Satis as fast-response missions for Apophis during its Earth close encounter in 2029

Introduction:Satis is an ESA Phase A Planetary Defense mission study targeting (99942) Apophis, a Potentially Hazardous Asteroid (PHA) with a diameter of about 350 m. The mission consists of a stand-alone 12U-XL CubeSat that aims to rendezvous with Apophis prior to its Earth Closest Approach (ECA) at a distance from Earth's surface of 31,500 km on Friday, 13th April 2029. In addition, Satis serves as pioneer to demonstrate a completely new class of small, fast-response missions for planetary defense. Here, we present the Satis science objectives and the overall mission definition.Science Objectives:The flyby of Apophis in April 2029 will be a unique opportunity to observe a PHA closely. Its gravitational encounter with Earth will enable the direct observation of changes in the asteroids rotation, possible surface changes, as well as its long-term orbit. Satis will observe these parameters to assess the effects of an ECA on the evolution of asteroids.Apophis is a tumbling body or a non-principal axis (NPA) rotator. The gravitational forces on Apophis during the ECA will likely change Apophis rotational speed significantly, by more than 30%, and will cause a wobble in the rotation 1-5. These changes in rotation will allow us to infer the ratios of moments of inertia and provide information on internal mass distribution and structure (Sci-1). The interior structure of an asteroid carries the imprint of its collisional and accretion history and is important for planetary defense mitigation attempts and Hypothetical impact assessment 6.The tides and strong rotational forces have the potential to cause changes in the surfaces due to geophysical processes, such as landslides. Investigating this possible resurfacing caused by the ECA is important for interpreting present observations and to understand the evolution of asteroid surfaces (Sci-2). Surface strength and cohesion are the main drivers for surface geophysical processes and are also critical for planetary defense, as they dictate the response of the asteroid to kinetic impacts.The orbital perturbation from Earth's gravity on the Apophis orbit is likely to increase and change the orbit by increasing the semi-major axis and the perihelion distance (Sci-3). The change in the orbit due to ECA is expected to be large, but exact predictions are not possible due to uncertainties in the asteroids initial orientation. Flyby-induced changes to Apophis spin state and surface will also affect the asteroids Yarkovsky acceleration, which is a relatively small perturbation but relevant for long-term orbit and planetary defense. Fig. 1: The modelled changes in Apophis gravity field due to ECA 1 Mission definition:The feasibility of the Satis mission concept was studied by an ESA CDF study 7 and since then the mission and system design further advanced 8. The mission is designed to observe the asteroid before, during, and after the ECA in order to detect any changes induced by the effects of Earths gravity on this PHA, thus providing unique data for Planetary Defense purposes. The mission starts with an April 2027 launch on a dedicated micro-launcher equipped with a kick stage. The kick stage will be used to inject the CubeSat onto the required escape velocity vector. Following commissioning, the CubeSat will use a high-performance miniaturized electric propulsion system for the 2-year interplanetary transfer to achieve the rendezvous with Apophis. The mission duration will cover operations close to the asteroid two months before and at least two months after the ECA. Proximity operations will lead the spacecraft very close to Apophis surface to enable detailed analysis of the asteroid. Communication and navigation will be performed using a miniaturized X-band transponder interfacing with ESTRACK deep space ground stations. The payloads considered include a visual camera, thermal imager, radio science experiment as well as Hyperspectral, VIS/NIR imagers to fulfil the science objectives. The Mission Definition Review has been successfully completed and completion of Phase A of the mission study is foreseen for end of August 2024 with a Preliminary Requirements Review.References: 1 Noiset, G. et al. (2023) 8th IAA Planetary Defence Conference, 3-7 April, 2003, Vienna. 2 Benson et al., (2023) Icarus, 390, 115324. 3 Scheeres, D. J. et al., (2006), Science, 314 (5803) 4 Souchay, J. et al (2018), Astronomy and Astrophysics, 617, 111. 5 Souchay, J. et al., (2014) Astronomy and Astrophysics, 563, 16. 6 Senel C. and Karatekin . 8th IAA Planetary Defence Conference, 3-7 April, 2003, Vienna. 7 Satis CDF study (2022) ESA-TECSYE-HO-2022-003030. 8 Fogliano, V. et al. (2024) Small Satellites Systems and Services (4S) Symposium, 27-31 May, 2024, Palma de Mallorca.