Defending Earth From Asteroids

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The Tunguska event, it was believed to be caused by the air burst of an asteroid or comet about 50-60 meters or 160-200 ft in size, at an altitude of 5-10 kilometers or about 3-6 miles. It was estimated that the asteroid had a kinetic energy of around 15 megatons, or the equivalent to that of an explosion of 1,000 Hiroshima-type atomic bombs. It’s even been estimated that the explosion had caused a deceleration of the Earth's rotation relative to the rest of the Solar System by 4 microseconds. This explosion, caused by a near-earth asteroid about 20 meters or 66 ft in size, was estimated to have released the energy equivalent of around 500 kilotons of TNT.
The Chelyabinsk event is a reminder of the destructive power of even small asteroids, and highlights both the frequency of these events as well as the importance of the need to to identify and track these potential threats.
PHO
PHO’s are defined as near earth objects, such as an asteroid or a comet, that have an orbit which approaches the earth at a distance of 0.05 astronomical units or 19.5 lunar distances, or less. 85% these asteroids are known as Apollo asteroids, as they hold an orbit that keeps within the inner solar system.
DETECTING POTENTIAL THREAT OBJECTS
They scan the sky slowly, on the order of once a month but produce deeper, more highly resolved data. Warning surveys, in contrast, utilize smaller telescopes to rapidly scan the sky for smaller asteroids that are within several million kilometers from earth. These dedicated survey installations first started to appear around the late 1990s and were initially clustered together in a relatively small part of the Northern Hemisphere. Initiated in 2015, This robotic astronomical survey and early warning system located in the Hawaiian islands is optimized for detecting smaller near-Earth objects a few weeks to days before they impact Earth.
Further NASA funding had brought the system to the Southern hemisphere with two additional telescopes becoming operational in early 2022 in South Africa. At present, several other southern hemisphere based surveys are also under construction. In addition to ground based surveys, the Wide-field Infrared Survey Explorer or WISE infrared telescope, in earth’s orbit, was tasked with a 4 month extension mission called NEOWISE, to search for near-earth objects using its remaining capabilities. While this initial extension occurred in 2010, NASA had reactivated the mission in 2013 for a new three-year mission to search for asteroids that could collide with Earth, and by July 2021, NASA would reactivate NEOWISE once again, with another PHO detection mission extending until June of 2023.
Currently, a replacement space-based infrared telescope survey system called the NEO Surveyor is under development with an expected deployment in 2026.
DART MISSION
DART was launched on November, 24 2021 on a dedicated Falcon 9 mission. The mission payload along with Falcon 9's second stage was placed directly on an Earth escape trajectory and into heliocentric orbit when the second stage reentered for a second escape burn. Despite DART carrying enough xenon fuel for its Ion thruster, Falcon 9 did almost all of the work, leaving the spacecraft to perform only a few trajectory-correction burns with simple chemical thrusters for most of the journey. On 27 July 2022, the DRACO camera detected the Didymos system from approximately 32 million km or 20 million mi away and began to refine its trajectory.
These captured images were transmitted in real time to earth using the RLSA communication system. A few minutes before impact DART performed final trajectory corrections. This ultimately changes the overall orbit of the asteroid system. An asteroid on a hypothetical collision course with earth would only require a path shift of 6,500 km to avoid the earth, a tiny amount relative to 10s the millions of kilometers it would travel orbiting the sun.
LICACUBE
Built to carry out observational analysis of the Didymos asteroid binary system after DART's impact, it was the first deep space mission to be developed and autonomously managed by an Italian team.
HERA - FOLLOW UP
In October 2024, the ESA will launch the Hera mission with its primary objective being the validation of the kinetic impact method to deviate a near-Earth asteroid in a colliding trajectory with Earth. Hera will fully characterize the composition and physical properties of the binary asteroid system including the sub-surface and internal structures. Hera is expected to arrive at the Didymos system in 2026.
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