Earth's first mission for planetary defense

Over the next decade, both NASA and the European Space Agency (ESA) will take the first steps toward a defensive plan against asteroids.

The solar system is full of giant asteroids hurtling around, and even though space is big and mostly empty, there's a decent chance that some of those asteroids are on a collision course with the Earth. If a large enough asteroid hits the Earth in the right spot, it could wipe out millions of people, which is why it's so important to learn how to prevent asteroid impacts.

Planning for humankind's first mission to a binary asteroid system has entered its next engineering phase. A binary asteroid is a system of two asteroids orbiting their common barycenter (the center of mass of two or more bodies that are orbiting each other). ESA's proposed Hera mission would also be Europe's contribution to an ambitious planetary defense experiment. Named for the Greek goddess of marriage, Hera would fly to the Didymos pair of Near-Earth asteroids: the 780 m-diameter mountain-sized main body is orbited by a 160 m moon, informally called 'Didymoon', about the same size as the Great Pyramid of Giza. Hera's manager Ian Carnelli explains "Such a binary asteroid system is the perfect testbed for a planetary defense experiment but is also an entirely new environment for asteroid investigations. Although binaries make up 15% of all known asteroids, they have never been explored before, and we anticipate many surprises. The extremely low-gravity environment also presents new challenges to the guidance and navigation systems. Fortunately we can count on the unique experience of ESA's Rosetta operations team which is an incredible asset for the Hera mission."

A NASA mission called the Double Asteroid Redirection Test, or DART, is due to collide with it in October 2022. The impact will lead to a change in the duration of Didymoon's orbit around the main body. Ground observatories all around the world will view the collision, but from a minimum distance of 11 million km away. By the time Hera reaches Didymos, in 2026, Didymoon will have achieved historic significance: the first object in the Solar System to have its orbit shifted by human effort in a measurable way. The smaller Didymoon is Hera's main focus: the spacecraft would perform high-resolution visual, laser and radio science mapping of the moon, which will be the smallest asteroid visited so far, to build detailed maps of its surface and interior structure. Hera will also demonstrate the ability to operate at close proximity around a low-gravity asteroid with some on-board autonomy similar in scope to a self-driving car, going on to deploy Europe's first deep-space CubeSats, and potentially also a micro-lander, to test out a new multi-point intersatellite link technology.

Hera will measure the crater left by DART to a resolution of 10 cm, accomplished through a series of daring flybys, giving insight into the surface characteristics and internal composition of the asteroid.NASA's DART mission meanwhile has passed its preliminary design review and is about to enter its 'Phase C' detailed design stage. All of this means that Hera will benefit our understanding of binary asteroids along with advancing our planetary defense technology while operating in deep space. It's a lot for one spacecraft to accomplish, but it's almost certainly up for the task.