Asteroids and the threat to Earth

 

NASA just shut down a planetary defence mission that tracks asteroids. Now what?

Artist’s impression of NEOWISE spacecraft. NASA/Caltech-JPL

Steven Tingay, Curtin University

Launched in 2011, NASA’s NEOWISE mission operated in Earth’s orbit until late last week. It detected more than 3,000 near-Earth objects or NEOs – asteroids or comets whose orbits can bring them close to Earth, even with the possibility of a collision. NEOWISE was shut down on August 8.

Surveying the population of NEOs is central to the emerging concept of planetary defence. That is, understanding and mitigating the risk of collision from asteroids large enough to do significant damage to Earth.

NEOWISE has made fundamental contributions to establishing the knowledge base for planetary defence, with more than 200 of the 3,000 objects it studied not known to us previously.

Now at mission end, and commanded by NASA to shut itself down, NEOWISE will re-enter Earth’s atmosphere before the end of this year. Where does that leave us with defending our planet?

All Known Asteroids in the Solar System (1999–2018)

From astrophysics to planetary defence

NEOWISE started life as a different mission, simply called WISE (Wide-field Infrared Survey Explorer). It was designed to study the infrared radiation from distant galaxies in the universe.

Infrared means “beyond red” – infrared light sits just past the red end of the spectrum of colours humans can see. We know infrared radiation better as the heat from the Sun, for example, or from a radiator keeping us warm in winter.

Infrared light is just outside the part of the spectrum that the human eye can see. brgfx/Shutterstock

When the coolant on the WISE mission ran out and these sensitive observations of galaxies couldn’t be carried out any more, NASA granted a mission extension under the NEOWISE name. They realised the telescope system was still sensitive enough to detect asteroids and comets that come close to Earth and the Sun, thereby having a very strong infrared signal.

NASA has an extraordinary history of squeezing extra life out of missions that reach completion. In this case, NEOWISE represented an entire second life, in an entirely different area of research.

How will we defend Earth now?

As well as the discovery and study of thousands of NEOs, NEOWISE established the foundation of knowledge that has informed a new, dedicated planetary defence mission. NASA’s NEO Surveyor will be launched in 2027.

NEO Surveyor’s goal is to discover approximately two thirds of all NEOs larger than 140 metres in diameter, over a five year baseline survey. This is a big step toward fulfilling the mandate United States Congress has provided to NASA: to discover 90% of all NEOs in this size range.

If they hit Earth, asteroids of this size could cause mass casualties if the impact were over a large metropolitan region.

You might think this poses a bit of a risk – shutting down NEOWISE three years before launching NEO Surveyor. What happens if one of these big asteroids comes our way in the next few years?

The risks are very small, as estimates show asteroids 140 metres in diameter impact Earth only approximately every 20,000 years. So, we would have to be extremely unlucky to have one in any given three-year period, especially impacting a place that would cause a large amount of damage. Only around 3% of Earth’s surface is occupied by urban areas.

NASA doesn’t really have much of a choice with the end of NEOWISE. The Sun’s 11-year activity cycle is picking up and causing Earth’s upper atmosphere, the ionosphere, to thicken. NEOWISE is flying through this ionosphere and can’t raise its orbit, so the ionosphere is inevitably dragging NEOWISE back to Earth.

NEO Surveyor started construction in 2023, so a 2027 launch is pretty impressively rapid, which is minimising the gap between NEOWISE and NEO Surveyor.

NEOWISE is scheduled to enter Earth’s atmosphere before the end of the year, but we don’t know precisely when.

Weighing almost 700kg, some of NEOWISE itself is likely to impact the surface of Earth. Hopefully it stays away from populated areas in the process – some recent re-entry events have resulted in space debris falling reasonably close to populated areas.

An asteroid is coming! What next?

Knowing about an asteroid on a collision course with Earth is one thing. Doing something about it is another thing altogether.

Huge steps toward planetary defence occurred two years ago, when the DART mission flew to an asteroid, impacted it, and changed its trajectory. This demonstrated it’s possible to change the course of asteroids, which could be used in the future to protect Earth from a collision.

Predicting potential Earth impacts as far as possible in advance, years preferably, gives the DART-style technology approach a chance.

The pioneering work of NEOWISE, and the upcoming comprehensive observations of NEO Surveyor, will place an enormous amount of information in our scientific bank, which will never go out of date and will be the basis for planetary defence for perhaps hundreds of years into the future.

Steven Tingay, John Curtin Distinguished Professor (Radio Astronomy), Curtin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Eyes in Space - Canada's sentinel in the sky

Canada's NEOSSat

The surprise explosion of a meteor over Russia on 15th February this year, again served as a reminder of the vulnerability of the planet and its' inhabitants to contact with near-earth objects. The meteor was undetected and estimated to be approx 17 metres wide delivering a 470-kiloton blast with a shock wave that damaged 3,000 buildings and injured over 1,500 people in the Chelyabinsk region of Russia.

Canada has added to the relatively small number of assets available for space monitoring with the launch on 25 February 2013 of the Near-Earth Object Surveillance Satellite (NEOSSat). The suitcase-sized NEOSSat orbits approximately 800 kilometres above the Earth, searching for near-Earth asteroids that are difficult to see using ground-based optical telescopes. NEOSSat is not limited by the day-night cycle but operates round the clock. It circles the globe every 100 minutes, scanning space near the Sun to pinpoint asteroids that have proximity to the Earth. Despite the additional surveillance there remains a significant number of unseen objects as shown below. 

Size in metres	Unknown %
1,000 upwards	7%
150-999	67%
40-149	99%

Asteroid collisions - size doesn't matter after all

The idea that asteroid collisions with Earth pose any more a direct threat than science fiction films from Hollywood would portray ('Deep Impact' ) is a common belief not readily shared by many astronomers. This conventional and common understanding is grounded in a perception of a giant asteroid hurtling to earth in an event similar to the extinction of the dinosaurs. In reality, much smaller and more readily Near Earth Objects (NEO) which traverse similar orbits can, in fact, do extraordinary damage without actually striking the planet.

This was demonstrated all too effectively by the Tunguska event where a very powerful explosion occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, in the early morning on June 30,1908. It is estimated the asteroid,measuring 70 meters in diameter entered Earth's atmosphere travelling at a speed of approx 33,500 miles per hour and during its rapid descent, the space object heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and explode, producing a fireball and releasing energy equivalent to about 185 Hiroshima atomic bombs (NASA). 2,000 square kilometers of forest was destroyed. The above ground explosion is in fact referred to as an ‘air burst’ and constitutes the more likely threat to the Earth from asteroids. The Tunguska asteroid is now considered to have been much smaller than first estimates and was probably 30-50 meters in diameter. Measurement of sizes of asteroids suggests that even relatively small objects with the correct mass, trajectory and speed would be able to create a downward directed blast from an air burst with devastating results.The images in this post show the effects of the 1908 blast, many years after it occurred.

Scanning the Heavens

The recent exercise in December 2008 by the US Air Force to assess the ability to cope with a collision with a Near Earth Object again highlighted how limited is the Earth's capability to detect or prevent such an event taking place. Recently however a Near Earth Object did indeed come into contact with the Earth in October 2008. Designated 2008 TC3 this car sized object exploded over the Sudan but was only spotted by a telescope (the Catalina Sky Survey) with 20 hours notice and at a distance of 500,000 kilometres from our planet. Although the likelihood of a catastrophic asteroid collision is extremely rare, the potential for a strike from a smaller but still destructive near earth body has much greater plausibility.

This situation again raises the need for a greater overall surveillance of the space around Earth and in relation to the Earth's orbit around the Sun. The proposal to increase surveillance through the Panoramic Survey telescope and Rapid Response System (Pan-STARRS) as well as the proposed Large Synoptic Survey Telescope in Chile (providing the project is completed by 2015) will complement the existing monitoring programs. The next question is what action to take if a large body is located on a trajectory to Earth.

Asteroids and the threat to Earth

The question of what action to take when there is the possibility of an asteroid collision with the Earth periodically surfaces from time to time in science and astronomy circles. With impact craters clearly visible on most continents (and most recently discovered in the Yukatan Peninsula in the Gulf of Mexico) and the theory that an asteroid strike led to the demise of the dinosaur age (at the end of the Cretaceous period), this matter is one which won't disappear. The options of how to prevent a collision are not numerous but in practical terms only a handful have any chance of success. Asteroid Ida (pictured) is 35 klms in length and is located in the asteroid belt and does not cross Earth's orbit but represents the size and scale of many of these bodies floating in relatively nearby space.

One option often canvassed is to actually destroy an asteroid through the detonation of a nuclear device below the surface however the most obvious pitfall is without knowing precisely the composition and density of the material, the only result may be to splinter the asteroid and create additional possible impacts on the Earth. More favourably considered are strategies of 'nudging' an asteroid away using nuclear explosions in near proximity or alternatively a concentrated range of laser beams using solar power from reflected rays of the sun with the same effect.

Whatever the possible solutions may be, the principal concern is to be able to actually detect an asteroid before it's too close. In that respect space surveillance programs have fallen on hard times and its more luck than intention when an asteroid is located travelling nearby.