Space telescope spots thousands of hazardous asteroids
2021 Over a period of four years, NASA’s Near-Earth Object Camera (NEOCam) is to identify two thirds of all asteroids and comets on a collision course with Earth. The telescope will be placed between Earth and the Sun.
The pastel morning sky lights up, and a few seconds later, a deafening bang follows. It is slightly past nine o’clock in the morning on 15 February 2013, when a some 20-m-long asteroid explodes in the sky above the Russian city of Chelyabinsk on the frontier between the Ural Mountains and Siberia. The energy discharge injures 1,600 people, damages 3,600 buildings, and activates car alarms.
The asteroid struck Earth without being identified by one single telescope. The fear of an unknown asteroid or comet measuring up to several km one day striking one of the major cities of the US have made the American space agency NASA introduce a 7 stage strategy, aiming to prepare scientists and authorities for identifying and destroying threatening asteroids and comets – and at worst alleviating the consequences of an impact.
EARTH CLOSE TO COLLISION THIS YEAR
NASA is continuously watching out for NearEarth Objects (NEOs) via telescopes on Earth and in space. Whereas the vast majority of very large asteroids with diameters of 1+ km have been identified, scientists estimate that about 300,000 unknown NEOs of 40+ m exist, which could make up a threat.
The 37-m-long 2000 SG344 asteroid is an example of how much damage even small asteroids can cause. The asteroid is one of those which are monitored by NASA, because it might strike Earth. If that were to happen one day, the force would be the equivalent of about 70 Hiroshima bombs. The effect of an impact depends on how the asteroid explodes. The Chelyabinsk
asteroid was porous, causing it to burst when it entered the atmosphere, which functions as a brake. Known as an air burst, this type of impact produces a shock wave, which can be more destructive than when an asteroid strikes Earth’s surface.
In January 2017, Earth only just avoided a collision with the 2017 AG13 asteroid, which passed by us at a speed of about 58,000 km/h at a distance of less than half the distance between Earth and the Moon. The asteroid, which is estimated to have been up to 35 m long, was spotted by telescopes only two days before passing close by our world.
The threat came from the Asteroid Belt between Mars and Jupiter. The about 180-million-km-wide belt – i.e. slightly more than the distance from the Sun to Earth – consists of hundreds of thousands of known rocks of all sizes. The actual number is estimated to be much higher, however. Just like planets, the rocks are orbiting the Sun, but the gravitational pull of planets’ elliptical orbits around the Sun sometimes push some of the rocks into paths which bring them close to Earth – known as Near-Earth Asteroids.
The comets, which mainly consist of ice, come from a place even further away. Some of them derive from the Kuiper Belt, which also includes the dwarf planet of Pluto, whereas others come from the Oort Cloud – a huge cloud of ice lumps, which is believed to surround all planets. Seen from our own perspective, comets and asteroids become potentially hazardous objects (PHOs), when they are about 7.5 million km from Earth’s orbit. SMALL ASTEROIDS ARE VERY HARMFUL In 2005, the US authorities ordered NASA to identify 90 % of all Near-Earth Asteroids measuring 140+ m before 2020. Asteroids of this size are easier to detect and keep an eye on than the smaller ones and have such an impact that they could destroy entire cities and regions. But in 2017, the size of the objects will probably be adjusted – not least because the only 20-m-long asteroid, which struck Chelyabinsk, demonstrated that much smaller objects can also cause severe damage.
So far, astronomers have only identified 28 % of the asteroids and comets of 140+ m, which probably exist in the Solar System, but NASA aims to launch a new infrared space telescope, the Near-Earth Object Camera (NEOCam), which is going to find more of the hidden killers orbiting the Sun from 2021.
It may seem as if Earth is under constant bombardment, but luckily, major impacts causing severe damage are relatively rare. The largest recorded impact in recent history was in 1908, when an asteroid of an estimated length of 40 m exploded in the atmosphere above the Tunguska River in Siberia, destroying a forest the size of Tokyo.
This type of impact statistically only occurs once every millennium, but Earth’s surface is ripe with scars caused by much more extensive prehistoric impacts. 190 craters have been recorded in the Earth Impact Database. The most destructive impacts, such as the one which may have caused the dinosaurs to go extinct, take place at intervals of averagely 100 million years. COLLISIONS WITHOUT WARNING Asteroids and comets strike with very different warnings. Some collisions can in principle be predicted centuries before they occur, whereas others happen at a few days’ or no notice, and consequently, an important element of NASA’s strategy is to develop and
test methods for bending or destroying the threats, before they strike.
In the early 2020s, NASA hopes to send a spacecraft to an asteroid to collect a fragment of it. The Asteroid Redirect Mission will, moreover, try to bend the asteroid’s path by entering into an orbit beside it for months, using its gravitational pull to push it. This will be one of the first experiments with moving an asteroid or comet away from its path.
ASTEROID WEAPONS ON STANDBY
If NASA learned about an asteroid heading directly towards Earth today, it would take up to five years to prepare and launch a craft to try to prevent the collision. In many cases, that would be too long, and so, NASA recommends that the preparation time be limited by always having the necessary weapons on standby.
In some cases, asteroids and comets however strike so suddenly that we cannot prevent it. If the authorities only have a few days or weeks, it is a question of calculating the impact site so accurately that all affected people can be evacuated in time. Hundreds of thousands or millions of wounded and displaced people reSquire emergency hospitals and temporary living quarters of an unknown scale, and infrastructure such as electricity grids and roads must be reconstructed as quickly as possible. Such major operations require different national and global emergency management than what we have today.
“It is highly unlikely that we will see an impact of the kind which destroys civilizations in the next couple of centuries, but the risk of smaller, but still disastrous impacts is a real one, and we do not have a complete strategy for handling such an event at this point,” NASA says in its strategy plan.
The introduction of the seven stages is hence only the first step on the way towards avoiding the feared scenario. Although Earth has been spared in recent history, events elsewhere in the universe demonstrate that the risk is still a real one.
As recently as in 1994, fragments of the Shoemaker-Levy 9 comet measuring up to 2 km hit Jupiter, causing an energy discharge corresponding to about 400 million Hiroshima bombs. Perhaps Earth is the next one to come under attack.