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This image shows different elevation orbits for a potential mission to the bone-shaped asteroid Kleopatra, shown in the center of the image. Image Credit: Tortora et al. 2025
The European Space Agency is considering a mission to a metal-rich asteroid. The target is Kleopatra, an M-type asteroid with two moons. These asteroids are relatively common, but they're difficult to observe because they're in the middle and outer regions of the main asteroid belt.
These asteroids are more than just another part of the asteroid belt. They're more than just another part of nature or another clue to how the Solar System came to be. These iron-rich asteroids intersected with human cultures in fundamental ways.
Iron meteorites that fall to Earth likely came from these metallic asteroids. Ancient Egyptians, the Inuit, and other peoples used this meteoric iron in important ways, both ceremonially and practically. One Inuit group made tools out of the Cape York meteorite for centuries without ever learning metallurgy. For that group, this chunk of iron that fell from the sky bolstered their chances of survival in a fundamental way.
Ancient Egypt's King Tutankhamun was buried with a ceremonial dagger made of meteoric iron. During his time, iron smelting was rare, and meteoritic iron was more valuable than gold and was given as gifts to royalty. Inscriptions on the walls of some pyramids show that the Ancient Egyptians knew iron came from the sky. Maybe they thanked Nut, the goddess who embodies the sky.
The asteroid belt is basically a debris belt. It's populated by a single dwarf planet, Ceres, but the vast majority of it consists of collisional debris. Some of these pieces of debris could be planetary cores, and scientists think that's what some metallic asteroids are. Repeated powerful collisions stripped away a protoplanet's outer layers, leaving only the iron-rich core behind.
The target of the ESA's new proposed mission, 216 Kleopatra, is large for an M-type asteroid, at about 120 km in equivalent-volume diameter (Approximately 276km × 94km × 78km.) It has an extremely high density compared to other small Solar System bodies. It's also shaped like a dumbbell and has two moons with diameters of about 7 and 9 km. For comparison, Mars' moons Phobos and Deimos are 22 and 12 km in diameter, respectively.
216 Kleopatra is a dense, odd-shaped asteroid with two sizeable moons not seen in this image. It doesn't look much like a piece of planetary core left over from massive collisions. Image (screenshot) Credit: By RunningNonsenseMan - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=113743040
A 2011 paper in Astronomy and Astrophysics said this about it: "Kleopatra is a puzzling multiple system due to the unique characteristics of the primary. This system certainly deserves particular attention in the future, with the Extremely Large Telescopes and possibly a dedicated space mission to decipher its entire formation history."
The ESA's proposed mission, Heavy Metal, can hopefully satisfy our curiosity about Kleopatra. One of the big questions scientists have about the asteroid concerns its mass and its gravity field. Measuring these two properties will help determine if Kleopatra is the remnant of a protoplanetary core. Without close-up measurements, they can't be certain it's as dense as our measurements-from-a-distance say it is.
A group of researchers from Italy's Università di Bologna and the Swedish Institute of Space Physics have tackled the problem by simulating its visit to Kleopatra. The research is "Radio Science Investigations for the Heavy Metal Mission to Asteroid (216) Kleopatra." It's been presented at scientific conferences and is available on the pre-print server arxiv.org. The lead author is Prof. Paolo Tortora from the Università di Bologna.
"Its high bulk density and metallic content, highlighted in multiple studies, provide an extraordinary opportunity to investigate the composition and geology of a possible protoplanetary core or re-accreted asteroid," the authors explain. "Its shape also suggests a complex formation mechanism and may shed light on the formation of binary and contact-binary asteroids."
If the HM mission is chosen, it will launch during a window in 20367-37. It would consist of a main spacecraft and a smaller sub-craft. After a 3 to 5-year journey, it would reach Kleopatra, and the main spacecraft would spend about one year there.
The Heavy Metal (HM) mission design includes the radio science experiment (RSE), which this paper simulates to see how effective it can be. The RSE involves the tracking link between the spacecraft and Earth, optical navigation (OPNAV) images from the spacecraft and an inter-satellite link (ISL) between the main spacecraft and the smaller spacecraft. Together, the RSE should be able to determine the composition and geology of Kleopatra and its two small moons and differentiate between different formation and composition models.
When it first reaches the asteroid, it will enter into a circular orbit about 5,000 km away. At that distance, Kleopatra's gravity field will already be affecting the spacecraft. At this distance, the RSE will characterize the two moons' orbital elements.
These figures show the orbits of spacecraft and satellites around Kleopatra (Credit OHB). Left: The primary spacecraft's proximity orbits, corresponding to KCOs (Kleopatra Circular Orbits) with 45° inclination and a semi-major axis of 700 km (blue), 550 km (green), 400 km (white), and 260 km (pink); Right: The sub-craft proximity orbits, corresponding to KCOs with 90° inclination, except for the initial injection orbit and semi-major axis of 650 km (blue), 499 km (green), and 260 km (white); Transfer orbits are depicted in red, while orbits of the two moons Alexhelios and Cleoselene are shown in yellow. Image Credit: Tortora et al. 2025
After its primary 5,000 km orbit, the main spacecraft will descend to 700 km altitude, placing it just outside the moon Axelhelios. This will also put Kleopatra into different illumination conditions relative to the main spacecraft, allowing it to capture detailed images of the asteroid's surface.
During this phase, the smaller sub-craft will separate and enter a polar orbit. The smaller craft's main mission is to perform low-altitude fly-bys of the two moons and Kleopatra. These will complement the main spacecraft's measurements and create a better understanding of the system's gravity, electric, and magnetic fields.
The smaller craft's mission will last about three months as it gets progressively closer to Kleopatra. The closer it gets, the riskier it becomes, and eventually, it will perform a controlled crash into the asteroid's polar region.
During this time, the main spacecraft will continue to support the sub-craft's operations via ISL from higher-altitude orbits. After mission personnel are satisfied that the dynamical environment has been characterized with a sufficient level of detail through close-up observations and gravity measurements, the primary spacecraft will descend further.
This table shows the reference timeline and simulated orbits for the Heavy Metal mission. Image Credit: Tortora et al. 2025
Kleopatra is an oddball, and nature's oddballs are full of lessons for scientists. Its unusual dumbbell shape and extraordinarily low density are strange for an iron-rich remnant of a planetary core. Its low density indicates that there's substantial porosity, not something you'd expect to see in a planetary core. Its shape and its porous structure indicate that Kleopatra could be a sort of hybrid. The asteroid's outer layers could've been stripped away by collisions, and then the object could've re-accreted more material. That could explain both its shape and its porosity.
In fact, Kleopatra's strange shape and potential rubble-pile nature also indicate that the moons could've formed from debris ejected during collisions and impacts. The study indicates that Heavy Metal's RSI will meet the mission's goals.
"The main result of the presented work is that the radio science experiment for the proposed Heavy Metal mission to the asteroid (216) Kleopatra has proved feasible," the authors write in their conclusion. "Preliminary results indicate that the asteroid mass can be retrieved with a relative accuracy up to 10^-7, while the extended gravity field can be estimated up to degree 10 with a sufficient accuracy to discriminate between internal structure models, satisfying the scientific goals of the mission."
The HM mission should also reveal the nature of Kleopatra's pair of moons. "The structure and composition of the two moons will help determine if they originated from the main body or were captured and provide insight into the formation of asteroid satellites," the authors write.