Hayabusa2 reaches tiny 1998 KY26

Hayabusa2's encore: a 30-m asteroid spinning once every 11 minutes — the first close look at such a body.

The story

After dropping 5.4 g of Ryugu into the Australian desert in December 2020, Hayabusa2 went into extra time on its remaining propellant. The target is 1998 KY26 — about 30 m across and spinning once every 10.7 minutes — with arrival planned for July 2031.

Objects of this size are the most common members of the "city-threatening" asteroid population, yet none has ever been observed up close. The body that exploded over Chelyabinsk in 2013 is estimated at 20 m, so this is the first look at one of that class — valuable data for planetary defence.

A rotation of eleven minutes would fling loose surface rock off by centrifugal force, so the asteroid must be a single solid piece rather than a rubble pile to hold its shape — the opposite structure from Ryugu and Bennu, to be compared with the same instruments.

The extended mission was precisely designed, not a case of "fuel left, one more stop". It uses two Earth flybys (2027 and 2028) and, before them, a high-speed pass of the asteroid Torifune (2001 CC21) in July 2026 — a double bonus. The spacecraft works eleven more years after delivering its capsule. The orbit view follows Hayabusa2's track.

Why it matters

Asteroids of about 30 m strike Earth roughly once a century. Chelyabinsk (2013) was that size, and Tunguska in 1908 is estimated at a larger 50–60 m. The internal structure of this class — solid rock or rubble pile — is the key variable setting the burst altitude and ground damage of an impact, yet none has ever been observed.

A fast rotator is a natural experiment in asteroid strength. The shortest spin at which a rubble pile can hold together is about 2.2 hours; KY26 turns twelve times faster. Photographs of its shape and surface fractures will give the first observational basis for estimating the tensile strength of asteroid material.

The high-speed Torifune pass, in which Hayabusa2 photographs a 700 m asteroid while sweeping by at 5 km/s, is a rehearsal for observing objects that appear at short notice at high relative speed — the situation for a body like Apophis in 2029.

How to watch

At 30 m across, 1998 KY26 is too faint for almost any telescope, and Hayabusa2 cannot be seen from Earth. Select Hayabusa2 in the orbit view for its current position, the Earth-flyby waypoints and the planned 2031 arrival point.

The orbit replay runs from the capsule separation in December 2020 through the 2026 Torifune pass, the 2027 and 2028 Earth flybys and arrival at KY26 in 2031; at high speed the orbit can be seen adjusted at each Earth pass.

Ryugu and Bennu appear in the orbit view with asteroids switched on. Placing the three orbits side by side shows what they share — all are near-Earth objects crossing close to Earth's orbit.

Background

Hayabusa2 launched in December 2014, reached Ryugu in June 2018, collected samples in two touchdowns, departed in November 2019 and delivered its capsule to Earth on 6 December 2020. The samples contained some twenty amino acids and traces of water.

Two candidates were considered for the extension, and in September 2020 KY26 was chosen on the basis of ion-engine propellant and arrival timing. The cameras and laser altimeter are healthy, but the sampling mechanism has been used, so this visit is observation only.

After arriving in July 2031 the spacecraft will stay beside KY26 to observe its spin and shape and, propellant permitting, attempt a close descent for high-resolution imaging. Because of the rapid rotation no touchdown is planned.

1998 KY26 was discovered in June 1998 by the Spacewatch survey in Arizona, and that same month, passing within 800,000 km of Earth, it was observed by radar, which measured its 30 m size and 10.7-minute spin. Spectra suggested it may be unusually rich in water for an asteroid, another factor in its selection. Because its near-Earth orbit required only a small change in velocity to reach, the remaining propellant was enough — the decisive reason it could be chosen.

FAQ

How big is 1998 KY26?

About 30 m across — the class of the Chelyabinsk body (20 m) — spinning once every 10.7 minutes.

Will it bring back another sample?

No. The sampling mechanism was used at Ryugu; this visit is observation only.

When does it arrive?

July 2031, after Earth flybys in 2027 and 2028.

Why this asteroid?

It is the most common threatening size yet never observed up close, and its rapid spin makes it a natural test of asteroid strength.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.