The closest flyby of a large asteroid in modern history is getting closer, and the European space community wants to be there when it happens. Asteroid Apophis, approximately 340–375 meters long, will pass within the belt of geostationary satellites on April 13, 2029, at an altitude of about 31–32 thousand kilometers above the Earth's surface. This is an event that, according to estimates, occurs for objects of this size only once in thousands of years and which, under favorable conditions, will be visible to the naked eye to inhabitants of Europe and Africa. That is why the European Space Agency (ESA), through its Space Safety Programme, is preparing a rapid mission RAMSES – Rapid Apophis Mission for Space Safety – to monitor the asteroid before, during, and after its dramatic encounter with our planet.
Why the 2029 Apophis flyby is so important for science and safety
Unlike previous probe visits to smaller bodies in the Solar System, this event will allow for the observation of the dynamic reaction of an asteroid to a strong but brief external influence: passing through Earth's gravitational field. In practice, this means it will be possible to track changes in rotation, nutation, orientation of the spin axis, micro-faults on the surface, and even the possible rearrangement of debris in the regolith cover. These are all parameters crucial for understanding the behavior of small bodies near planets – knowledge that directly feeds into planetary defense protocols.
Although Apophis in 2029 poses no danger of impacting Earth, a flyby of this magnitude at such close proximity is an excellent "natural experiment" from which models and procedures can be derived for cases where the trajectory of a dangerous object might one day need to be altered. Rotational changes (the so-called YORP effect), the small push from solar radiation (the Yarkovsky effect), and the gravitational "tug" of a planet during a close encounter – all these factors influence the evolution of an orbit and spin. Systematic measurement before and after the flyby is therefore the only way to quantify the overall effect and separate temporary from permanent behavior.
What RAMSES aims to achieve and what the operations will look like
The proposed RAMSES mission is designed as a fast, agile platform that launches approximately one year before the flyby, arrives near the asteroid a few months earlier, and then "accompanies" it through the most critical period of the close encounter. At the time of writing this text (October 14, 2025), the project is in a preparatory phase, with an emphasized goal for Europe to demonstrate its ability to rapidly design, launch, and manage a mission to an object of exceptional interest for public safety and science.
The mission plan envisages a primary orbiter equipped with a combination of optical and infrared cameras, a LIDAR for high-precision topographic imaging, a radiometer, and – depending on the final configuration – radar or radio science experiments to investigate the internal structure. Additionally, the orbiter is expected to release two small systems in the CubeSat category directly near or onto the surface of Apophis. Their role is to capture the micro-relief, measure regolith properties, local variations in gravity and magnetic field (if any), and record seismic signals caused by tidal stresses during the flyby past Earth.
The "three acts" of the scientific campaign
- Before the encounter: building a detailed 3D model of the geometry and mass, precisely tracking rotation and orientation, measuring the thermophysical properties of the surface, and narrowing down orbital uncertainties. This part creates a reference "zero point."
- During the closest approach: a high-frequency series of imaging to register short-term changes – e.g., acceleration of rotation, shifts in the position of boulders, regolith slides down slopes, the appearance of micro-landslides, or dust emissions.
- After the flyby: comparison with the initial state, assessment of permanent changes in spin and surface structures, and additional orbital dynamics including the effects of Earth's gravity and thermal forces.
How a close encounter helps us in asteroid defense
The measurements from RAMSES ideally build upon the conceptual framework established by the DART and Hera missions. While DART and Hera proved it is possible to change the orbit of a small body with a "kinetic impactor," Apophis provides an opportunity to study the "other side of the equation": how a massive body (Earth) reshapes an asteroid's rotation and surface through its transient influence. This is directly important for the calculations of future deflection missions, as the final trajectory depends not only on the initial "push" but also on subsequent "tiny" effects that accumulate over the years.
In the context of operational safety, the rapid exchange of data with a network of ground-based observatories and radars will allow for continuous updating of ephemerides and risks. Even when there is no impact risk – as is the case with the 2029 flyby – building a procedural "routine" (from detection and assessment to a coordinated response) raises the level of readiness for scenarios with a real threat.
What we know about the asteroid itself
Apophis belongs to the group of so-called near-Earth asteroids, with an orbit that periodically crosses Earth's. Its size is estimated to be around 340 meters in mean diameter, with a possible elongation along the longer axis up to approximately 450 meters. The density and porosity, along with the thermal inertia of the surface material, will determine how the object behaves during the flyby. If the surface is rich in loose regolith, tidal forces may cause local rearrangements; if it is a more monolithic body, the changes will be more evident in its spin dynamics than in its geomorphology.
The movement of Apophis on April 13, 2029, will be visible as a fast-moving point in the night sky, and under optimal conditions, it is expected to have an apparent brightness of a few magnitudes, allowing observation without a telescope. Observers in Western Europe and most of Africa will have the best geometry, and over several hours, the asteroid will cross a significant portion of the sky, making it an ideal target for coordinated campaigns by amateur and professional astronomers.
Planned instruments and measurements
To achieve its scientific goals, the mission plans to combine complementary instruments:
- High-resolution cameras in the visible and near-IR spectrum for mapping morphology, assessing granulometry, monitoring dust ejections, and obtaining a photometric light curve to derive its spin.
- LIDAR for precise digital elevation models (DEMs), enabling the detection of sub-meter changes on slopes and crater rims between the "before" and "after" phases.
- A thermal radiometer for mapping thermal contrasts, determining thermal inertia, and modeling the Yarkovsky acceleration.
- A radio science link to determine the gravitational field and mass distribution, which helps in creating models of the internal structure (monolith vs. rubble pile).
- A seismometer or micro-accelerometer on a CubeSat to record transient jolts during perigee, if engineering margins permit a landing on the surface.
The role of small spacecraft: "pocket laboratories" on and around Apophis
The two planned CubeSats represent the closest "contact" with the asteroid. One is designed for close hovering and stereo imaging with very high spatial resolution, while the other is being considered as a hopping probe that would collect dust samples, measure the mechanical properties of the regolith, and send data back to the orbiter through brief contact with the surface. This distribution of risk – where critical experiments are offloaded to cheaper modules – reduces the complexity of the main spacecraft and allows for a greater scientific return in the limited time around the closest approach.
Timeline and key milestones
According to the currently considered scenario, the launch would take place during 2028, with arrival near the target in early 2029, well before the perigee on April 13, 2029, to establish the nominal scientific configuration and calibrate the instruments. After the "culmination," RAMSES would continue to monitor the object in its departure phase to record any delayed changes in spin and surface. Meanwhile, the international community is coordinating with other planned missions and telescope campaigns to cover the widest possible temporal and spectral domain of observation.
European technological capability and industrial approach
RAMSES is conceived as a "light-footed" mission that makes maximum use of the technological heritage of previous projects (from navigation in microgravity to autonomous CubeSat operations), shortening the development cycle and confirming that Europe can respond quickly to a unique scientific and security opportunity. Through streamlined procurement processes, collaboration with commercial partners, and a modular design, the project aims to reduce risks by distributing them across multiple smaller, interoperable components.
What ground-based networks will observe and how they fit into the story
While the orbiter deals with the "microphysics" on-site, ground-based radars and optical networks will track the macrodynamics: the precise ephemeris, changes in brightness, and any potential dust events. The combination of data from space and the ground will allow for cross-validation – for example, a radar shape profile can be matched with a photogrammetric model from the LIDAR, and photometry from RAMSES images can be directly linked to light curves from ground-based observatories.
Visibility in the sky from Croatia and the wider region
For observers in Croatia, as well as in most of Europe, the flyby on April 13, 2029, will take place in the evening hours local time, provided the sky is clear and there is minimal light pollution. Since Apophis will be moving relatively quickly and changing its brightness, it is recommended to choose a dark location, avoid city lights, and use simple aids like sky charts or apps for tracking celestial objects. For photographers, short burst exposures and a stable tripod can capture the bright "streak" crossing the starfield.
How changes on the asteroid's surface will be monitored
One of the key questions is whether tidal forces will cause local landslides – small slips of regolith down slopes, especially at the transitions between smooth and steep regions. Such micro-events can be detected by comparing before/after mosaics with a resolution of tens of centimeters per pixel. If recorded, scientists will be able to determine the critical slopes (the so-called "angles of repose") for regolith on small bodies, which has implications for the design of future landers and sample collection mechanisms.
Rotation, resonances, and the "dance" through the gravitational field
During the close encounter, there is a subtle exchange of angular momentum between the asteroid and Earth. Although the changes will not be drastic, precise radio-tracking and photometry can show an increase or decrease in the rotation period by a fraction of a percent, as well as changes in the moment of inertia if mass is redistributed. Special attention will be paid to possible resonant interactions during the flyby – brief phases where small mechanical "kicks" add up and leave a measurable trace in the spin.
Public aspect and educational potential
The Apophis flyby is an excellent opportunity for science popularization. In the weeks leading up to April 13, 2029, a series of public viewings, workshops, and online broadcasts are expected. The role of RAMSES will be not only scientific but also educational: visualizations of 3D models, timelines of changes, and interesting "before/after" comparisons will bring the complex dynamics of small bodies closer to a wide audience. Combined with open data and collaborative projects, a rich set of materials will be created for schools, universities, and science clubs.
Comparison with previous and parallel missions
Unlike missions targeting binary systems or small, relatively "quiet" objects, RAMSES fully relies on the temporarily "disturbed" state of the asteroid due to its flyby past a planet. This builds a reference base of knowledge about what a "gravitational bell toll" means for the structure and rotation. This base, along with the results of earlier experiments (impact deflectors, precise radio determination of masses and densities), will become part of the standardized calculations when engineers design future planetary defense missions.
What are the engineering risks and how are they mitigated
Operating a spacecraft near a small body requires sophisticated autonomy: relative navigation using surface landmarks, resilience to dust clouds, and strong changes in illumination during rapid geometric evolution. RAMSES relies on multiple levels of redundancy and "fail-safe" modes that allow for a quick retreat to a safe orbit if circumstances change. The CubeSats, as the "experimental wings" of the mission, carry a higher operational risk but also open up space for bolder maneuvers and local experiments that the main spacecraft could not perform without compromising safety.
Broader significance for European space policy
The success of RAMSES would be a clear demonstration of Europe's ability to respond in the short term to a globally important event and provide data of public interest. The Space Safety Programme strengthens the institutional network between space agencies, industry, and scientific institutions, and also encourages greater public involvement in understanding the risks from small bodies. In the long term, missions like this lay the foundation for interoperable protocols and joint responses in scenarios of real danger.
What follows until April 2029
In the coming years until the perigee (from today's date, October 14, 2025), radar and optical campaigns will continue to improve the orbital model of Apophis and monitor for any variations. on the industrial side, the selection of the final platform, integration of instruments, and qualification of the CubeSats will take place in accelerated testing cycles. In parallel, networks of observatories will agree on common protocols for synchronized observations to make the most of the short but extremely informative flyby window.
Topic map for readers and observers
- Ground-based observation: where to find a dark sky, how to prepare, what to expect in the sky above Croatia and neighboring countries.
- Safety: why the flyby poses no danger and how risks are assessed in real time.
- Instruments in focus: what exactly cameras, LIDAR, and a radiometer measure; what the "before – during – after" cycle looks like.
- CubeSats: what small auxiliary bodies can do better and faster than the main spacecraft.
- Planetary defense: where RAMSES fits into the bigger picture, from monitoring the NEO population to possible future deflection missions.
Key terms for understanding the flyby
Perigee is the moment of closest approach of the asteroid's orbit to Earth; for Apophis in 2029, this occurs on April 13. Geostationary orbit is located at ≈35,786 km above the equator; Apophis will pass below it, between that altitude and Earth. The Yarkovsky effect is a weak but measurable thrust caused by the uneven thermal radiation from the surface of a rotating body, and YORP is a related torque that changes the rotation speed and axis tilt. Regolith is the fine-grained material (dust, sand, debris) that covers many asteroids and the Moon, and its mechanics are key to determining the reaction to tremors and tidal forces.
Areas open for further monitoring
Will the rotation speed change enough to be measured with high statistical confidence? Will the CubeSats manage to get close enough to record local seismic events? How permanent will the changes in topography be after the flyby, and how will these changes be reflected in the future evolution of Apophis? The answers to these questions will form a new generation of models for the behavior of small bodies near planets and will become the starting point for the design of future missions that may one day have to make a decision about deflecting an object from Earth's path.