Tracking satellites around our Moon seems like a simple task, but it’s surprisingly complicated. Patches of strong gravitational fields can destabilize orbits, slowly nudging satellites off their predicted orbits. Meanwhile, the bright surface of the moon can obscure the presence of small satellites, just like how a brilliant star can wash out the dim reflected light of nearby planets.
The Deep Space Network (DSN) is an array of enormous radio antennas used to communicate with satellites outside of Earth’s orbit. The antennas are spaced around the planet – near Goldstone, California; Madrid, Spain; and Canberra, Australia — to provide continuous coverage despite the Earth’s rotation. While the DSN is usually tasked solely with transferring data to and from satellites, scientists are learning to use the antennas in new ways. First, DSN helped with observing near-Earth objects when an asteroid zipped close past our planet. Now, they’re looking closer to home – around our moon.
Computer-generated image of Chandrayaan-1’s location when it was observed by the Goldstone radar on July 2, 2016. Inset records strength of radar echo. Credit: NASA/JPL-Caltech
Pointing one of the DSN’s powerful antennas at the moon tests out new capabilities: Can these antennas designed for deep-space communication also be used to track satellites where optical telescopes fail? On July 2, 2016, scientists at Jet Propulsion Laboratories directed the 70-meter antenna DSS-14 at DSN’s Goldstone Deep Space Communications Complex in California at the moon. They found one well-tracked active satellite, NASA’s Lunar Reconnaissance Orbiter [LRO], confirming that the technique worked. The radar scientists then moved on to a more challenging target and found the Indian Space Research Organization’s Chandrayaan-1 spacecraft, abandoned in 2009.
“We have been able to detect NASA’s Lunar Reconnaissance Orbiter [LRO] and the Indian Space Research Organization’s Chandrayaan-1 spacecraft in lunar orbit with ground-based radar,” said principal investigator Marina Brozovic in a press release. “Finding LRO was relatively easy, as we were working with the mission’s navigators and had precise orbit data where it was located. Finding India’s Chandrayaan-1 required a bit more detective work because the last contact with the spacecraft was in August of 2009.”
Radar image of Chandrayaan-1 spacecraft on July 3, 2016, with radar echo strengthening and weakening as the beam reflects off the satellite’s flat surfaces. Credit: NASA/JPL-Caltech
Chandrayaan-1 is roughly the size of a smart car — 1.5 meters per side. Launched in 2008, the satellite was India’s first lunar probe. Shortly after the mission officially ended in 2009, it malfunctioned and stopped communicating with the Earth. The last scientists knew of Chandrayaan-1 was that it was in a stable orbit about 124 miles above the Moon, making one full rotation every two hours and eight minutes. Utilizing the 70-meter antenna at Goldstone, the researchers found Chandrayaan-1 after the spacecraft crossed the paths of microwave beams directed from Earth toward the Moon’s north pole.
Ryan Park, the manager of JPL’s Solar System Dynamics group, used the observations of Chandrayaan-1 to make an updated estimate for its location. “It turns out that we needed to shift the location of Chandrayaan-1 by about 180 degrees, or half a cycle from the old orbital estimates from 2009,” Park explained in the press release. “But otherwise, Chandrayaan-1’s orbit still had the shape and alignment that we expected.”
DSS-14, aka “Mars,” the 70-meter antenna at Goldstone Deep Space Communications Complex. Credit: NASA/JPL-Caltech
Over the next three months, researchers were able to confirm Chandrayaan-1’s location by listening to echoes bounced off the spacecraft and detected by NASA’s ground-based Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico.
This successful proof-of-concept opens up new capabilities for how large radio telescopes can be utilized to track and monitor lunar spacecraft traffic—a functionality that will gain importance as plans to send humans and spacecraft to the moon ramp up in 2018 and beyond.