More on Neptune Missions

When I have a question or a connection to someone related to a future mission study, I'll send them a preview of my write up or a link to a posted one.  For the Neptune mission concept study, I did the latter with the technical team leader, Tom Spilker of JPL.  He and I met at the AGU conference at his poster on a Saturn ring observer concept (which will be the subject of a future blog entry).  I asked Tom if he had any corrections or additions he might suggest.  In the course of a couple of emails, he expanded on several topics.  With his permission, I have quoted extensively from those two emails and changed them only to connect topics together.

Side note:  In this field, I am an interested amateur.  I notice that a lot of readers are in locations near major centers of development for planetary missions.  If anyone has corrections or additions to what I write, I will publish them.  I presume annonimity unless you specifically give permission to use your name or quote directly.

From Tom:

You might want to mention the advantage of going to Neptune before continuing to a KBO:  you can use Neptune's gravity to change the spacecraft's outbound trajectory significantly (up to 60 degrees bending angle), to steer to a large, scientifically interesting KBO, rather than one that happens to be within 0.9 degrees of the trajectory to Pluto.  This is not to minimize the importance of New Horizons!  The first view of a KBO that is still a part of the main population (for example, hasn't had a close encounter with Neptune) is an important milestone and will teach us much about the Kuiper Belt.  But the ability to choose one that should teach us the most about the primordial Kuiper Belt is also important.

Concerning the trajectory flexibility, if you specify one single KBO and won't settle for any other, then indeed you lose a lot of flexibility from the trajectory in the Neptune system.  If you specify that you must have an optimal or near-optimal radio science flyby at both Neptune and Triton, then you narrow greatly the number of KBOs you could encounter.  The radio science investigations would involve determining the gravity fields (and thus the distribution of mass in Neptune's and Triton's interiors), and possibly atmospheric occultation experiments.  The study found that doing a "best compromise" mission still gets good science returns at all three destinations, you just don't get really hi-quality gravity field data at both Neptune and Triton.

That said, one of the advantages of the Neptune flyby is that the choice of which KBO will be visited can be made before launching, so considerable effort can be put into a trade study examining the many different KBOs we might visit, and, for each one, the trajectory options within the Neptune system and their potential science value.

Concerning the "understatement" of the science value of the flagship-class missions, you could point out that the Giant Planets Panel's interest was primarily in the lower-cost concepts, thus the science objectives the concepts were being judged against were more tuned to the lower-cost missions.  The flagship-class missions did a better job achieving these science objectives and thus scored higher than the low-cost missions, but if you added to the science value assessment the science objectives that only a flagship-class mission could address, the scores of the flagship concepts would increase significantly while those of the lower-cost concepts wouldn't change much.

Until NASA's Deep Space Network (DSN) makes some significant upgrades, the upper limits to practical telemetry rates from the outer solar system are likely to be somewhat greater than those available to Voyager 2, but unfortunately not "much greater".  The increase might be a factor of 2, not a factor of 10.  While the DSN has gained by going to the higher-frequency Ka-band, they have moved to smaller antennas (34 meter diameter, as compared to the 70 meter ones available for use at X-band), somewhat offsetting the gain... There's some uncertainty in the telemetry system data rates from Neptune-like distances, mostly because you can't be sure, this early in the concept's studies, how much electric power can be devoted to the telecommunications system;  in the business we shorten that to "telecom".  But given the likely range, downlinking 256 Gb could take anywhere from 5 months to well over a year.

In the list of instruments, "RSCM" means "Radio Science Celestial Mechanics", a mouthful of words that means measuring the gravity field of a planet (or other object) via very accurate tracking of the spacecraft's trajectory.  This is accomplished using the radio link between the spacecraft and ground stations, specifically by measuring the Doppler shift the spacecraft's motion imposes on the frequency of the radio signal.

I would call Triton a "possible" ice-ocean moon, not a "likely" one.  If Triton is captured, and if it were captured billions of years ago, not millions of years ago, we think there's not enough tidal dissipation there to keep part of it thawed until the present.  Enceladus surprised us, but it's only 4 Saturn radii from Saturn's barycenter where Triton is more than 14 Neptune radii from Neptune's barycenter;  the largest-scale tidal effects go as one over r cubed.

Although the terms, "hydrogen-helium giants" and "methane-rich gas giants" are certainly reasonable, the planetary science community uses different terminology. In that community Jupiter and Saturn are known as the "gas giants" because their compositions are dominated by hydrogen and helium that were delivered to the forming planets as gases.  Out in the frigid outer solar system, where compounds such as water and ammonia occur as "ices" (solids; unless they are in deep atmospheres that are hot at depth, where they can be gases or liquids), Uranus and Neptune appear to have far greater abundances of such compounds, that we think were delivered to those planets as ices, so they are called the "ice giants".

Editorial Note: I was aware of the term 'ice giants,' but wanted to emphasize the compositional difference without taking the space to add the background explanation.  The Uranus orbiter mission concept study (page 5) has a nice explanation of the difference between the two groups of large outer planets: "Uranus and Neptune represent a distinct class of planet. Their composition and interior structure are known much less well than those of the gas giants Jupiter and Saturn. While Jupiter and Saturn are composed mostly of hydrogen (more than 90% by mass) with hydrogen envelopes thought to extend all the way to relatively small rock/ice cores ... Uranus and Neptune possess much smaller hydrogen envelopes (less than 20% by mass)... The bulk composition of these planets [Uranus and Neptune] is dominated by much heavier elements... Since these species are thought to have been incorporated into proto-planets primarily as ices... Uranus and Neptune are often referred to as 'ice giants.'   However, it is thought that there is currently very little ice in these planets, a supercritical fluid being the preferred phase of H2O at depth."