Asteroid Defense: What to do if an asteroid threatens the Earth?

Imagine an asteroid on a collision course with Earth. What should we do? In order to answer this question Nasa and Esa plan to redirect an asteroid.


Hera planetary defence mission
Hera planetary defence mission.


  • Asteroids can be dangerous to the earth.
  • There is still no plan how to fend off an asteroid.
  • The space agencies ESA and NASA want to distract an asteroid together.
  • The "AIDA" mission consists of two parts: n"DART" (NASA) should hit an asteroid, "Hera" (ESA) investigate the results.
  • To do this, European politics must first approve and finance the mission "Hera".
  • On November 27th and 28th, the meeting of the Council of Ministers, which rejected a similar mission in 2016, will meet again.


What can be done if an asteroid is heading directly to Earth and threatens to cause considerable damage? 

The dinosaurs 66 million years ago could not defend themselves - the result is known: They became extinct. Theoretically mankind has other possibilities at its disposal.

But so far there is no concrete plan for the scenario, only ideas. 

But that will change soon, if the European space organization Esa and its US counterpart Nasa are to succeed. The two space agencies are planning space missions to test for the first time whether and how the trajectory of an asteroid can be changed.

For the mission "AIDA" (Asteroid Impact & Deflection Assessment) the researchers have chosen the near-earth double asteroid (65803) Didymos. The asteroid has a diameter of about 800 meters and is orbited by a small moon. It bears the unofficial name Didymoon, has a diameter of about 170 metres and orbits the asteroid at a distance of about 1.1 kilometres.


Asteroid the size of Didymoon could destroy a city.

170 meters in diameter - an asteroid this size would be able to destroy a city on Earth. Esa experts estimate that the impact of an asteroid about 140 meters in diameter would cause economic losses of 42 billion euros. 


Hera planetary defence mission 2.
Hera planetary defence mission.


Included in this value are the numerous deaths and injuries to be expected, as well as physical damage, for example to the infrastructure. An asteroid with a diameter of one kilometre could result in economic losses of 3.24 trillion euros, according to Esa estimates (PDF).

At any time in the future a dangerous asteroid could be discovered that threatens Earth. In this case, the major space agencies want to be prepared. Several missions and large telescopes search the sky automatically for asteroids, there are lists on which the previously known dangerous asteroids are listed. 

But what to do when things really get serious? The best ideas and calculations are only ideas and calculations if you don't know how an asteroid is actually made and how it reacts.

Asteroid defence: Esa and Nasa want to do the operational test

That's why Nasa and Esa want to do the real world test together. The plan is for Nasa to send a space probe called "DART" (Double Asteroid Redirection Test) to the double asteroid Didymos in 2021.


The Asteroid Impact Mission (AIM).
The Asteroid Impact Mission (AIM).


The Double Asteroid Redirection Test (DART)

DART" should arrive there in 2022 and hit the small moon Didymoon in October while the asteroid system is relatively close to Earth. Then the European space probe "Hera" comes into play: It is supposed to be launched in 2024 and reach the two asteroids in December 2026 - at least that is the current plan. "Hera should measure the effect of the impact very accurately," explained Dr. Patrick Michel, who is involved in the mission.

Among other things, "Hera" is to examine the crater formed by the impact of the "DART" probe on the small celestial body. Hera" will also examine details of Didymos and Didymoon and, of course, the researchers will also be interested in whether it was possible to divert the asteroid at all. 

According to Michel, the mission is important for planetary defence and for science. "Simulations of asteroid diverting by impact are only as good as the knowledge we have put into them," stresses Dr. Kai Wünnemann, head of the Department of Impact and Meteorite Research at the Museum für Naturkunde in Berlin. "With "Hera" and "DART" we have the unique opportunity to test our simulations and provide additional information," continues Wünnemann.

Asteroid mission "Hera" up for debate at ESA Conference.

There is a reason why the researchers are so committed to the "Hera" mission and have even launched the "Support Hera" campaign: the Esa Conference will take place on 27 and 28 November 2019. 

About every three years, the ministers responsible for aerospace in each of the Esa member states meet to decide which missions have priority and what funds should be allocated. 

At the previous Council of Ministers conference in 2016, the ministers of the Esa's Asteroid Impact Mission (AIM) refused financial support. This mission should - just like now "Hera" - observe how "DART" hits the small asteroid and then measure the effect. But politics said no - and science had to come up with something new. The result: "Hera".

More than 1200 scientists and citizens have signed an open letter to support the "Hera" mission. "The "Hera" mission is the core of the knowledge we need to discover and ultimately divert dangerous asteroids from Earth," explains Grig Richters, co-founder of Asteroid Day, which aims to draw attention to the danger potential of asteroids.

Space research to raise awareness of the danger posed by asteroids.

Since the 2016 Council of Ministers Conference, the Space Agency has been striving to raise political awareness of the possible dangers of an asteroid impact*. "This changes the narrative," said Esa Director General Jan Woerner in an interview with spacenews.com last year. The public relations work was also expanded.

There is a mission video for which it was possible to recruit a true rock star: Queen guitarist Brian May. The former "Apollo" astronaut Rusty Schweickart also supports the "Hera" mission. Schweickart co-founded the non-governmental organization "B612 Foundation", which is dedicated to the exploration of near-earth objects and planetary defense.

Currently, there are 901 objects known as "Near-Earth Objects" (NEOs) with a diameter of at least one kilometer. A total of 21,538 near-Earth objects of all sizes are currently known (status: 21.11.2019), including the asteroid "Apophis", which was thought to be likely to hit Earth briefly in 2029. "New NEOs are now being discovered at a rate of about four per day," explains Patrick Michel. "We need a coordinated international strategy to reduce the impact of near Earth objects."

"Hera" would be the first spacecraft to visit a double asteroid.

If the politicians approve the "Hera" mission at this year's ESA Council of Ministers meeting, the entire "AIDA" mission should be a major step in this direction. At the same time, "Hera" will be the first spacecraft to visit a double asteroid. 

The mission results might become "important for the rescue of our planet" one day, as Queen guitarist Brian May explains in the Esa video. May knows what he's talking about: He is not only a rock star, but also a doctor of astrophysics.

Astronomy's never-ending story with asteroids.

The fact that asteroids have wiped out the dinosaurs is nothing new. But now we also know what happened immediately after the asteroid impact 66 billion years ago, which wiped out the dinosaurs, and it's clear that the asteroid was solely to blame for the dinosaurs' extinction.

Asteroids have a negative side - but sometimes they can have positive effects: For example, the collision of two asteroids 466 million years ago had surprisingly positive effects on Earth. The researchers are currently particularly interested in another asteroid: The asteroid (6478) Gault behaves extremely unusually.

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