Near Earth Objects Dangers from Space

Near Earth Objects – Dangers from space

Bernhard Hofer

From times immemorial, Earth has been in the line of cosmic bullets; and even in our time, the danger for our planet isn’t averted in any way. Asteroids and comets are still numerous and can impact any time. Additionally, too little observation facilities have been established yet and there are few models of what would happen in case of emergency.The pivotal questions are now: How devastating would be the effects of an impact of a really big object? How likely is this to happen? And are there any mitigation strategies?In this article we show the danger from space through cosmic bullets and we discuss the possible effects of an impact.Frighteningly, we had to discover, that it seems that politicians and population are underestimating the danger through asteroids because of the rareness of bigger impacts. But a view into the past and our cosmic neighborhood prove that the threat is real as well, as it has the potential of a global and existence-threatening cataclysm. Our research has shown that existing warning systems don’t suffice by far to detect possible upcoming impacts early enough and additionally there are no realistic practical chances to prevent a future impact.Furthermore, our investigation shows clearly that many research, development and information need to be done. It illustrates that it is necessary to set up international cooperation to ensure a sufficing lead time and adequate reactions to a future threatening.

Our solar system is full of small objects, so-called asteroids. While most of them are on stable orbits far away from Earth, some of them come very close or even cross the Earth orbit. These are called Near Earth Object (NEOs). Comets,which are a conglomerate of stone, dust and ice come from depths of space and are far less predictable in their orbit. Both sorts of NEOs can be so called Potentially Hazardous Asteroids (PHAs), Objects that can potentially impact on Earth.We will show which classes of NEOswe know and were most of them are located in our solar system. Astronomers try to find

as much NEOs as possible and calculate their orbits, so they make sure none of them impacts on Earth unpredicted. But because most of these cosmic objectsare very small and dark, they are very hard to detect, so actually very big telescopes would be needed to find almost all of them. Unfortunately, politicians don’t give them the funding to do so. Therefore it remains the questions how many NEOs there are in our solar system and how likely they will impact on Earth. Additionally, it remains unclear, what the effects of an impact would really be, sowe tried to make some predictions what might happen, depending on our current knowledge. Finally, we asked the question if we can actually effectively react if we become the target of a PHA. We validated the practicability of large-scaled evacuations and even mitigation strategies. What can we do depending on the scale of the object and in which way do the properties of the asteroid affect our possibilities? Is an atomic bomb really capable of saving us from an asteroid?In order to figure this points out we investigated previous impacts in history of Earth. Additionally we looked at the results of missions to investigate asteroids, like the “Deep Impact” mission of NASA in 2005. Our article will be structured in 4 Sections:Sec. (1) will present the different classes of NEOs and were they are located. In Sec. (2) we will give an overview over the existing observation facilities and the methods of finding NEOs. Sec. (3) will focus on the effects of an impact and the likelihood they will happen. Finally in Sec. (4) we will show what we need to know to avoid an impact and how convertible mitigation strategies can be.

Sec. (1): NEOs

There are 2 main types of NEOs:

Asteroids are small objects orbiting the Sun. The larger ones are called planetoids. Asteroids are too small to assume a spherical form;their diameters vary from a few meters to more than 100 km. In comparison to comets they are not outgasing near Sun.The biggest accumulations of Asteroids are the Main belt between Mars and Jupiter and the Kuiper belt beyond Neptune.

Comets are small icy objects of average 10-20 km diameter that heat up when they come close to Sun, begin to outgas and displaying a visible atmosphere called “coma” or even a tail. Their main contents are rock, dust, water ice, and frozen gases such as methane, ammoniac, cyan and other hydrocarbons [1]. Therefore, American astronomer Fred Whipple called them “dirty snowballs”. Most comets orbit sun in widely elongated elliptic, almost parabolic, orbits. Because of this and the fact that they lose material every time they come close to Sun, they have to come from far away from the center of the solar system, so astronomer Jan Hendrik Oort assumed the hypothetic Oort cloud in about 1,6 Light-years distance as their origin[2].

Sec. (2): Detection and tracking

In astronomy, images of the starry sky are most of the times made with CCD-cameras (Charged Coupled Devices). That’s a checkered semiconductor-oxide interface converting incoming photons into electron charges. The advantage of this method is that no dark room is needed, as the image is directly processed by the computer, so astronomers can save a lot of money. With long exposure time, imaging the same spot of the sky, asteroids can be detected, being noticed as stripes on the image, while the fixed stars remain points. With further observation, the orbit of the Objects can be calculated then. But unfortunately, telescopes can’t observe the whole sky, as looking in the direction of the Sun makes it impossible to detect any objects there, because they are outshined by the star. Comets are very tricky to find, because they have such elongated orbits that they seem to come from the depths of space and are very dark before coming near Sun and therefore they often remain undetected before they are very close. If they even come from the direction of the sun, they are nearly undetectable for astronomers.

In 1998, US-congress charged the NASA to detect 90% of all NEOs with more than 1 km diameter until 2008 [3]. For this purpose, several telescopes have been installed like the CSS, LINEAR and Spacewatch. Still, these were not enough to fulfill this goal and additionally, smaller NEOs are extremely dangerous as well and even harder to detect. While not only for detecting NEOs, in 2009, the Wide-Field Infrared Survey Explorer (WISE) was send to space. NASA funded an additional mission to help ground based telescopes in detecting NEOs and measuring their diameter. Because WISE detection methods are infrared based, it is better for finding low reflecting objects than ground based telescopes. Currently the NASA is searching for NEOs with 140m diameter or more until 2020 [3], but larger and unfortunately more expensive telescopes would be needed to detect most of them.

Sec. (3): Danger through NEOs

Thankfully there wasn’t an apocalyptic impact in human history yet, therefore it is not easy to suggest what exactly would happen. So we tried to figure it out, looking at previous impacts in Earth history. The most famous impact was the asteroid/comet that is said to have erased the dinosaurs. Because of the tracks in geological layers and the according 200km diameter Chicxulub impact crater that was found in Mexico, Yucatán, experts can estimate the effects of the impact[3]. The asteroid was estimated to ~ 10km in diameter and the impact energy would have been as much as millions of atomic bombs. Earthquakes with magnitude of around 10 raged near the impact region and because the impact was near to the ocean, mountain-high tsunamis flooded all coasts of the planet[4]. Even worse, the raised dust darkened Earth for many years and poisoned gases in the atmosphere lead to sour rain. As a result, there was a mass extinction of plants which lead to a mass extinction of animals as well. These frightening effects are the result of the big mass of asteroids and the high average velocity of 42 km/s. That leads to a huge momentum and energy. Experts are not sure how big an asteroid has to be to lead to global climatically effects, but most of them estimate a size of around 1 km. It is estimated that such an impact occurs once in around 100.000 years. An impact like in Yucatán 65 million years ago is expected once in 100 million years.

But even smaller object can lead to devastating impacts. 1908 in Tunguska, an asteroid of just a few ten meters diameter exploded a few kilometers over the Siberian tundra. The stenght of the explosion was between 10-15 megatons of TNT, in comparison: the Hiroshima bomb of 1945 had only 0,015 megatons of TNT[5]. An asteroid like that can easily wipe out a big city like New York and is estimated to impact on Earth once in about 1000 years. The good news is that most of the times these objects will impact somewhere in wilderness or the ocean.

Fig. 1: Danger-model by Falko Langenhorst (2002)[6]

Sec. (4): Mitigation strategies[3]

If an asteroid aiming towards Earth is detected, it is very important to know of which material they are made of, what their shape is and how dense they are to react in an effective way. For this propose, astronomers can use radio telescopes to make 3 dimensional models of the object. But to find out density and material they are made of, space missions to the asteroid must be done. Therefore, the NASA launched the “Deep Impact” mission[7]: In 2005, a space probe impacted on comet temple 1 with a velocity of 10,2 km/s while observed by a second probe. About 15.000 tons material was ejected from the comet and an impact crater with 100m diameter was left on the surface. Therefore, the outer layers of the comet are made out of very loose material. A similar mission of the ESA called “Don Quijote” is planned in order to test if the impact of a space probe on an asteroid can change its orbit.

Assuming we got all data needed from the NEO, what mitigation strategies have been developed yet and how effective can they be? If the asteroid isn’t too big and if a warning time of a few decades is provided, “slow push-pull methods” are a good choice. For example it would be possible to send a spacecraft to the asteroid, pulling the object with his gravitation a little bit over a long time and changing its orbit just enough not to hit Earth. The advantage of this method would be that no contact to the asteroid is needed and it would be very precise, but unfortunately it can only work with small asteroids and need a lot of time, which is not really realistic. A more realistic scenario would be a warning time of a few years or even months. Depending of the size and composition of the asteroid,launching several small probes to impact on the asteroid can be an option or even an atomic bomb. To the latter need to be said, that an atomic bomb hasn’t the same effect in space as it has on earth, because there is no atmosphere, so only the radiation can transfer momentum to the asteroid. Additionally, if the asteroid is very dense, material can be ejected or it can burst entirely. Then a swarm of small asteroids would hit Earth which would have an effect similar of a shotgun and be even worse than one big bullet. If the asteroid is too less dense, it would possibly consume too less momentum and still hit Earth. That’s why it is so important to know the composition of the NEO. Still, an atomic bomb would be the only possibly effective way to mitigate a bigger asteroid in short time, even though it has only been tested on computer simulations yet, so there remains uncertainty regarding practicability.

In any case we came to the conclusion that humanity only has a chance, if effective observation can provide enough warning time and most important, all nations work together. If an emergency occurs, there will be no time for disputing who is going to lead and who is going to pay, so these questions should be answered before. We believe that a permanent international committee should be established that takes responsibility when an asteroid is detected and decides what to do. Furthermore all information about NEOs has to be shared to all astronomers in the world and politicians should consider a higher budget to provide better observations facilities, maybe an international “NEO-observation budget” by the UN could solve the financial problems so no nations has to pay more than the others.Cause we shouldn’t forget the following: We all live on the same planet!

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