Astronomers and space missions have long charted asteroid encounters, from the historic Galileo flyby of asteroid Ida in 1993 that first revealed the first confirmed satellite of an asteroid to modern spacecraft charting unusual shapes and executing high-precision laser ranging.
Space exploration history is punctuated by unexpected discoveries during routine planetary flybys. When NASA’s Galileo spacecraft hurtled past the main-belt asteroid Ida at nearly 28,000 mph on August 28, 1993, it came within 1,500 miles of the asteroid’s surface while en route to Jupiter.
That close approach yielded an unexpected milestone.
Discovering Dactyl and Proving Asteroid Moons Exist
Before the Galileo encounter, astronomers suspected that space rocks could possess natural satellites, but definitive proof remained elusive. The flyby clinched the theory that asteroids could have moons when imaging team member Ann Harch subsequently discovered a tiny moon orbiting Ida while reviewing data months later. Asteroid 243 Ida was actually the second asteroid ever to be visited by a spacecraft, following Galileo’s flyby of the asteroid Gaspra in October 1991.
The International Astronomical Union named that newly found satellite Dactyl, after mythological beings from Mount Ida.
“It was previously thought that natural satellites of asteroids could form, but they probably weren’t common,” Torrence Johnson, Galileo’s project scientist, once said of the flyby in a NASA statement. “Having found one fairly quickly, we can say that they’re probably more common than previously thought.”
Ida is an S-type asteroid made mostly of silicate rock and sits in the main asteroid belt, suspected by NASA to be leftover debris from an ancient crash of two much larger objects. It was originally discovered on Sept. 29, 1884, by Austrian astronomer Johann Palisa at the Vienna Observatory.
Modern Laser Ranging and Planetary Defense Preparations
Decades after Galileo’s historic journey, asteroid exploration has advanced from simple imaging to high-precision measurement techniques crucial for planetary defense. A Japanese spacecraft that departed Earth more than a decade ago managed to do something no other spacecraft before it did: perform laser ranging during an asteroid flyby. The spacecraft that managed to do this is called Hayabusa2. It left our world back in 2014, targeting an asteroid called Ryugu. Operated by the Japan Aerospace Exploration Agency (JAXA), the small vessel reached its destination in 2018, collected samples, and became the first human-made spacecraft to create an artificial crater on the surface of an asteroid before delivering its samples back in 2020. As it speeds towards its next target, the near-Earth asteroid 1998 KY26 which it intends to study by 2031, Hayabusa2 reached an Apollo-class piece of rock called Torifune at the beginning of July to conduct laser target practice.
Meanwhile, international space agencies are preparing for an unprecedented observational campaign targeting the near-Earth asteroid Apophis. Discovered in 2004 while traveling on an alarming potential collision course with our planet, Apophis is now better known as the best example yet of our world dodging a cosmic bullet. Apophis is set to pass within 32,000 kilometers of Earth on April 13, 2029, swooping below the altitude of geostationary satellites. While initial calculations suggested the roughly 340-meter-diameter rock had a one-in-37 chance of impacting Earth in 2029, careful tracking subsequently dropped those odds to zero. Even so, the close approach will still offer a once-per-millennium natural experiment,
according to Massachusetts Institute of Technology planetary scientist Richard Binzel at a recent workshop. Back in 2020, at the inaugural workshop dedicated to the encounter, no formal plans existed to study Apophis during this encounter, but Binzel noted at the third iteration of the workshop in Padua, Italy, that the situation had completely transformed into thinking about air traffic control
for Apophis.
Coordinated International Fleets Target Apophis
To monitor the 2029 flyby, three major missions are now set to get up close and personal with Apophis from NASA, the European Space Agency (ESA), and the Japan Aerospace Exploration Agency (JAXA). Coordinated with and complementing one another, the trio of missions will act as an international relay at the asteroid, and they could be joined by a smaller, student-led Chinese CubeSat mission bound for Apophis as well. JAXA’s spacecraft DESTINY+ is part of this coordinated effort.
Complementing these flyby milestones, recent ground-based observations have revealed astonishing structural anomalies in other main-belt objects. The asteroid known as (44) Nysa, or just Nysa, was captured spinning through the main asteroid belt located between Mars and Jupiter using the Very Large Telescope in Chile and the Large Binocular Telescope in Arizona. First observed in 1857, Nysa was only recently resolved to have a unique “trilobateor three-headed structure, appearing to be around 47 miles (75 kilometers) in circumference at its widest point with two narrow
necks.” Astronomers also resolved that a 0.6-mile-wide (1-km-wide) moon, currently known as S/2026 (44) 1, orbits the asteroid at a distance of around 106 miles (170 km).
“The images reveal a remarkably unusual object,” said Kate Minker of Arizona’s Lowell Observatory in a statement announcing the discovery. “The most likely explanation is that Nysa is either a contact trinary, consisting of three connected components, or an extremely irregular coherent body unlike anything we’ve previously observed.”
