Centauri Dreams

I’ve always had a great interest in Iceland, stemming from my studies of Old Norse in graduate school, when we homed in on the sagas and immersed ourselves in a language that has changed surprisingly little for a thousand years. There’s much modern vocabulary, of course, but the Icelandic of 1000 AD is much closer to the modern variant than Shakespeare’s English is to our own. Syntactically and morphologically, Icelandic is a survivor, and a fascinating one.

New Horizons’ journey to Kuiper Belt Object MU69 occasions this reverie because the mission team has named the object Ultima Thule, following an online campaign seeking input from the public that produced 34,000 suggestions. The word ‘thule’ seems to derive from Greek, makes it into Latin, and appears in classical documents in association with the most distant northern areas then known. In the medieval era, Ultima Thule is occasionally mentioned in reference to Iceland, and sometimes to Greenland, and may have been applied even to the Shetlands, the Orkneys and, probably, the nearby Faroes. Northern and on the edge, that’s Ultima Thule.

The new Ultima Thule is a natural coinage, as New Horizons’ principal investigator Alan Stern (SwRI) has noted:

“MU69 is humanity’s next Ultima Thule. Our spacecraft is heading beyond the limits of the known worlds, to what will be this mission’s next achievement. Since this will be the farthest exploration of any object in space in history, I like to call our flyby target Ultima, for short, symbolizing this ultimate exploration by NASA and our team.”

Hence the beauty of space exploration. On Earth we eventually reach our Ultima Thule, whichever place we want to assign the name, whereas in space there’s always the next one. And indeed, New Horizons may get the chance to go after another Kuiper Belt Object after MU69. Future explorations will always find more distant targets in the cosmos.

Image: Artist’s impression of NASA’s New Horizons spacecraft encountering 2014 MU69, a Kuiper Belt object that orbits 1.6 billion kilometers beyond Pluto, on Jan. 1, 2019. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben.

Now 6 billion kilometers from Earth, New Horizons has exited hibernation as of 0212 EDT (0612 UTC) on June 5, with all systems in normal operation. We’re now in the process of uploading commands to the computers aboard the spacecraft to begin preparations for the Ultima Thule flyby, including science retrieval and subsystem and science instrument checkouts. Things are heating up — we’re not that far from August, when New Horizons will begin making observations of its target, imagery that will provide information about any needed trajectory adjustments.

But back to that hibernation, which this time around lasted 165 days. New Horizons is now fully ‘awake’ and will remain so until late 2020, when all data from the Ultima Thule encounter should have been sent back to Earth. Hibernation itself was an ingenious innovation that would maximize efficiency by reducing the cost of mission control staffing. After all, a sleeping bird requires only a skeleton crew to maintain basic communications during this period.

The sheer ingenuity of the New Horizons design comes across here. No other NASA mission has attempted hibernation, but the experience of missions like Voyager demonstrated how useful it could be. Voyager required about 450 people to run flight operations, according to David Grinspoon and Alan Stern in Chasing New Horizons. Contrast that with a New Horizons flight staff of fewer than 50 people.

The numbers are striking when you look at how the project team changed after launch as well. In the four years before New Horizons’ 2006 departure, more than 2500 people were involved in building, testing and launching the spacecraft. They included those working on the Radioisotope Thermoelectric Generator (RTG) that converts radioactive decay into electricity, the ground systems necessary to monitor the mission, and of course the rocket that would launch it.

Within a month after launch, all that had changed. “The big city that was New Horizons was reduced to a small town,” write Grinspoon and Stern. As the book memorably states:

During the long years of flight to Pluto, only a skeleton crew of flight controllers and planners, a handful of engineering ‘systems leads,’ the two dozen members of the science team, their instrument engineering staffs, and a small management gaggle was needed. Alan [Stern] recalls, “Just weeks after launch nearly everyone went their own way, and the project was reduced to a little crowd of about fifty belly buttons. All of a sudden I looked around and it hit me: there are just a few of us — a tiny team — and we’re the entire crew that’s going to fly this thing for a decade and 3 billion miles and plan the flyby of a new planet.”

Image: Flight controllers Graeme Keleher and Anisha Hosadurga, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, monitor New Horizons shortly after confirming the NASA spacecraft had exited hibernation on June 5, 2018. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Mike Buckley.

When New Horizons reached Pluto, 9.5 years had passed since launch, but because of hibernation, most of the craft’s primary systems only had 3.5 years of operational time clocked against them, which means the spacecraft was, for all intents and purposes, years younger than it would otherwise have been. Early hibernation periods tested out the concept not long after launch, easing into a process that soon increased hibernation periods to months at a time.

As New Horizons left its last hibernation period before the Pluto/Charon flyby, Alan Stern chose a ‘wake-up song’ for the occasion, a tradition dating back to Gemini 6 when flight controllers played ‘Hello Dolly’ to wake up astronauts Wally Schirra and Thomas Stafford. Stern chose ‘Faith of the Heart,’ a theme from the TV series Star Trek: Enterprise, with its lyric “It’s been a long road, getting from there to here.” Little did the team know at the time that the ‘heart’ of the title would be echoed by a famous feature on the surface of Pluto itself.

If you haven’t read Chasing New Horizons (Picador, 2018), I can’t recommend it strongly enough. This is the best inside account of a space mission I’ve yet read. Tomorrow I want to dig a little deeper into the book and talk about the New Horizons mission in context as we now begin the exciting process of preparing the craft for yet another encounter.

The longer we can keep a mission going in an exotic place, the better. Sometimes longevity is its own reward, as Curiosity has just reminded us on Mars. After all, it was only because the rover has been in place for six years that it was able to observe what scientists now think are seasonal variations in the methane in Mars’ atmosphere. Thus the news that Juno will remain active in Jupiter space is heartening, and in this case necessary. The mission is now to operate until July of 2021, an additional 41 months in orbit having been approved. More time on station allows Juno to complete a primary science mission that had appeared in jeopardy.

The reason: Problems with helium valves in the spacecraft’s fuel system resulted in the decision to remain in the present 53-day orbit instead of the 14-day ‘science orbit’ originally planned, and that has extended the time needed for data collection. Thus the lengthening of operations there not only allows further time for discovery but essentially enables the spacecraft to achieve its original science objectives. NASA has now funded Juno through FY 2022, allowing for the end of prime operations in 2021 and data collection and mission close-out carrying into 2022.

“This is great news for planetary exploration as well as for the Juno team,” said Scott Bolton, principal investigator of Juno, from the Southwest Research Institute in San Antonio. “These updated plans for Juno will allow it to complete its primary science goals. As a bonus, the larger orbits allow us to further explore the far reaches of the Jovian magnetosphere — the region of space dominated by Jupiter’s magnetic field — including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. We have also found Jupiter’s radiation environment in this orbit to be less extreme than expected, which has been beneficial to not only our spacecraft, but our instruments and the continued quality of science data collected.”

In its present 53-day polar orbit, Juno moves as close as 5,000 kilometers from the Jovian cloud tops and backs out as far as 8 million kilometers. It’s an orbit that minimizes exposure to Jupiter’s radiation belts even as it allows the craft to study the planet’s entire surface over the course of its time there. The latest work on data collected during these orbits comes in two papers, one in Nature, the other in Nature Astronomy, that look at Jovian lightning and how it is produced.

The first analysis draws on data from Juno’s Microwave Radiometer Instrument (MWR), which can record emissions at a wide range of frequencies. Because lightning discharges emit radio waves, Juno can keep an eye on lightning activity on the gas giant. Jovian lightning has also been detected by optical cameras aboard spacecraft as localized flashes of light. Shannon Brown (JPL), lead author of the paper on this work, points out that until Juno, the radio signals spacecraft have detected all came from the Galileo probe, Cassini and the two Voyager flybys, but these were all found in the kilohertz range of the radio spectrum, despite attempts to find signals in the megahertz range. The reason for the discrepancy has been a mystery.

After all, terrestrial lightning emits a broad signal over the radio spectrum up to gigahertz frequencies. Juno is helping to resolve the discrepancy, detecting Jovian lightning ‘sferics’ (broadband electromagnetic impulses) at 600 MHz. That implies that the planet’s lightning discharges are not fundamentally distinct from the lightning we experience on Earth. During Juno’s first eight orbits of Jupiter, the spacecraft detected 377 sferics, finding them prevalent in the polar regions and absent near the equator, with the most frequent occurring in the northern hemisphere at latitudes higher than 40 degrees north.

“We think the reason we are the only ones who can see it is because Juno is flying closer to the lighting than ever before,” says Brown, “and we are searching at a radio frequency that passes easily through Jupiter’s ionosphere.”

But what would account for the fact that Earth’s lightning activity is highest near the equator, while Jupiter’s is most frequent in the polar regions? Brown and company suggest that Jupiter’s poles allow more warm air to rise from within because there is less upper-level warmth from sunlight. Possibly the heating from sunlight at Jupiter’s equator can stabilize the upper atmosphere to inhibit warm air rising from below as it does at the poles. If this is the case, we would expect the polar regions to experience the convective forces that lead to lightning.

From the paper’s abstract:

Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.

Image: This artist’s concept of lightning distribution in Jupiter’s northern hemisphere incorporates a JunoCam image with artistic embellishments. Data from NASA’s Juno mission indicates that most of the lightning activity on Jupiter is near its poles. Credit: NASA/JPL-Caltech/SwRI/JunoCam.

What scientists now have to explain, as this JPL news release points out, is why the north pole is so much more active than the south. Our understanding of energy flow and circulation on Jupiter is clearly a work in progress, something the Juno data trove may help us untangle. Meanwhile, Ivana Kolmašová (Czech Academy of Sciences, Prague) and colleagues have offered what NASA is calling ‘the largest database of lightning-generated low-frequency radio emissions around Jupiter (whistlers) to date.’ The dataset includes more than 1600 signals collected by Juno’s Waves instrument, 10 times the number recorded by Voyager 1.

We’re not only further along in detection technology than in the Voyager days, with advances in microwave and plasma wave instruments to sense lightning amidst Jupiter’s emissions, but we’re also dealing with a spacecraft that has come closer to Jupiter than any other craft in history, allowing a vast increase in signal strength. The knowledge that Juno will now be able to proceed through its entire primary data collection mission is thus a cause for celebration.

The papers are Brown et al., “Prevalent lightning sferics at 600 megahertz near Jupiter’s poles,” Nature 558 (2018), 87-90 (abstract); and Kolmašová et al., “Discovery of rapid whistlers close to Jupiter implying lightning rates similar to those on Earth,” Nature Astronomy 6 June 2018 (abstract).

Things always get interesting when the American Astronomical Society meets, which it is now doing in Denver, in sessions that will run until June 7. There should be no shortage of topics emerging from the meeting, but the first that caught my eye was a different approach to the putative world some are calling Planet Nine. Teasing out the existence of a planet at the outer edges of the Solar System has involved looking at gravitational interactions among objects that we do know about, and extrapolating the presence of a far more massive body.

But the methodology may be flawed, if new work from Ann-Marie Madigan and colleagues at the University of Colorado Boulder is correct. At a press briefing at the AAS meeting, the team presented its view that objects like Sedna, an outlier that takes more than 11,000 years to complete an orbit around the Sun, should be considered in relation to other so-called ‘detached bodies.’ Almost 13 billion kilometers out, Sedna is one of a collection of such objects that appear in some ways to be in another category from the more conventional inner worlds.

Image: An artist’s rendering of Sedna, which looks reddish in color in telescope images. Credit: NASA/JPL-Caltech.

Sedna and its ilk come nowhere near the larger planets of our system, and their orbits may tell a tale. As this CU-Boulder news release explains, it was an undergraduate student named Jacob Fleisig who began to model a significant pattern known as ‘inclination instability’ that Madigan had previously described in the literature. Fleisig’s computer modeling illustrates how inclination instability can ease Sedna’s orbit from oval to circular over time.

In the model, accumulating gravitational forces drive growth in the orbital inclinations of objects in eccentric orbits. From the new work:

…secular (orbit-averaged) gravitational torques between orbits in the disk drive exponential growth of their inclinations. As the orbits’ inclinations grow, they tilt in the same way with respect to the disk plane. This leads to clustering in their angles of pericenter and the initially thin disk expands into a cone shape. Concurrently, the orbital eccentricities decrease and perihelion distances increase.

If such a mechanism is at work in our own system, we would expect it to occur between minor planets originally scattered to large orbital eccentricities via interactions with the giant planets. Such objects then become gravitationally detached from those planets. The authors believe this mechanism can explain the orbits of high perihelia objects like Sedna. Current studies posit anywhere from 1 to 10 Earth masses of cometary material existing at hundreds of AU from the Sun. The authors see such objects being originally scattered by the giant planets and their orbits decoupled by perturbations from cluster gas and nearby stars.

Perhaps this is all we need to explain the orbits of detached objects. There would be no need for a ‘Planet Nine’ at the edge of the Solar System. Instead, the detached objects achieve their present orbits through a series of small-scale interactions.

“There are so many of these bodies out there. What does their collective gravity do?” asks Madigan. “We can solve a lot of these problems by just taking into account that question.”

A great deal of work is ahead as the authors apply findings from their current computer simulations — “focused on the linear phase of the inclination instability in an idealized set-up” — to the outer Solar System. The paper notes their intent to model the gravitational influences of the giant planets and to focus on individual minor planets — especially those whose orbits become retrograde — instead of averaging their results for many hypothetical objects.

The paper is Madigan et al., “On the Dynamics of the Inclination Instability,” submitted to The Astrophysical Journal (preprint).