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The exploration of asteroids involves the study of these small, rocky bodies within the Solar System. Since the discovery of 1 Ceres, which is now classified as a dwarf planet, asteroids have garnered significant interest from the astronomers. These celestial objects are seen as key to understanding the early Solar System and hold potential for future resource utilization. Ongoing and planned missions aim to mine asteroids for valuable materials and further investigate their contributions to our planetary neighborhood.

Technical requirements
The technical requirements for missions to explore asteroids involve several key considerations and advanced technologies. Efficient propulsion systems are essential for navigating the vast distances of space. Chemical propulsion is commonly used for initial launch and major trajectory changes, while ion propulsion provides efficient long-duration thrust for deep-space maneuvers. Sufficient fuel reserves are crucial for trajectory adjustments, course corrections, and potential return trips, with the amount needed varying based on the specific asteroid's distance and orbital characteristics.

Detailed trajectory planning is necessary for optimizing fuel use and mission duration, often employing gravity assists from planets to boost the spacecraft’s speed and alter its trajectory without expending additional fuel. Accurate navigation systems are required to guide the spacecraft to the asteroid, utilizing star trackers, inertial measurement units, and onboard computers capable of processing real-time navigational data.

Maintaining communication with Earth over vast distances necessitates high-gain antennas, which ensure the transmission of scientific data and telemetry between the spacecraft and mission control. A reliable power supply, often provided by solar panels or radioisotope thermoelectric generators (RTGs), is essential to power the spacecraft's instruments and systems throughout the mission. Additionally, asteroid missions require a suite of scientific instruments tailored to study the asteroid's composition, structure, and other characteristics, providing valuable data for both scientific research and potential future resource utilization.

Pioneer 10
In August 1972, Pioneer 10 incidentally flew by an unnamed asteroid en route to Jupiter, marking the first encounter of a man-made object with an asteroid. Later that year, in December, it flew past 307 Nike.

Galileo
On October 29, 1991, at 22:37 UTC, the Galileo spacecraft conducted a flyby of the S-type asteroid 951 Gaspra, passing at a distance of 1604 km. During the flyby, Galileo captured 57 images of Gaspra, with the closest approaches reaching within 5300 km. The highest-resolution images obtained have a resolution of 54 meters per pixel (177.16 ft).

During the flyby of Gaspra, Galileo measured the asteroid's surface area to be approximately 525 km2, comparable to the size of Hong Kong or Guam. Galileo studied Gaspra and discovered that its surface composition is primarily made up of olivine and pyroxene. Additionally, Galileo found that Gaspra's surface is relatively flat, with one of the largest areas being approximately 200 m, the same diameter as Pontikonisi in Greece.

On August 28, 1993, Galileo visited another S-type asteroid, 243 Ida. It discovered that Ida's surface was heavily cratered and primarily composed of olivine and pyroxene. Additionally, Galileo found that Ida was split into two sections, with the top, ball-like section named Pola Regio. This formation was possibly the result of a previous impact from a larger body that did not travel fast enough to disrupt Ida's shape. Additionally, Galileo discovered the first asteroid moon, Dactyl.

NEAR Shoemaker
On June 27, 1997, the NEAR Shoemaker spacecraft flew by asteroid 253 Mathilde en route to 433 Eros. NEAR discovered Mathilde was heavily cratered, with the largest crater being roughly 33.4 km in diameter, roughly the same diameter as the island of Vieques in Puerto Rico. NEAR discovered that Mathilde is predominantly composed of carbonaceous material, similar to CI1 or CM2 carbonaceous chondrite meteorites, with a surface rich in phyllosilicate minerals.

On December 20, 1998, NEAR performed an orbital insertion to enter orbit around 433 Eros. However, the insertion was immediately halted, and the spacecraft entered safe mode. During the failed orbital insertion, NEAR consumed 37 kg of its total fuel. The original mission had planned for four burns after insertion around Eros. However, due to the spacecraft’s unexpected condition, these subsequent burns were impossible to execute effectively.

On February 14, 2000, NEAR successfully entered orbit around Eros, marking the first time a spacecraft had inserted an orbit around such an asteroid. Prior to this, NEAR had completed a 13-month heliocentric orbit with the intention of rendezvousing with Eros. Beginning on January 24, 2001, the spacecraft conducted approaches to Eros's surface, coming as close as 5 km away, in preparation for a safe landing. On February 12, 2001, NEAR descended to Eros's surface, surprising astronomers by safely landing. Following the extension of its high-gain antenna, NEAR's gamma-ray spectrometer was reprogrammed to analyze the composition of Eros's surface. NEAR transmitted its final signals from Eros at 7 p.m. EST on February 28, 2001.

Deep Space 1
On July 29, 1999, the Deep Space 1 spacecraft encountered 9969 Braille en route to 19P/Borrelly. At the time of the encounter, Deep Space 1's ultraviolet spectrometer had malfunctioned, but it managed to capture two CCD images during the flyby. Despite a planned close approach of 26 km from Braille, the images were actually taken from an estimated distance of 14000 km due to tracking system issues. This encounter primarily served as a test of Deep Space 1's technology before its subsequent flyby of 19P/Borrelly.

Cassini-Huygens
On January 23, 2000, the Cassini–Huygens spacecraft inadvertently passed within 1500000 km of asteroid 2685 Masursky while en route to Saturn. Cassini estimated the asteroid's diameter to be just under 15-20 kilometers based on an angular diameter of 0.81–1.08 arcseconds observed hours before its closest approach. However, due to Cassini's distance from Masursky, the spacecraft did not gather vital information about the asteroid.

Stardust
On November 2, 2002, the Stardust spacecraft flew by 5535 Annefrank at a distance of 3079 km, en route to sample the coma of 81P/Wild and solar wind. Stardust discovered that Annefrank's size was double the previously estimated diameter and that it had a shape resembling a triangular prism. Additionally, Stardust found that Annefrank was heavily cratered. Debate ensued over whether Annefrank was a contact binary after scrutiny of the images captured by Stardust.

Hayabusa
On September 15, 2005, JAXA’s Hayabusa, Japanese for peregrine falcon, spacecraft took a low-resolution colorized image of its target, 25143 Itokawa. By October 4 of the same year, Hayabusa had reached its Home Position, 7 km away from Itokawa. On November 12, Hayabusa deployed its MINERVA probes to analyze Itokawa's surface. However, due to an error during deployment while Hayabusa was moving away from Itokawa, the MINERVA probe reached escape velocity and drifted into heliocentric orbit. On November 19, Hayabusa landed on Itokawa’s surface, but issues arose when its high-gain antenna couldn't be used during the final stage of sampling. Additionally, a power outage caused complications during the handover of the ground station antenna from the Deep Space Network (DSN) to the Usuda Station. Consequently, it was proposed that Hayabusa hovered 10 meters above Itokawa’s surface for 30 minutes before retreating due to a command from ground control. By the time communications were reestablished, Hayabusa was 100 km away from Itokawa and entered safe mode to stabilize attitude control. After analyzing the reestablished communication data, JAXA announced on November 23 that Hayabusa had indeed landed on Itokawa's surface.

On November 25, JAXA initiated another attempt to land on Itokawa’s surface. It was proposed that the samplers were activated, but subsequent analysis revealed another issue that prevented sampling. After another communication reestablishment, the probe was put into safe mode. On November 27, Hayabusa experienced a power outage, likely due to a fuel leakage. Three days later, JAXA announced that communication with Hayabusa had been reestablished, although issues with the probe's reaction control system remained. Mission control worked to fix the issue before Hayabusa’s launch window back to Earth. On December 2, an attitude correction was initiated, but the thrusters failed to generate sufficient force. On December 3, it was found that the probe's Z-axis was 20 to 30 degrees from the Sun direction and increasing. As an emergency measure on December 4, xenon propellant from the ion engines was used to correct the spacecraft's spin, and the maneuver was successful. By December 5, attitude control was corrected, reestablishing communication with Hayabusa through its medium gain antenna, which allowed mission control to confirm that the probe's samplers had not been penetrated and still contained samples. On December 6, Hayabusa was 500 km away from Itokawa, and by December 8, an orientation change was observed, leading to the loss of communication.

On March 7, 2006, Hayabusa reconnected with the DSN. The spacecraft was ahead of Itokawa by a mere 13000 km, traveling at 3 m per second. On March 5, 2010, it was announced that Hayabusa had passed lunar orbit. By March 27, Hayabusa was on a trajectory to pass within 20000 km of Earth. On June 13, the reentry capsule was launched, containing minerals and tholins from Itokawa.

On June 13, 2010, at 13:51 UTC, the reentry capsule and Hayabusa entered Earth’s atmosphere. The capsule, weighing 510 kg, landed in the Woomera Prohibited Area in Southern Australia and was recovered at 07:08 UTC (16:38 local) on June 14, 2010.

Rosetta
On September 5, 2008, the Rosetta spacecraft flew by 2867 Šteins from a distance of 800 km at a speed of 8.6 km/s en route to 67P/Churyumov–Gerasimenko, limiting the encounter's duration to only a 7-minute flyby. However, 15 scientific instruments aboard the Rosetta spacecraft obtained vital informational data about Šteins in this short period. The flyby occurred while Šteins was illuminated by the sun from the spacecraft's perspective, resulting in clearer images. However, due to the rapid flyby, the images taken were blurred.