The most elongated asteroid of its time

What the 1994 Goldstone radar images and two-magnitude light curves revealed about Geographos.

Published 2026-10-23

Long before anyone saw its shape, astronomers suspected Geographos was unusual.^[2]

A wildly varying light curve

Since the 1970s, many light curves have been recorded. They show a rotation period of about 5.22 hours and a brightness variation of 1.02 to 2.03 magnitudes — the asteroid can appear several times brighter side-on than end-on.^[2]

Radar confirms it

During its 1994 close approach, the Deep Space Network’s Goldstone station in California studied Geographos with radar.^[2][5] The images showed it to be the most elongated object in the Solar System, about 5.0 × 2.0 × 2.1 km.^[2][1]

Inside and on the surface

• Its interior is probably a rubble pile.^[2]
• High thermal inertia suggests a surface mix of fine grains, large rocks and boulders.^[2]
• It rotates retrograde, and several studies agree on a spin axis near ecliptic longitude 55°, latitude −46° to −52°.^[2]

Being reshaped by sunlight

The YORP effect is slowly speeding up its spin, and the Yarkovsky effect shrinks its orbit by about 27 m per year.^[2][7]

Numbered citations link to the sources list on the Geographos overview page.