Juno’s Do-or-Die Moment: What to Watch For on July 4

Juno engine burn.jpg
Juno's engine will burn for 35 minutes to place it in orbit around Jupiter.

When NASA’s spacecraft Juno enters into orbit around Jupiter on Monday evening, July 4, there won’t be any picture postcard transmitted to Earth, no message saying, “I’m here!”

Engineers in mission control at the Jet Propulsion Laboratory in Pasadena will learn only that a 35-minute engine burn designed to slow the spacecraft from a blistering 130,000 miles per hour (relative to Earth) has ended. The critical event is called Jupiter Orbit Insertion, or JOI.

Confirmation that the engine burn has completed will come as a three-second tone, a signal sent 48 minutes 19 seconds earlier by Juno from 540 million miles away. On July 4, it will take that long for a radio signal to travel between Jupiter and Earth.

About 23 minutes after the burn ends, Juno is expected to begin transmitting telemetry signals, allowing the Juno team to confirm its orbital trajectory.

For Rick Nybakken, Juno’s project manager at JPL, Monday’s critical event will carry promise and peril. “If we don’t fire the main engine long enough, we won’t slow down enough to go into orbit around Jupiter,” he says. If that happens, Juno will pass right by the planet and enter into its own long orbit around the sun—and probably never see Jupiter again. Juno’s exploration of Jupiter will end before it’s begun.

Juno has traveled five years to reach Jupiter, and its main engine has fired successfully twice before. But this time the spacecraft will be flying through Jupiter’s intense radiation zone. During the burn, certain features of Juno’s main computer will be disabled to decrease the possibility of a computer restart or other anomaly that could interrupt the main engine burn. However, if that happens, Juno has a pre-programmed “auto-restart” procedure to reboot the computer within eight minutes and resume the burn.

“We’re fairly confident that we’ve designed the spacecraft and the sequence to handle everything Jupiter can throw at us when we need to do that JOI burn,” Nybakken says.

Once the burn begins, scheduled for 8:18 pm Pacific Time, there is nothing mission control can do except wait for that three-second tone. However, mission teams at JPL and at Lockheed Martin Space Systems outside of Denver will be monitoring progress during the burn from signals captured by 70-meter Deep Space Network antennas in Goldstone, California and Canberra, Australia.

All science instruments aboard Juno will be turned off from five days before JOI to 50 hours after JOI.

The countdown to Jupiter Orbit Insertion begins in earnest at 6:13 pm Pacific Time Monday evening. That’s when the spacecraft switches to its medium gain antenna to communicate with Earth, and begins transmitting the tones that will signal when it’s completed each maneuver in the JOI sequence.

Juno has five telecom antennas: a high-gain antenna, medium-gain antenna and forward low-gain antenna at the “front” of the spacecraft, and an aft low-gain antenna and a toroidal low-gain antenna at the “back” of the spacecraft, where the main engine is located. Which antenna is used depends on the needed rate of data transfer to Earth, as well as Juno’s orientation.

Below is a timeline of key events for the rest of JOI. All times are “Earth Received Time”—that is, the time that signals from Juno, traveling at the speed of light from 540 million miles away, arrive at Earth. The actual events take place about 48 minutes before the times noted.

6:16 pm:  Juno begins a slow turn away from the sun, to assume the proper attitude for entering into orbit around Jupiter.

7:28 pm:  Juno begins a faster and larger turn to position itself more precisely.

7:56 pm:  Juno begins increasing its spin from 2 rpm to 5 rpm, which is intended to keep the spacecraft stabilized during the 35-minute JOI burn. This is expected to take about five minutes.

8:18 pm:  The critical moment: Juno fires its main engine and begins a 35-minute burn designed to slow the spacecraft down. Juno will enter orbit around Jupiter at 130,000 mph, relative to the Earth.

8:53 pm: The 35-minute engine burn ends. JOI is expected to use about 36 percent of Juno’s remaining fuel.

8:55 pm:  Juno begins spinning down from 5 rpm to 2 rpm, a maneuver that will take about five minutes.

9:07 pm:  The spacecraft begins turning back toward the sun, to maximize the amount of sunlight falling on its solar array.

9:11 pm:  Juno switches to its medium-gain antenna. Tones marking each step of JOI are no longer received at mission control.

9:16 pm:  Juno begins transmitting telemetry signals to JPL. It’s expected to take 20 minutes or more for mission control to lock into Juno’s signal.

With a lock on Juno, mission control will confirm that the spacecraft has entered into the intended orbit around Jupiter. Juno’s first complete orbit around Jupiter will take 53.5 days. That will be followed by a second 53.5-day orbit. Both orbits are highly elliptical, limiting the amount of time the spacecraft flies close to the planet to its close approaches over the poles.

This first “Capture Orbit” phase will end on October 14, and five days later Juno will execute a 22-minute engine burn to enter into a much smaller 14-day orbit around Jupiter. While all science instruments aboard Juno are expected to be turned on sometime during the capture orbit phase, the official science mission begins on October 19 with the first of 32 highly elliptical science orbits.

At that point, Juno will be ready to get down to work.