CUBESATS JOINING HERA MISSION TO ASTEROID SYSTEM
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.
“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.”
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.
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.
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.
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.
CLOSE ENCOUNTERS: PLANNING FOR EXTRA HERA FLYBY
ESA’s proposed Hera mission will already visit two asteroids: the Didymos binary pair. The Hera team hopes to boost that number by performing a flyby of another asteroid during the mission’s three-year flight.
The opportunity arises because Hera will be flying out to match Didymos’ 770-day orbit, which circles from less than 10 million km from Earth to out beyond Mars, at more than double Earth’s distance from the Sun.
In the process Hera will pass both multiple near-Earth asteroids and the inner edge of the main Asteroid Belt. Initial studies at ESA’s European Space Operations Centre have turned up dozens of candidate asteroids across different mission scenarios.
“Ideally we would like a flyby of another binary asteroid, to enable comparisons with Didymos,” explains ESA’s Hera project scientist, Michael Küppers.
“We would choose something of a different taxonomic type from the S-type asteroids like Didymos. We would also prefer a larger object: its greater size would allow us to resolve it meaningfully from further away.”
Take as an example one body researchers would like to see: the 2121 Sevastopol binary pair in the inner belt has an 8.6 km diameter main body with a 3.5 km diameter moon.
This system is a member of the poorly understood ‘Flora’ family of main belt stony asteroids, produced by a collision event a relatively recent 100 million years ago – theorised to be associated with the Chicxulub impact that killed the dinosaurs.
The next step would be to create a shortlist of targets, which could then be the subject of ground-based observations to determine more about their properties and sharpen knowledge of their orbits before Hera’s launch in late 2023.
ESA’s Rosetta comet-chaser performed two asteroid flybys as it passed through the main belt during its decade-long flight to comet 67P/Churyumov–Gerasimenko, passing the 5-km diameter diamond-shaped 2867 Šteins and the mammoth 120-km diameter 21 Lutetia.
“To make flybys happen, we have to know where our trajectory will pass relatively close to asteroids if we do nothing,” notes Michael Khan, heading Mission Analysis at ESA’s Flight Dynamics division. “Then we tweak the trajectory to make a specific difference to that distance, bringing us much closer.
“With Rosetta we had a lot of capability, because it was a large spacecraft with extra fuel in in the tanks to get the mission back on track in case something went wrong. Plus we were performing lots of gravity-assist flybys around Earth and Mars, and massaging those flybys slightly gave us a lot of freedom to manoeuvre.
“Hera is not Rosetta, however: this will be a smaller mission with a shorter cruise phase and lower performance limits. We will still try, but the constraints are such that we won’t know for certain which asteroids we could target until after Hera’s launch. It will come down to what day within Hera’s launch window that we take off, and also the precision of that take-off – it is possible that any extra fuel earmarked for asteroid flybys might be needed to fine-tune our trajectory to Didymos. But any flyby would be an excellent opportunity to boost Hera's science return even further.””
To compare the two missions, Rosetta was lorry-sized, while Hera will be the scale of a desk. But any asteroid flyby would benefit its end mission as well as offering plentiful bonus science.
Michael Küppers was also part of the Rosetta team: “These hours-long asteroid flybys were quite dramatic events, and our opportunity to try out our scientific instruments and obtain scientific results from these unknown objects, preparing for our main goal of 67P/Churyumov–Gerasimenko.”
Hera’s lead scientist Patrick Michel, CNRS Director of Research of France’s Côte d'Azur Observatory, hopes Hera would indeed achieve a flyby: “Any object would be valuable. Each time we’ve encountered a new asteroid we’ve discovered something unexpected.”
Hera, Europe’s contribution to an international planetary defence experiment, is currently under study to be presented for approval by ESA’s Space19+ Council meeting of European space ministers.