Socorro, NM — When the Galileo Probe becomes the first spacecraft to enter the atmosphere of Jupiter on Dec. 7, a New Mexico radio telescope will be watching. In a technical feat thought impossible when Galileo was launched in 1989, the National Science Foundation’s Very Large Array (VLA) will record the faint radio signal from the probe to help scientists measure the giant planet’s winds. The VLA observations will dramatically improve estimates of Jupiter’s wind speeds and complement other measurements studying the climate of Jupiter. The Galileo probe will transmit information to the main spacecraft as it descends toward a searing death under tremendous heat in Jupiter’s lower atmosphere. The main spacecraft will later relay the probe’s data to Earth. No Earth-based reception of the probe’s radio signals was planned originally. The probe’s antenna will be pointed at the main spacecraft, not the Earth.
However, in 1991, Robert Preston and William Folkner of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, CA, were discussing Earth-based reception of data from a similar probe under design for a planned mission to Saturn. “I thought, why not do this for Galileo,” Folkner said.
“They were planning to build this capability into the spacecraft for Saturn,” Folkner explained, “and they thought it couldn’t be done with the Galileo spacecraft already enroute to Jupiter. I didn’t know it couldn’t be done, so I worked it out and found that we could do it.”
According to Preston and Folkner’s calculations, the direct reception of the probe’s signals by the VLA and a similar radio telescope in Australia will make the measurement of Jupiter’s winds ten times more precise as long as the probe radio signal can be detected. In addition, the direct reception also greatly improves scientists’ knowledge of the probe’s position as it enters the Jovian atmosphere. This will allow more effective use of the measurements of the probe radio signal by the main spacecraft to determine atmospheric properties.
The VLA observations will record the shift in frequency of the probe’s radio signal as Jupiter’s winds buffet the probe. This Doppler shift in frequency will allow scientists to calculate the wind speeds.
Scientists expect the 746-pound probe to send information about Jupiter’s atmosphere for up to 75 minutes during its parachute-slowed descent. Preston and Folkner, who are working with Jose Navarro of the National Radio Astronomy Observatory (NRAO) in Socorro, NM, expect to receive the probe’s signals with the VLA for the first 20 or 30 minutes of the descent.
The technical difficulties in directly receiving the probe’s signal are challenging. The probe has only a 25-watt radio transmitter. The probe’s directional antenna is aimed at the main Galileo spacecraft, nearly 90 degrees away from the direction of the Earth. This effectively reduces the power to 7 watts or less toward the Earth. At Jupiter, the probe is more than half a billion miles distant from Earth.
Only a large radio telescope is capable of receiving this faint signal, more than 100,000 times weaker than the faintest signal a home FM radio can pick up.
Even using a radio telescope as large as the VLA, the scientists may have to wait for the main Galileo spacecraft to send the probe’s data back to Earth before they can recover the signals they recorded. With the relayed data in hand, they can “reconstruct” the probe’s radio signal and use that reconstructed signal to help their computers find the weak recorded signal on the VLA tapes. A preliminary relay of the probe’s data from the main spacecraft is planned in December.
During its descent, the Galileo probe will send information about the chemical composition of Jupiter’s atmosphere at different altitudes. It is expected to encounter winds of up to 200 m.p.h.