Arming against the asteroids

In this Q&A session Professor Francesco Topputo from the Politecnico di Milano, discusses the Italian university’s department of aerospace science and technology involvement, in developing a CubeSat for the European Space Agency’s Hera space craft. The mission, to the binary asteroids of Didymos and Dimorphos, is part of the AIDA project with NASA to explore the possibility of deflecting dangerous asteroids away from a collision course with the Earth.

In this Q&A session Professor Francesco Topputo from the Politecnico di Milano, discusses the Italian university’s department of aerospace science and technology involvement, in developing a CubeSat for the European Space Agency’s Hera space craft. The mission, to the binary asteroids of Didymos and Dimorphos, is part of the AIDA project with NASA to explore the possibility of deflecting dangerous asteroids away from a collision course with the Earth.

 

Q) How did the selection process take place for the University to be part of this ESA Hera project? What is your department's experience in developing these kinds of systems before?

The Deptartment of Aerospace Science and Technology (DAER) has a long-lasting track record of ESA projects. Most notably, it participated to ESA’s Rosetta mission and is currently involved in the preliminary design of future exploration missions to small celestial bodies. The research group at DAER has a renowned expertise in the field of astrodynamics and guidence, navigation and control (GNC) design for small spacecraft.

To be selected for the Hera project we took part in an international consortium that was awarded a public call issued by ESA. Our mission proposal seemed to be stronger and more credible than those of our competitors.

Q) What are the aims of the mission? How will the CubeSats collect data and what will it be used for? Why was the asteroid Didymos and its moon chosen?

The Hera mission is the European contribution to the Asteroid Impact and Deflection Assessment (AIDA), a joint mission between ESA and NASA, the first ever planetary defence mission. In this context, the goal of AIDA is to test the effectiveness of the ‘kinetic impact’ technique, which aims at deflecting the orbital path of a potentially hazardous asteroid by means of a high-velocity impact on its surface. The technique is based on a momentum exchange between the impactor and the asteroid and is effective for asteroids below few hundreds of meters in size. This is very effective in the case of Didymos binary asteroid: an impact on its small moon (Dimorphos) will cascade into a larger effect to the orbital motion of the whole binary system.

Hera and the CubeSats will study the Didymos system after the kinetic impact and will collect crucial data on the dynamics, surface properties, and physical composition of Didymos and its moon. One of the scientific goals of the CubeSats is to collect data through a spectrometer in the optical and near infrared bands. This is aimed at providing a global coverage of the two asteroids’ surfaces at medium-high resolution, and take close-up images at very high resolution of the impact crater and other notable surface features. In addition, the CubeSats will collect data on the dust environment in the proximity of Didymos, by means of a thermogravimeter, which will measure the properties and density of dust particles. Finally, the CubeSat will contribute crucially to the radio-science experiment of Hera, which aims at reconstructing the gravitational field produced by Didymos.

Q) How will the guidance system work on the CubeSats so far away from the Earth? Is there a degree of AI being created into the systems so the satellites can manoeuvre independently and make autonomous decisions? What is the GNC algorithm? What are the challenges when controlling craft over such long distances? How do you mitigate problems such as power failures, solar interference, damage etc.?

The CubeSats that will accompany Hera and collect data

At this stage of the design the GNC alghoritm is currently under development. One of its crucial components will be the image processing and navigation, for which we are still investigating a couple of options. The possibility to include a degree of AI alghoritms, especially in the optical navigation, is on the table concurrently with more traditional techniques that have been developed. Because of the long distance, a CubeSat cannot be operated in real-time. This translates into challenges to keep it at safe distances from the other bodies (Hera, Juventas, Didymos, Dimorphos) and to be able to point toward the asteroids even with uncertainties. The latter is important to maximise the scientific outcome and to have a robust optical navigation to reconstruct onboard the state of the CubeSat. Because of this we are currently investigating semi-autonomous or autonomous capabilities for the pointing maneouvers.

Q) How do you assess and test your software, electronics etc. before final assembly and installation? Does it involve many simulated iterations? Has simulation speeded up the process of developing space craft and their guidance systems like this?

Simulations are definetely important in the spacecraft development process. Thanks to them we can not only test the robustness of the designed mission, but also simulate the performances of different setup. On the software side simulations are fundamental to quickly speed up the development of the GNC system, especially for navigation and image processing. Because we can use realistic synthetic images and hardware-in-the-loop facilities we can make sure our alghoritms are robust when deployed on the spacecraft and will work with real images. On the hardware side there exists a series of rigorous tests that are done to make sure the spacecraft will survive in the harsh space environment. These involve radiation, thermal, vibrational, electronic, behavioural and integration tests.

Q) How will the communication between Hera and the CubeSats work?

The mission will be the first to test an interplanetary Inter-Satellite Link (ISL) between the CubeSat and the mothercraft Hera. In fact, the CubeSats will not communicate directly with ground stations, but through Hera’s relay. In terms of hardware, the ISL consists of patch omnidirectional antennas, which will allow the CubeSat to be continuously in touch with the mothercraft, within a range of 60km. The information provided by the ISL will be crucial to the effective execution of Hera’s radio-science experiment to reconstruct the gravity field of Didymos.

Q) Can you explain a little more about how the kinetic impactor will work? Is it realistic that dangerous asteroids can be identified and a system like this launched in time?

The impact is carefully designed to infer a small velocity change to the orbital path of the asteroid. In the case of Didymos, the velocity of Dimorphos will be changed by approximately 1mm/s. This small change will produce a notable effect in the long-term, which can be quantified into thousands of kilometers over a few-year time.

Professor Francesco Topputo

Bigger asteroids would receive a smaller velocity change and thus would require a longer time to reach safe deflection values and viceversa, smaller asteroids will require shorter time after identification. The practical implementation of such technique is realistic. An efficient monitoring system is currently active among the major space agencies, to continuously detect and classify potentially hazardous asteroids from ground. Due to their size, bigger asteroids are detected earlier, typically several years (or decades) prior their potential impact. This would leave sufficient time for a small kinetic impact deviation to deflect the path of a potentially hazardous asteroids headed towards our planet.

Q) What potential do CubeSats have to make space/galaxy exploration more cost-effective?

In the next decades the Cubesats will play a crucial role in the exploration of the Solar System. They provide low-cost, but high-tech platforms, able to enhance the capabilities of bigger spacecraft beyond current technological limitations. Their lower cost enables aiming at high-risk/high-gain objectives, both in terms of scientific return and in terms of new technology development.

Q) How have all the stakeholders in the project such as the University performed? Does it seem to you that the ESA/NASA collaboration is working well?

The ESA/NASA collaboration on the AIDA mission was established several years ago and is now consolidated. Since then, the broad AIDA community has worked together towards the accomplishment of its scientific goals. Both at European and international level, all scientific partners are committed towards an efficient and transparent collaboration.

Politecnico di Milano www.polimi.it/en

ESA/Hera www.esa.int/Safety_Security/Hera

 

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