Question

Near Earth Objects (NEOs) is anything in space that comes too close to Earth (within 1.3 AU) and is sufficiently large. Describe two challenges astronomers face in finding NEOs.

Additionally, scientist distinguish between potentially hazardous objects (140m or larger) and hazardous objects (larger than 1km). What would happen to Earth if an object greater than 1km collided with the planet?

What would happen to Earth if a potentially hazardous object collided with the planet?

Answer 1

Observations to determine size of NEOs are rarely made, because they require specialized techniques such as radar or thermal infrared radiometry; rather, the size of an NEO is estimated from its brightness. Sizes estimated this way are uncertain by about a factor of 2 to 4. In most cases, sufficient observation of an object will establish that the chances of its colliding with Earth are negligible. This happens when the object is small and discovered while passing very close to Earth; it quickly becomes too faint to observe further. Even a larger and more distant object can be lost because of poor weather. Without the observations needed to compute a reliable orbit, prediction of the object’s future close approaches to Earth is highly uncertain.

On the other hand, the orbit of the NEOs are uncertain because it may change the path due to small change in gravitational force due to satellite and other NEOs, etc. So, you need continuous observation of a NEO to determine the path and expected entry into the Earth´s orbit. There are several thousands of NEOs of different size and shape. It is difficult to track all these NEOs to predict the impact on Earth.

The energy released by an impactor depends on diameter, density, velocity, and angle. The diameter of most near-Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of two based on the asteroid brightness. The density is generally assumed because the diameter and mass are also generally estimates. Due to Earth's escape velocity, the minimum impact velocity is 11 km/s with asteroid impacts averaging around 17 km/s on the Earth. The most probable impact angle is 45 degrees. The more energy is released, the more damage is likely to occur on the ground due to the environmental effects triggered by the impact. Such effects can be shock waves, heat radiation, the formation of craters with associated earthquakes, and tsunamis if water bodies are hit. If you consider, the density of NEOs, ρ = 2600 kg/m³; velocity, v = 17 km/s; and an angle of 45°, then the energy released considering the kinetic energy equivalent to TNT (Approx.) will be ~40 megaton for potentially hazardous objects (140m or larger) and ~50,000 megaton for hazardous objects (larger than 1 km). You can imagine the effect compared to the kinetic energy released by the atomic bomb dropped on Hiroshima approximately 16 kilotons.