A Companion for Near-Earth Asteroid Apophis

Contrary to initial fears, asteroid Apophis will not collide with Earth. Instead, it will welcome a visitor: ESA is sending a space probe to accompany it on its journey.

To the point:

• Millennium event: On April 13, 2029, the near-Earth asteroid Apophis will fly past Earth at a distance of just under 32,000 kilometers.
• Space probe Ramses: ESA’s Ministerial Council has now decided to accompany the flyby with a space mission.
• Cosmic encounter: Ramses will study the asteroid at close range for six months and witness it racing past Earth. 
• Göttingen contribution: The Max Planck Institute for Solar System Research is sending a particle spectrometer along for the ride.

For a short time in December 2004, it looked as if asteroid Apophis could pose a threat to Earth: an impact in 2029 seemed possible – and could have devastated the region it hit or triggered tsunamis. In the meantime, the all-clear has long since been given. Researchers have calculated the trajectory of Apophis with greater accuracy and can rule out a collision with certainty.

Instead, on April 13, 2029, Apophis will pass by Earth at a distance of just under 32,000 kilometers, less than a tenth of the distance between Earth and the Moon. Statistically, such an event occurs only once every 5,000 to 10,000 years. The European Space Agency (ESA) has now decided to monitor Apophis' flyby of Earth over several months with a space probe called Rapid Apophis Mission for Space Safety (Ramses) in collaboration with the Japanese Space Agency (JAXA). An instrument from the Max Planck Institute for Solar System Research will also be on board.

Were it not for its almost intrusive proximity to Earth, Apophis would not stand out significantly from its peers of near-Earth asteroids. With a length of about 450 meters and a width of about 170 meters, Apophis is of medium size. Numerous similar bodies are known to exist and regularly intersect Earth's orbit on their path around the Sun. In order to protect our planet from impacts by such bodies in the future, it is crucial to know their properties as accurately as possible. With this knowledge, it might be possible to develop a space mission that could divert an approaching asteroid from its collision course.

Plasma cloud and radiation belt

The space probe Ramses will be doing valuable groundwork for this. Equipped with eight scientific instruments and accompanied by two CubeSats, Ramses aims to learn as much as possible about the near-Earth visitor, such as its internal structure, composition, density, and porosity. In addition, the instruments will document how Apophis, its shape, trajectory, and rotation change under the influence of Earth's gravity. The Ramses Plasma Spectrometer (RPS) will measure the energy and distribution of the ions and electrons that surround the space probe and interact with the asteroid's surface material. It is contributed to the mission by the MPS in collaboration with the Institute for Space Physics in Kiruna, Sweden.

Among other things, the researchers are interested in the thin cloud of charged particles that surrounds every asteroid. When solar wind particles hit its surface, they knock particles out of it, creating a plasma cloud. The particles detected by RPS provide clues about the composition of the asteroid – on the surface and in the layers just below it. Only RPS can provide this indirect “view” beneath the surface. The instrument thus also contributes to a deeper understanding of how the surfaces of asteroids gradually change as they travel through space. Researchers refer to this process as space weathering.

The close flyby of Earth will take Apophis even through Earth's outer radiation belt for a short time. There, captured by Earth's magnetic field, charged particles whiz around our planet at high speeds. The researchers are eager to see how the plasma environment of an asteroid behaves in this extreme environment.

Proven technology under time pressure

Compared to the typical development time for space missions, the lead time until the encounter with Apophis is extremely short. Ramses must launch into space in spring 2028 in order to intercept the near-Earth visitor at exactly the right time, two months before its flyby, and then accompany it for six months. ESA is therefore relying on instruments whose measurement principle has already proven itself, for example in ESA’s Hera mission to the near-Earth asteroid Dimorphos, which was launched last year.

RPS has a slightly different history. The instrument is an almost identical twin of the PEP-JEI instrument, which is currently traveling to Jupiter's icy moons aboard ESA's JUICE spacecraft. “The conditions in the plasma environment of Jupiter's large moons are similar to those near Earth. RPS is therefore well positioned for its future task,” explains RPS team member Markus Fränz.

Once it reaches its destination, the Ramses space probe will not be the only spectator. It is planned that shortly after Apophis' flyby of Earth (and thus about two months later than Ramses), OSIRIS-APEX will arrive. Two years ago, the NASA probe, under its former name OSIRIS-Rex, brought a sample from asteroid Bennu back to Earth. OSIRIS-APEX is now already on course for its encounter with Apophis. On its way to asteroid Phaeton, the space probe Destiny Plus from the Japanese space agency JAXA will also fly past Apophis. Ramses and Destiny Plus will be launched into space together on a Japanese H3 rocket.