7 January 2019

When ESA’s planned Hera mission journeys to its target binary asteroid system, it will not be alone. The spacecraft will carry two tiny CubeSats for deployment around – and eventual landing on – the Didymos asteroids. Each companion spacecraft will be small enough to fit inside a briefcase, as compared to the desk-sized Hera.

CubeSats are nanosatellites based on standardised 10 cm-sized units. Hera has room to deliver two ‘six-unit’ CubeSat missions to the Didymos asteroid system – a 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.

The Hera mission received proposals for CubeSats from across Europe, and an evaluation board has now made the final selection.

CubeSat shot of Mars

“We’re very happy to have these high-quality CubeSat missions join us to perform additional bonus science alongside their Hera mothership,” explains Hera manager Ian Carnelli.

“Carrying added instruments and venturing much closer to our target bodies, they will give different perspectives and complementary investigations on this exotic binary asteroid. They will also give us valuable experience of close proximity operations relayed by the Hera mothercraft in extreme low-gravity conditions. This will be very valuable to many future missions.”

APEX CubeSat

Paolo Martino, Hera spacecraft lead engineer adds: “The idea of building CubeSats for deep space is relatively new, but was recently validated by NASA’s InSight landing on Mars last November, when a pair of accompanying CubeSats succeeded in relaying the lander’s radio signals back to Earth – as well as returning imagery of the Red Planet.”

The first CubeSat companion is called the Asteroid Prospection Explorer (or ‘APEX’), and was developed by a Swedish/Finnish/Czech/German consortium. It will perform detailed spectral measurements of both asteroids’ surfaces – measuring the sunlight reflected by Didymos and breaking down its various colours to discover how these asteroids have interacted with the space environment, pinpointing any differences in composition between the two. In addition, APEX will make magnetic readings that will give insight into their interior structure of these bodies.

Juventas CubeSat

Guided by a navigation camera and a ‘laser radar’ (lidar) instrument, APEX will also make a landing on one of the asteroids, gathering valuable data in the process using inertial sensors, and going on to perform close-up observations of the asteroid’s surface material.

The other CubeSat is called Juventas, developed by Danish company GomSpace and GMV in Romania, and will measure the gravity field as well as the internal structure of the smaller of the two Didymos asteroids.

Didymos binary asteroids

In close orbit around Didymoon, Juventas will line up with Hera to perform satellite-to-satellite radio-science experiments and carry out a low-frequency radar survey of the asteroid interior, similar to performing a detailed ‘X-ray scan’ of Didymoon to unveil its interior. The adventure will end with a landing, using the dynamics of any likely bouncing to capture details of the asteroid’s surface material – followed by several days of surface operations.

Hera is set to be humankind’s first mission to a binary asteroid system. As well as testing technologies in deep space and gathering crucial science data, Hera is designed to be Europe’s contribution to an international planetary defence effort: it would survey the crater and measure orbital deviation of Didymoon caused  by the earlier collision of a NASA probe, called DART. This unique experiment will validate the asteroid deflection technique referred to as kinetic impactor, enabling humankind to protect our planet from asteroid impacts.

DART mission profile

Next, the two CubeSats will have their designs refined and interfaces with their mothership finalised, in line with continuing design work on the Hera mission itself, which will be presented to ESA’s Space19+  meeting towards the end of this year, where Europe’s space ministers will take a final decision on flying the mission.