On November 29, 2021, an Air Force Research Laboratory (AFRL) Starfire Optical Range (SOR)* telescope on Kirtland Air Force Base near Albuquerque, New Mexico, recorded an image of asteroid (22) Kalliope, and its natural satellite Linus. A confirming image was taken four nights later. What is unique about these observations is the small size of the telescope used, only 1.5 meters in diameter.
Normally the purview of large 8 to10-m diameter telescopes on mountain tops in Hawaii or Chile, asteroids are faint to begin with – and their satellites even fainter – orbiting very close to their parent. Detecting them requires large telescopes, since faintness limits are proportional to telescope mirror area and resolution is proportional to telescope diameter. In all cases, adaptive optics (AO) is required to defeat the turbulence of the atmosphere by making the point sources (stars) much smaller and brighter.
Also participating in studies of asteroids and their satellites is the rather small, by comparison, SOR 3.5-m telescope. This telescope, which had previously been the smallest telescope to image and follow an asteroid and its moon over months, uses AO and a sodium laser to produce an artificial or guide star as a point reference.** Now, the 1.5-m telescope, right next door to the 3.5-m, has the record after taking images using AO, but without a laser.
“It was tough closing the AO loop because Kalliope was so faint,” said Dr. Odell Reynolds, a senior engineer at the SOR. “We had to slow down the system correcting the wavefront, hoping that it would still keep up with the changing atmosphere.”
Scientists at the SOR were surprised and delighted that their smallest telescope captured the image.
“In the end, after fitting the dominant Kalliope as a Lorentzian point spread function, and then subtracting it, Linus just popped out,” exclaimed SOR astronomer Tanya Tavenner. “I couldn’t believe we imaged a moon around an asteroid with such a small telescope. It’s really a tribute to the observing and engineering skills of Odell.”
Tavenner was pessimistic that there was anything in the field other than Kalliope when she first reviewed the images.
“Linus was so faint next to the much brighter Kalliope that I missed it the first time around,” Tavenner said. “Kalliope is 63 times fainter than the naked eye limit, and Linus was found to be 23 times fainter still than Kalliope, and would be located within the spread of the point source for Kalliope produced by the atmosphere without AO.”
Dr. Mala Mateen, the principal investigator for the Closely Spaced Object program at the SOR, was excited about the image.
“This demonstrates that we can perhaps push high-contrast imaging to smaller telescopes in order to separate two close objects in space,” Mateen said.
The asteroid Kalliope resides in the Main Belt of asteroids between Mars and Jupiter, and is about 161 kilometers in diameter. Its satellite Linus, one of two known to orbit Kalliope, was discovered in 2010 with the Keck 10-meter telescope on Mauna Kea, Hawaii, and is about 34 km in diameter. Linus orbits Kalliope at a distance of 1100 km in 3.6 days.Internet Explorer Channel Network