Planetary Defense – What do we Know So far

Planetary Defense

Many movies have explored the innate human fear of an asteroid crashing into Earth and wiping out humanity. While these movies often have creative solutions to divert the asteroid, such as having oil miners drill into the asteroid and plant nuclear bombs to split it in two; in reality, protecting the planet is much simpler, though not easy. 

Near-Earth Objects

Near-Earth Objects

A near-Earth Object, or NEO, is a comet or asteroid that passes close to the Earth’s orbit, within 45 million kilometers. To put that in perspective, the Moon is only 384,400 km away from the Earth on average. Asteroids and comets orbit the Sun just like the planets. Some of the smaller moons of planets may be captured asteroids.

The majority of our solar system’s asteroids reside in the asteroid belt, between Mars and Jupiter. Most NEOs come from the inner part of the asteroid belt. Over their lifetimes of millions of years, their orbits have been altered by the gravitational influence of Mars and Jupiter, or by collisions with other asteroids, causing them now to pass closer to Earth.

Thankfully most NEOs are small enough that if they do enter our atmosphere they will disintegrate before reaching the surface. However, NEOs that are larger (30 – 50 meters) could cause irreputable damage when they impact the surface. 

Not all NEOs are considered to be at risk of causing a cataclysm if they impact Earth. Potentially hazardous objects (PHOs) are NEOs that are larger than 140 meters in size and whose orbits are within 7.5 million km of Earth’s orbit. 

How Often Are NEOs Close to Earth?

How Often Are NEOs Close to Earth

Several times a month astronomers detect small asteroids, a few meters in size, passing between the Earth and the Moon’s orbit. Small meteoroids hit Earth’s atmosphere every day. Some burn up in our atmosphere causing shooting stars, while others make it all the way to the ground as the size of a grain of sand, pebble, or even sometimes a small rock.

Check out our previous article to learn about the difference between asteroids, meteors, and comets. A close approach table is updated daily by the Jet Propulsion Laboratory’s Center for NEO Studies. 

Are There Any Large Asteroids That Currently Cause a Threat to Earth?

Are There Any Large Asteroids That Currently Cause a Threat to Earth

No known asteroids pose any significant risk to Earth in the next 100 years. Of the known asteroids the highest risk is 1 in 714, or 0.14% chance of impact. The Jet Propulsion Laboratory’s Center for NEO Studies also maintains a Sentry Impact Risk Table to monitor the risk of known and new NEOs. 

How Are We Protected?

How Are We Protected

Thankfully, of all natural disasters that can occur, an asteroid impact is the only one we may be able to prevent. The United Nations Office for Outer Space Affairs (UNOOSA) works to promote international cooperation for the exploration of Space.

Per UNOOSA recommendations, the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG) were established in 2014 and are important mechanisms in coordinating planetary defense. Both NASA and the ESA have programs and collaborate on missions to explore methods of deflecting asteroids. 

NASA has a few methods for diversion they are studying. One is called a gravity tracker. A space craft would rendezvous with the threatening asteroid but not land on its surface. This craft would maintain a relative optimal position and use the gravitational attraction between the asteroid and the craft to change the path of the asteroid and steer it off course. The craft could even increase its gravitational pull by increasing its mass by plucking a boulder off the asteroid’s surface. 

Another method NASA is experimenting with is kinetic impact. It is the simplest method and most technologically mature methods currently available in our defense against asteroids. This technique consists of launching a spacecraft that will crash directly into the asteroids surface at a calculated location at several km/s in speed. This will knock the asteroid off course and prevent it from colliding with the Earth. 

A method of last resort that is being explored is nuclear explosion. If the asteroid is very large or our warning time is short, launching a nuclear device is currently the most effective option. An asteroid can be most controllably and predictably deflected with a detonation of a nuclear device.

This method involves detonating a nuclear device a few hundred meters above the surface of the asteroid. The energy from the nuclear explosion is mainly X-rays which will instantly strike the surface of the asteroid. This will superheat and vaporize the material in the top layers of the asteroid and expel it from the surface. This will cause a momentum push on the asteroid and deflect it into a new trajectory, away from Earth. 

What Are the Current Missions Exploring Planetary Defense?

What Are the Current Missions Exploring Planetary Defense

NASA and ESA are collaborating on a mission called AIDA (Asteroid Impact and Deflection Assessment), to redirect a moon of a NEO. There is an asteroid named Didymos whose orbit passes near Earth every few hundred years. Didymos is 780 km in diameter and lies between the orbits of Earth and Mars, less than 10 million kilometers away from Earth at its closest point. Didymos has its own moon, making it a binary system.

This moon, Dimorphos, is 160 m in diameter and is the target of this mission. This binary asteroid system is currently at its closest point to Earth in its orbit, and it won’t be nearby again for over a hundred years. While Didymos’s orbital path is not expected to cause it to collide with Earth, because it is so close, it is classified as a PHO. 

AIDA Mission Overview

AIDA Mission Overview

The goal of this mission is to demonstrate our ability to change the trajectory of an object on collision course with Earth. 

AIDA has two stages:

Stage 1: Asteroid Impactor – NASA’s Double Asteroid Redirection Test (DART)

DART’s goal is to deflect Didymos’s moon, Dimorphos, using kinetic impact. The DART probe will impact Dimorphos at 23,760 km/hr, causing it to redirect its path. Alongside DART, will fly an Italian CubeSat called LICIACube to observe the impact. 

Stage 2: Follow-up asteroid rendezvous spacecraft – ESA’s Hera

After DART’s impact, Hera will rendezvous with Dimorphos to study the aftereffects. Hera has two briefcase-sized CubeSats that will act like drones. These CubeSats will collect detailed measurements of both physical properties of Dimorphos and Didymos, as well as DART’s impact site. Most importantly Hera will analyze Dimorphos’s new orbit as a result of the DART impact.  

What Makes this Mission So Important

What Makes this Mission So Important

While kinetic impact is not as showy as a nuclear explosion, the AIDA mission is making history. Dimorphos will be the first natural body in our solar system to have its orbit changed by humans. Stage two of the AIDA mission, Hera, will be ushering in new autonomously navigating technology. This mission is a validation mission that we can change the trajectory of a large potentially dangerous object in space.  

Mission Status

NASA launched DART in November 2021. DART successfully impacted Dimorphos on September 26th, 2022, at 7:14 pm EDT. Below are DART’s final images before impact. 

AIDA Mission status

Didymos (bottom left) and Dimorphos approximately 2.5 minutes before impact. Image Credit: NASA/Johns Hopkins APL 

large asteroid

Dimorphos 11 seconds before impact. Image Credit: NASA/Johns Hopkins APL

asteroid close up

Last complete image before impact. Image Credit: NASA/Johns Hopkins APL

DART demonstrated NASA’s ability to navigate and intentionally collide a spacecraft with an object to change its trajectory. 

Until the arrival of Hera, astronomers will observe Dimorphos with ground-based telescopes. From the ground they will be able to confirm if DART changed Dimorphos’s orbit around Didymos. Researchers expect to have changed Dimorphos’s orbit around Didymos by approximately 1%, decreasing it by about 10 minutes. 

ESA’s Hera spacecraft will launch in October of 2024. Hera will conduct a detailed survey on both Didymos and Dimorphos, with a particular focus on the crater left by DART. 

While space can seem like a very scary and unpredictable place, we are lucky to have astronomers and scientists tirelessly working to keep us and the planet safe from space calamities. 


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Cassie Hatcher

Cassie Hatcher

Cassie is a lifelong learner with a passion for communicating high level science in a conversational matter. She holds a B.S. and M.S. in physics and has written two astronomy theses, one of which is published. She earned an internship at NASA’s Goddard Space Flight Center in 2016 and got the chance to see the James Webb Space Telescope while it was being built.

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