Explained: Why NASA and ESA want to hit an asteroid called Didymos

european space agency, hera, aida, dart, nasa, asteroid collision, planetary defence
Scientists are trying to find ways to deflect asteroids from a collision course with Earth. (Image: Artist’s concept of Hera, source: ESA)

The European Space Agency (ESA) has approved the budget of Hera, the European component of the mission to slam a spacecraft into an asteroid. The project aims to study the effectiveness of an impact to ward off an impending asteroid threat.

Amidst the growing concern of need of a planetary defence mechanism, scientists are studying asteroids and trying to find ways to deflect them from a collision course with Earth. One such project is the Asteroid Impact and Deflection Assessment (AIDA), which includes NASA's Double Asteroid Redirection Test (DART) mission and the European Space Agency's (ESA) Hera.

Why we need a planetary defence mechanism

There are around 25,000 near-Earth objects (NEOs) that orbit the Sun on a trajectory that brings them close to our planet's orbit. NASA tracks such near-Earth objects to ensure they do not become threats. However, certain near-Earth objects have been classified as "potentially hazardous" which are 140 metres or more in size and come within 0.05 AU (astronomical unit) to Earth.

The distance in space is usually measured in astronomical units where 1 AU is the distance between Earth and the Sun, which is around 93 million miles or 150 million kilometres.

european space agency, hera, aida, dart, nasa, asteroid collision, planetary defence
Schematic of the DART mission shows the impact on the moonlet of asteroid Didymos. (Image: NASA/Johns Hopkins Applied Physics Lab)

According to NASA JPL’s Centre for NEO studies, as of now, there are about 900 near-Earth objects measuring more than 1 km. An impact from one of these NEOs can bring devastating effects to Earth. That is why scientists are working on a number of planetary protection initiatives to deflect asteroids if they threaten to impact the Earth. However, the Asteroid Impact and Deflection Assessment (AIDA) is the most drastic measure of all times.

Why AIDA is aiming for Didymos?

The twin-asteroid system Didymos is a binary near-Earth asteroid. According to NASA, while the primary body of Didymos is approximately 780 meters across, its secondary body or "moonlet" is about 160-meters in size, which is more typical of the size of asteroids that could pose the most likely significant threat to Earth. So, Didymos makes a suitable target for NASA and ESA's mission.

european space agency, hera, aida, dart, nasa, asteroid collision, planetary defence
Simulated image of the Didymos system, derived from photometric lightcurve and radar data. (Image: Naidu et al., AIDA Workshop, 2016)

DART and Hera's mission

Last year, NASA announced that it had started the construction of DART, their component of the AIDA mission. DART is scheduled to launch in 2021 with an aim to slam into the smaller asteroid of the Didymos system at around 6 km per second in 2022.


Hera will arrive at the Didymos system in 2027 to measure the impact crater produced by the DART collision and study the change in the asteroid's orbital trajectory. ESA was supposed to construct the complementary mission Asteroid Impact Mission (AIM) to study Didymos prior to DART's collision, but the mission was scrapped and ESA came up with an alternative mission Hera, which is scheduled to launch in 2024.

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