People in the space business always joke about the stress levels at any launch, but if you’re keeping tabs on a billion dollar spacecraft like Juno, I’d say the arrival can create just as many, if not more, gray hairs. Plenty of people are breathing easier this morning after Juno’s successful 35-minute engine burn and entry into orbit around Jupiter, confirmation of which came in just before midnight Eastern US time (03:53 UTC on July 5). Congratulations to the entire team.

All of this was part of a sequence of arrival events — Juno’s orbit-insertion phase (JOI) — that included spinning up the spacecraft from 2 to 5 revolutions per minute as an aid to stability, along with attitude changes in anticipation of the main engine burn, which began at 23:18 EDT. The latter decreased the spacecraft’s velocity by 542 meters per second to make orbital capture possible. Juno has already been turned again to allow its solar cells to work at full capacity.


Image: The Juno team celebrates at NASA’s Jet Propulsion Laboratory in Pasadena, California, after receiving data indicating that NASA’s Juno mission entered orbit around Jupiter. Rick Nybakken, Juno project manager at JPL, is seen at the center hugging JPL’s acting director for solar system exploration, Richard Cook. Credit: NASA/JPL-Caltech.

I always love control room photos when things are going well, remembering especially the New Horizons team last summer, images of which mingled joy with astonishment at what the doughty spacecraft was seeing. Juno is now in a 53.5 day orbit in preparation for an eventual 14-day orbit that will be achieved after a final engine burn on October 19. It’s at that point that the mission’s primary science collection period begins.

In this Cornell University news release, Jonathan Lunine, a member of the university’s Carl Sagan Institute, likens Juno’s work at Jupiter to an older discipline here on Earth, one that can help us understand the earliest days of the Solar System. What sort of materials, for example, did Jupiter take in as it grew into the gravitational behemoth it is today?:

[Jupiter is] a unique record for the outer solar system of what these protoplanets might have been like. We’re doing the astronomical equivalent of ‘broken pottery’ archaeology, trying to piece back together the original molecules and ice grains that got evaporated and dissociated inside Jupiter billions of years ago.”

We’ll have the opportunity to look at Jupiter in a number of new ways. The Galileo probe was unable, for example, to measure the water abundance in Jupiter beneath the clouds, but Juno’s microwave radiometer should be able to provide that measurement. Water abundance, in turn, tells us something about the materials that Jupiter absorbed early in its life, And by extension, we can apply this knowledge to the numerous gas giants we’re finding around other stars.

But it’s also going to be fascinating to learn whether or not the giant planet has a solid core. Juno’s gravity experiment will bring the spacecraft to within a few thousand kilometers of the cloud tops, allowing a measurement of the gravity field accurate enough to make the call. Bear in mind that the Cassini probe will burn up in Saturn’s atmosphere in 2017. During its final close flybys inside the rings, the same gravity experiment can be run. Says Lunine:

“[Cassini] will be passing just above the cloud tops like Juno does at Jupiter, underneath the rings of the planet, which will be pretty spectacular. The chance to be able to measure the core for both Jupiter and Saturn is really a tremendous opportunity.”

Bear in mind that even before Juno got to Jupiter, a number of science operations were already in progress, including work with the Jupiter Energetic Particle Detector Instrument (JEDI) to investigate the interplanetary medium as the spacecraft approached. Based at the Johns Hopkins University Applied Physics Laboratory, the JEDI team has been looking at ‘upstream ions,’ as explained by Dennis Haggerty, APL’s instrument scientist for the JEDI investigation:

“Jupiter is a very leaky planet. It has a unique particle identity, especially in terms of sulfur — which is not found in high numbers in the solar wind — and we’ve seen particles from Jupiter ‘upstream’ of the planet from missions including Voyager, Galileo and New Horizons.”

More in this APL news release, which describes the JEDI instrument and its upcoming work on Jupiter’s aurorae, which have a power density ten times greater than Earth’s, and an overall power that is greater by a factor of 100. JEDI will help explain how this system is energized.


Image: This artist’s concept depicts NASA’s Juno spacecraft above Jupiter’s north pole. Launched in 2011, the Juno spacecraft will arrive at Jupiter on July 4, 2016. As part of its instrument suites, it carries three Johns Hopkins APL-built Jupiter Energetic Particle Detector Instrument (JEDI) units to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, coming only 5,000 kilometers (about 3,000 miles) from the cloud tops at closest approach. Juno’s primary goal is to improve our understanding of Jupiter’s formation and evolution. Credit: NASA/JPL-Caltech.

What Lies Beneath

Juno invariably calls up memories of Arthur C. Clarke’s A Meeting with Medusa, a 1971 novella that Greg Benford mentioned in a conversation this past weekend. It’s been many years since I’ve read it, but I may have to revisit the work now that Alastair Reynolds and Stephen Baxter have produced a sequel, a timely arrival given the Juno activities. The Medusa Chronicles evidently presents Clarke’s character Howard Falcon with a host of new challenges, but I’ll want to refresh my memory of the Clarke before tackling it.

In Clarke’s tale, Falcon skippers a balloon craft making a slow descent through Jupiter’s upper atmosphere, where he runs into enormous life-forms, one the Medusa of the title. And here we go back to another Cornell physicist who changed our view of Jupiter, Edwin Salpeter. I’ll send you to Larry Klaes’ fine 2009 essay Edwin Salpeter and the Gasbags of Jupiter for a more detailed look, but I do want to at least mention Salpeter’s work with Carl Sagan on life in the atmospheres of gas giants on the morning of the Juno arrival.


The duo produced a paper titled “Particles, Environments, and Possible Ecologies in the Jovian Atmosphere” that appeared in The Astrophysical Journal in late 1975. Sagan would go on to discuss such life-forms in the popular TV series Cosmos. Let me quote Larry on the kinds of life Sagan and Salpeter imagined:

Sagan and Salpeter envisioned three main types of Jovian creatures. There were sinkers, small organisms which were constantly falling towards the deadly deep, dense, and hot layers of the planet but always managed to survive long enough to produce offspring that would stay up in the more habitable air layers to repeat their cycle of life. The other aerial residents of Jupiter were known as floaters, which Sagan would later describe as being “kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.” Floaters were seen as drifting across the vast alien sky in great herds, looking like a collection of immense balloons, which in essence there were, using the lighter elements of Jupiter’s atmosphere to stay aloft.

Image: Physicist Edwin Salpeter, whose work with Carl Sagan brought the idea of airborne life in Jupiter’s clouds to a wide audience.

Throw a class of hunter species into the mix and you have a vibrant and violent ecology, one that, the paper pointed out, could have been detected by the Voyager probes’ cameras if a floater of this sort existed and were high enough up in the cloud deck to be seen. Presumably Juno’s views will be spectacularly better, so perhaps some scrutiny of its imagery in search of unusual moving objects could be advocated. In any case, it’s a shame that Clarke, Sagan and Salpeter couldn’t be here to witness Juno’s arrival and the abundant imagery to follow of a realm they once imagined so vividly.