Progress Updates on the START Mission: Geocentric Reconnaissance of the 2029 Apophis Encounter in Support of Planetary Defense

1. Introduction

The 2029 Earth Close Approach (ECA) of the near-Earth asteroid (99942) Apophis presents a once-in-a-millennium opportunity to witness the real-time physical and dynamical evolution of a rubble-pile body under terrestrial tidal forces. During this encounter, Apophis will pass closer to Earth than geostationary orbit, providing a rare natural experiment for investigating how planetary tides modify the surface, spin state, and internal structure of a potentially hazardous asteroid. Although catastrophic disruption is not expected, the encounter may trigger localized surface changes, including regolith/boulder motion, fresh-material exposure, and seismic-related resurfacing.

International flagship missions such as ESA’s RAMSES and NASA’s OSIRIS-APEX are planned to provide comprehensive characterization of Apophis before and after the encounter. However, the scientific community would benefit from geometrically complementary, high-cadence datasets captured during the peak dynamic window around ECA. Here we present the Student-led Threatening Asteroid Reconnaissance of Tsinghua (START) mission, a low-cost, rapid-response SmallSat designed to observe Apophis from a geocentric perspective during its closest approach to Earth.

2. Geocentric Flyby Architecture and Science Return

START is a 200-kg SmallSat mission under active development at Tsinghua University in collaboration with commercial space partners. Operating within a low-cost mission framework of ~.8M USD, START leverages commercial components, private investment, and rideshare launch opportunities to demonstrate a scalable mission architecture. Rather than undertaking a conventional multi-year interplanetary cruise, START exploits the fact that Apophis itself will pass through near-Earth space during the 2029 encounter. The spacecraft is planned to launch as a rideshare payload into Low Earth Orbit in early 2028 and use a 60-mN xenon-based solar electric propulsion system to autonomously transfer over approximately 150 days to a 31,600-km-altitude circular mission orbit, matching Apophis’s ECA altitude.

Near Apophis’s Earth closest approach, START will perform a high-speed intercept with a relative velocity of 8.74 km/s and a closest approach distance of 7 km. The mission trajectory adopts a 55° inclination orbit to balance flyby velocity, nodal precession, and commercial launch accessibility. This design allows START to maximize launch opportunities while maintaining favorable observing geometry during the encounter. The spacecraft also requires enhanced attitude control capability to maintain stable target tracking during the high-speed flyby, with angular accelerations up to 0.608°/s².

The payload suite includes a high-resolution narrow-field camera, a wide-field navigation camera, and dual visible-to-near-infrared hyperspectral imagers covering 0.4–2.5 μm. During the flyby, START is expected to achieve a peak spatial resolution of approximately 8 cm/pixel, enabling the detection of tidally induced surface modifications such as localized mass flow, boulder displacement, and seismic-related resurfacing. The hyperspectral imagers will measure surface spectral properties and search for compositional variations associated with freshly exposed material.

START has completed system-level design and is now in the detailed design phase, with spacecraft manufacturing and integration planned to begin in late 2026. During Apophis’s closest approach, START will provide an independent close-range observing perspective that complements international flagship missions and contributes to the global Apophis campaign, while demonstrating a scalable, cost-effective architecture for future planetary-defense and small-body-exploration missions.

Fig1. START Mission Overview

References

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DellaGiustina, D. N., Nolan, M. C., Polit, A. T., ... & Wolner, C. W. V. (2023). OSIRIS-APEX: An OSIRIS-REx Extended Mission to Asteroid Apophis. The Planetary Science Journal, 4(10), 198. 

Morelli, A. C., Mannocchi, A., Giordano, C., ... & Topputo, F. (2024). Initial Trajectory Assessment of a low-thrust option for the RAMSES Mission to (99942) Apophis. Advances in Space Research, 73(8), 4241-4253. 

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