How the HERA mission is validating planetary defence

How the HERA mission is validating planetary defence

SciTech Europa caught up with the European Space Agency’s Dr Ian Carnelli to discuss what progress has been made in the HERA mission preparations thus far, and what the next stages will be.

The HERA mission – named after the Greek goddess of marriage – is an ESA mission that will be humankind’s first probe to rendezvous with a binary asteroid system, a little understood class making up around 15% of all known asteroids. It is the European contribution to an international double-spacecraft mission. NASA will first perform a kinetic impact on the smaller of the two bodies with its DART mission, and then the HERA mission will follow-up with a detailed post-impact survey that will turn this grand-scale experiment into a well-understood and repeatable planetary defence technique.

Due to launch in 2023, Hera would travel to a binary asteroid system – the Didymos pair of near-Earth asteroids. The 780m-diameter mountain-sized main body is orbited by a 160m moon, informally called ‘Didymoon’, about the same size as the Great Pyramid of Giza. This latter object is the focus of the HERA mission: the spacecraft will 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.

The HERA mission has recently been given the go-ahead to progress to detailed design and so SciTech Europa caught up with the European Space Agency’s Dr Ian Carnelli to discuss what progress has been made in the Hera mission preparations thus far, and what the next stages will be.

What makes the Didymos pair of Near-Earth asteroids perfect for a planetary defence experiment and as it is an entirely new environment for asteroid investigations, what challenges – and potential discoveries – are involved here?

There are many reasons for this. The first is perhaps that Didymoon is the exact size of what we see as being the most probable threat. That is, asteroids which measure 100-150m are the ones that we consider to be the most dangerous. That is because there are lots of them, but we don’t know just how many or where they are – this is opposed to the asteroids which measure a kilometre or larger, for which we know of around 90%. As such, Didymoon is representative of the asteroids we see as presenting the biggest threat to the Earth and is therefore where we are focusing our efforts. Didymoon will also enable us to validate DART’s terminal guidance system on a realistic scenario.

Didymos and Didymoon do indeed present us with a wholly new environment; we have never visited a binary asteroid before, and this continuously presents us with new and evolving challenges. For instance, Didymos’ gravity shield keeps changing because of the rotating moon, and this is something that is completely new for the HERA team members involved in the asteroid proximity navigation aspects of the mission.

The mission will certainly provide us with new discoveries. For example, several theories have been put forward to potentially explain how such binary systems form. The most probable of these posits that the primary (so Didymos, in this instance) experiences an increase rotation speed due to thermal effects. This causes material on the surface of the poles to begin moving towards the equator, where some of it is ejected before re-accumulating and forming a moon. Another theory suggests a collision could have caused the binary system. Nevertheless, only by actually travelling to the system will scientists actually be able to explain what happens.

Such processes also have wider implications with regard to the formation process of the solar system, meaning that there is a lot of scientific interest in this mission and that it promises to help answer some important questions.

One of the main challenges we will experience in sending the HERA spacecraft to the Didymos pair of Near-Earth asteroids will concern the navigation. This is due to the potential ejection of materials from around the equator as we are unsure what we will find. Indeed, our scientists have run simulations which have shown that some of the boulders in the area could actually be lifting up and falling back down again, so that is something we will have to watch out for.

The thermal radiation environment will also present a challenge. We will have to model that well because it has the potential to impact on the spacecraft we approach the surface – it could heat the spacecraft in locations that have not been designed to withstand it.
What challenges were there in ensuring that HERA was selected to progress to flight?
HERA is an optional programme, which means we have had to convince enough ESA Member States to reach a critical mass.

In the previously proposed mission, AIM, we had asked for €108m to cover industrial costs and formally approve the programme and so move forward; we reached €70m before the money was withdrawn. However, this demonstrated that there was interest in what we wanted to do, just not enough to get it started.

With HERA we are therefore working to ensure that all of the countries involved in ESA understand exactly how their industries and science teams can benefit from the mission. We also want to make it absolutely clear that this is a unique opportunity for them, not least because due to the fact that NASA is funding the DART mission, the experiments we will run with HERA will cost taxpayers just half of what it would normally be to validate this technique.

Alongside this, ESA has now established the Planetary Defence Programme within its Space Safety pillar. This is a specific programme that will act as a long-term roadmap and which thus contains additional activities. The fact that HERA is now a part of a big programme that will be present in ESA’s activities for some time, has also helped us to gain the support of the Member States.

The HERA mission has now entered its next engineering phase. What will this involve?

Yes, we are now in Phase B1, which is the detailed design phase. We have received about €7m for this phase which will enable us to design the system and sub-system of the spacecraft, as well as to identify all the equipment that we need to put into it.

In addition, we are also running some technology activities. Here, we are now manufacturing the on-board computer for the spacecraft. This will not be the flight model, however; it is cheaper version which we will use to test the software before placing it into radiation and thermal testing. While the computer is being built in Belgium, it will be shipped to Madrid, where the Guidance, Navigation and Control (GNC) team are based, and they will plug in the GNC software, as well as the navigation cameras, and put all of this in a lab with a mock-up of the asteroid before using robotic arms (which are controlled by the software that we are currently prototyping) to simulate the motion of Didymoon. This is a means to validate the whole chain of navigation and control of the spacecraft, and will also enable us to better understand how to move around this binary asteroid.

In parallel, we are also building engineering models the payloads that we don’t have already as flight spares. This includes the laser altimeter, and we are about to build the hyperspectral thermal camera and are in the detailed design phase of the two CubeSat’s. We are also building some engineering models for the more critical components of the CubeSat’s, such as the radar tomographer, and the deployment system for the antennas.
All of these activities will reassure our Member States that the critical technologies are all being taken into account and that we are trying to de-risk any all elements as far as possible.

What has been learned from the Hayabusa2 mission?

We have learned many things from this. Every mission to a small body brings with it completely new discoveries. With Hayabusa2, it revealed, for instance, that near-Earth asteroid 162173 Ryugu had no regolith. Rather, it had a very rocky surface with no loose material; it was also very dark. I don’t think anyone expected those things.

That has taught us that we need to expand the parameter range for Didymos and Didymoon’s properties so as to ensure that we are designing to the worst case scenario.
We are also learning from JAXA with regard to the operational procedures they have used. That is, they are using some trajectories that could be quite relevant for us, particular at the end of life, when we want to land on the primary, Didymos. Indeed, HERA’s trajectories at this time will be similar to those used by Rosetta at comet 67P/Churyumov–Gerasimenko, in that we will use arc trajectories that are intrinsically safe so that if anything goes wrong on the spacecraft it is not on a collision course.

As a sampling mission, Hayabusa2 will need to get close to the surface of 162173 Ryugu in order to deploy its rovers. From that perspective, HERA is a simpler and cheaper mission. But we can nevertheless learn from what they are doing with Hayabusa2 so that we can better plan the end of HERA’s life.

Have you yet been able to decide on what to use for the 40kg of payload capacity that was still available last time we spoke (such as a high-frequency radar for measurement of subsurface properties)?

Yes – we have just held a workshop in Berlin where 118 scientists came together to discuss a number of measurements that they would like to do with the payload we already have, as well as the options for additional payloads.

This workshop revealed that the community would like to have a thermal imager included in this mission. This is because thermal energy plays a very big role in the formation and evolution dynamics of these binary asteroids, and so thermal images would really help the scientists to better understand this.

We have also begun the development of a hyperspectral imager in order to extend its range to infrared wavelengths. In principle we have still some resources available to increase the mission return. We are opening dialogues with several partners, including JAXA about whether we could also incorporate any rebuilds from Hyabusa2 into the HERA mission.

Looking beyond the engineering phase that the HERA mission is now in, what’s next?

Next, we will prepare the programme proposal for the ministerial conference in 2019 which is called ‘Space 19+’. We are also in discussion with all the Member States to get a better understanding of what part of the HERA mission they be interested in funding. And with 22 Member States there is a lot of discussion to be had.

Alongside this, we will also be preparing for the next phase. Phase B1 will end with a preliminary system requirements review in August 2019, and the ministerial will take place in November, which means that we need a bridging phase in place for industry to keep. Thus, in the spring of 2019, we plan to ask our Member States for additional funding for the HERA mission so that we can keep the industry team alive while the ministerial takes place. Of course, we are also closely monitoring the progress of the DART spacecraft, which is now an approved mission at NASA, in Phase C.

Do you have any concerns about the debris that might be caused by DART’s impacts?

Absolutely, and this is one of the thigs we carefully analysed in the previous AIM project. Our simulations have shown that the very fine dust that results from DART’s impact will fly away very quickly (it will be ejected at km/s velocities), but that the bigger boulders could be floating around for two-to-three weeks. As HERA will be arriving at the Didymos pair a few years later, we don’t expect to find any debris when we get there.

We are a little more concerned about the fact that recent simulations have shown that DART could actually change the shape and structure of Didymoon, and if that happens then it will definitely have an effect on the motion of the moon around the primary, which would complicate things for us in that our measurements would be inaccurate. We will therefore have to completely understand the effect that DART has on the dynamics of Didymoon and will refine our computer models to be able to understand what is happening in more detail.

Ian Carnelli
Programme Manager
General Studies Programme (GSP)
European Space Agency (ESA)
Ian.Carnelli@esa.int
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