 % of the atmosphere still lay beneath it.
- For the better part of 10 seconds the asteroid hurtled through the air as a rigid body moving at a shallow angle, 17 degrees relative to the horizon and descending 1 km for every 3 km of flight.
- At about 45 km altitude the entry dynamics began to change. The dynamic pressure then built up from 0.7 Mpa (millions of Pascals, where 1 Pa = 1 atm equivalent), Within a couple more seconds, below 40 km, pressure on the now fracturing asteroid increased past 1 MPa, breaking it into a number of smaller fragments.
- As the pressure then grew exponentially the process cascaded and formed ever smaller fragments that rapidly increased the surface to volume ratio. As the fragments ablated the hot gas between them built up finally resulting in a chain reaction and a massive explosion converting the asteroid's kinetic energy into heat and pressure (yielding the shock wave that shattered windows).
All of the preceding is again relevant as learned of the recent close passage of the small asteroid 2012 TC4 over Antarctica at a too close distance of 27, 200 miles. TC4 was estimated to be travelling through space at roughly 16,000 mph - 4.5 miles a second.
Artist's conception of 2012 TC 4 passing in vicinity of Earth.
2012 TC4, was first spotted five years ago by the Pan-STARRS telescope at the Haleakala Observatory, in Hawaii, before disappearing as it orbits the sun. It then reemerged in July on a trajectory well inside our lunar orbit. A sketch of the object's trajectory is shown below:
Depiction of the trajectory of 2012 TC 4 on Oct. 12.
Observations reveal that 2012 TC4 is an elongated and rapidly rotating object that has been known to make many close approaches to Earth in the past. The space rock orbits the sun approximately every 1.67 years at a distance of about 1.4 AU. Astronomers estimate that 2012 TC4 has a diameter between 26 to 85 feet (8 to 26 meters). Note the size which is near the diameter range of the Chelyabinsk object. In other words, had TC 4 entered the Earth's atmosphere it could have delivered a similar air blast to that arising from the Chelyabinsk object.
Thankfully, TC 4's approach was planned for in advance as a test object for an asteroid early warning network - something I've been harping on for over three decades. Thus, long before it got within a tenth of the Moon's distance NASA had planned to use the flyby to test early warning for incoming space rocks. Observatories world wide - part of the International Asteroid Warning Network - had been focused in an TC 4 for weeks to test communication and coordination.
Before this close pass, researchers had relied on "tabletop" tests - or computer simulations with no actual asteroids involved. These sort of simulations make for nice abstract exercises but don't deliver much in terms of real time, actual asteroid threat factors.
What still remains is how one of these beasts might be stopped say if it's trajectory was determined to be headed for New York City, or New Orleans. Back in March, 2013 I noted the development of a high-powered 50kW laser by a German firm (http://www.dailymail.co.uk/sciencetech/article-2259639/The-groundbreaking-Star-Wars-laser-shoot-drone-sky-TWO-MILES-away.htmlIt had the power to knock down a drone from two miles away, and cut through a steel girder from 1kilometer away. Yet it was accurate enough to hit a target the size of a mortar round which got me to thinking of using multiple similar lasers in an effort to deflect small (< 50m dia.) asteroids. These would be similar to the Chelyabinsk object that recently exploded over central Russia. The German defense firm Rheinmetall Defence that developed it – could conceivably get its name in lights.
Think then of several Russian and U.S. craft carrying these devices to intercept an oncoming small (‘city buster’) asteroid before it can wreak havoc. If the detailed effects and dynamics can be worked out before hand, I see no reason why the oncoming threat can’t be stopped. Or, would we rather spend $20 billion more on missile “defense” systems that have been shown to be useless?
After the close pass of 2012 TC 4 this becomes much more than an abstract, academic exercise. The recent encounter shows me that it is crucial we not only be able to confidently track small asteroids, but also come up with ways to destroy them - say if one is bearing down on a major population center.
As the hackneyed phrase goes, "if we can send men to the Moon and back......."/
No comments:
Post a Comment