Missions to the Ice Giants Uranus and Neptune

[Comga]

As I see it, there was a new technology that would have had a major impact on outer planet missions:  The ASRG.

An ASRG would not make an ice giants mission better. It would have only reduced the Pu-238 requirement, not enhanced the science.

Now all the other technologies, like imagers, spectrometers, electronics, etc., will still advance on their own. There will be a better imager 10 years from now compared to today. But the science community largely decided that nothing entirely new is necessary to do this mission. A 30-year old imager at Uranus would provide great science we don't have today.

But isn’t the power to mass ratio on the ASRG sufficiently high that it enabled Nuclear Electric Propulsion (NEP) missions that wouldn’t need giant rockets?  Perhaps that’s less “mission enabling” than “budget enabling” but in the end those are the similarly important.

And while a 30 year old imager might produce scientifically valuable imagery, their power draw would be a problem for modern missions with less power. I am on a proposal right now where current digital focal planes will play a crucial role in meeting the power budget. But that’s one of those critical technologies that are in hand.

[vjkane]

But isn’t the power to mass ratio on the ASRG sufficiently high that it enabled Nuclear Electric Propulsion (NEP) missions that wouldn’t need giant rockets?  Perhaps that’s less “mission enabling” than “budget enabling” but in the end those are the similarly important.

And while a 30 year old imager might produce scientifically valuable imagery, their power draw would be a problem for modern missions with less power. I am on a proposal right now where current digital focal planes will play a crucial role in meeting the power budget. But that’s one of those critical technologies that are in hand.

I guess you could put enough ASRGs on a spacecraft to enable NEP, but you'd need a lot to replace giant rockets.  We are talking about 125 W'ish of power per unit.  NEP usually assumes fairly high levels of power. If you look at the SEP stages suggested for the ice giant missions (among others) they are talking about 25-kW levels of power.

(There have been proposals for low power EP systems that would make use of the small amounts of power available for very low thrust over very long periods of time.  EP doesn't inherently require high power levels if your craft is very small (CubeSats) or you are very patient.  But if you want to get somewhere fast, you probably want a lot of Pu.)

[jbenton]

Even should there be an outer planets' flagship mission that required or could benefit from ASRGs, they'd test the technology first on a cheaper, more expendable mission. Back when ASRGs had been fully funded, there were plans to make them available for Discovery missions - which could afford to be higher risk. This is why 2 of the 3 finalists for Discovery Mission 12 not only used, but required ASRGs to be successful (the TiME Titan boat-lander and the CHopper comet hopper)

And developing new technology has risks (cost, time, possibility of failure). So the community thought this out and it was pretty simple to them. They just want a mission, and it is more likely they will get a mission if the cost is kept down and nobody tries to do anything complex. It's an easy calculation driven by the desire to just do it.

I think it's also the case that it's hard to see how any new technology would substantially increase the science results. There are certain targets, like Venus or Europa, where you may need technology to get science results. But with the ice giants, orbiting them is not any different than orbiting Saturn, and most of the instruments you would use there are common to many different kinds of spacecraft (like imagers, spectrometers, magnetosphere instruments).

I read some material related to a New Frontiers competition that said "New Frontiers missions are not technology demonstrators"

What is true about New Frontiers missions is even more true of Flagship missions, which should never be tech demonstrators *cough* JWST *cough*

(except, I guess for the NF Dragonfly - though flying on Titan is eaiser on Titan than on Earth, and the EDL is simple so its a tech development, but it strikes me as low-risk - and also the 'copter and MOXIE on Mars 2020 - which may or may not get in the way of science, but can't lead to LOM and could be useful for future missions)

[jbenton]

Let's keep something in mind: EVERYTHING is a trade-off in multiple dimensions of mass, power, cost, schedule, operations, and risk.

So while the idea of adding smallsats to an ice giants mission might seem attractive at first glance, you have to consider how that is going to impact all the other aspects of the mission. If you add a smallsat, that takes mass away from something else, like fuel or redundancy. Is adding that smallsat worth shortening your main mission by a year?

And consider this simple fact--that smallsat is going to need its own power system and it is highly unlikely to be an RTG. So if it's a battery, then the smallsat has a short lifetime, probably measured in terms of days. Do you really want to trade tens of kilograms of mass for a mission that is only going to last a few days? What science is worth that kind of trade? The only missions that rise up in these kinds of evaluations are atmospheric probes and maybe a moon lander (particularly for Triton). Everything else is usually not even considered.

I hadn't thought about a mass penalty. I was assuming a LV large enough to have extra mass margin, but that of course can't be assumed. If this were to happen, it might be better to launch them seperatly on a cheaper LV.

SmallSats wouldn't make sense for the outer solar system, especially for flying to their own destination.  There are no small space-qualified RTGs.  The small antennas and limited power from a small RTG, if they existed, means the data that could be sent back would be minuscule.

Yeah, this idea doesn't work unless there are RTGs small enough to power them. I hadn't thought about calling home (I was thinking of them as normal daughter-craft that stay close to the larger probe. This seems impractical at the distances considered) There's a reason New Horizons' antenna is so huge 

[TripleSeven]

quote
I read some material related to a New Frontiers competition that said "New Frontiers missions are not technology demonstrators"

What is true about New Frontiers missions is even more true of Flagship missions, which should never be tech demonstrators *cough* JWST *cough*  off

at some point this has to change.

what got Webb was "to much" new technology (this is a theme song today...the Dreamliner, Ford CVN, F35 etc have all been snake bit by this) ....but no new technology to me is just as bad

the failure on Galileo with the antenna was not a technology failure (as the TDRSS and "other" satellites have used that antenna) but with managing the technology, particularly in the endless "road" trips.

just from an RF perspective antenna size (and related down link frequency) is a big issue with how much "time" the DSN has to "acquire" a satellite...and there is no better fix for that than more antenna

I tend to agree with "less technology" advancement on the instruments.  our knowledge of the outer planets is so "weak" that almost any "close" instrument does better...and at the current level of space advancement...well while the outer planet "systems"  are probably the resource key to the solar system...its unclear that we are getting any closer to using them for that.

[Blackstar]

What is true about New Frontiers missions is even more true of Flagship missions, which should never be tech demonstrators *cough* JWST *cough*  off

I think you're taking an overly simplistic view of what "technology demonstrator" means. For many flagship missions, the only way to do the mission is with new technology. We're not going to fly test payloads for much of that technology beforehand, we're going to develop it FOR the mission.

[Jim]

What is true about New Frontiers missions is even more true of Flagship missions, which should never be tech demonstrators *cough* JWST *cough*

JWST is not a flagship planetary mission. 

[Jim]

the folks who built the LM and a great stretch of Naval fighters including my old ride...are long gone

This is TRW legacy.  Northrop nor Grumman have little to do with this.  Grumman's space experience was gone by the end of the 80's.  Northrop had little in the first place.

and then there was a sunshield...

The sunshields on Galileo had nothing to do with the antenna.

if it had been on Cassini it would have changed the data time enormously

Wrong, it would not have.  Galileo's antenna was no larger than Cassini's.  The only "advantage" of Galileo's antenna over Cassini's was weight.

[Jim]

Their washers fall off in the standard vibration test and disappears into somewhere the rest of the spacecraft. They cannot even unfold their umbrella during the very last test without tearing it up and getting it stuck.

The washers were a spec issue. 
And the sunshield is one of the tech advances, so issues are expected

What has N-G been doing with all their billions all of these years??? I cannot imagine any alternative to blatant corruption and an attempt to charge the tax payers for incompetence.

Most of the money went to GSFC and not NG. 

[jbenton]

There was one new technology that popped up as useful for Neptune, and that was aerocapture. Doing that at Neptune is uniquely different than doing it at other planets, and we haven't done it at other planets yet anyway.

I think that one other area that would be challenging would be a Triton lander. That would require some autonomous capability that we haven't really demonstrated. Probably not all that difficult, but harder and it would have to be proven out. As much as I think a Triton lander (or rover?) would be cool, I don't see it happening. The cost and complexity is rather high.


Additional note: there has been a NIAC-funded study of a Triton hopper. That's a neat idea, but the technology readiness is really low. I don't see that happening for decades.

Is there something about Neptune that makes aerocapture more feasible or useful than for the other planets? (correct me if I'm wrong, but aerocapture is like aerobraking, but without using fuel?)

[redliox]

Is there something about Neptune that makes aerocapture more feasible or useful than for the other planets? (correct me if I'm wrong, but aerocapture is like aerobraking, but without using fuel?)

Aerocapture is aerobraking's big brother and utilized to enter orbit rather than merely adjusting it.  Saturn, Uranus, and Neptune could all utilize it but because Neptune is the farthest and more difficult to ensure fuel for that planet has the most to gain by slashing the fuel weight.  You could still run a Uranus mission fairly conventionally and was a reason the Ice Giant studies had 3 options for Uranus and only 1 for Neptune.  The problem for aerocapture is that any probe fast enough to reach Neptune ala New Horizons will be fast and trickier to ensure it doesn't explode in the atmosphere like the Mars Climate Orbiter in '98.  There are possibilities to use larger shields higher up in unconventional manners to mitigate the danger, but the tech (outside of old fashion heat shields) is under development at best.

[Blackstar]

Aerocapture has not been done for any planet (although EDL people have told me that it's not a very hard thing to do--and plenty of spacecraft have reentered Earth's atmosphere and landed on Earth, which is, well, a form of aerocapture).

The problem with doing it for Neptune is that--as I understand it--the atmosphere is rather slushy, and it's not easy to predict the density of any altitude or region ahead of time. That requires that the spacecraft do a lot more sensing and steering as it is flying through the atmosphere. It has to be pretty smart. Aerocapture at other bodies, like Titan, is more predictable and therefore not as demanding on your control system. 

[vjkane]

The midterm Decadal Survey assessment has just been posted. 

https://www.nap.edu/catalog/13117/vision-and-voyages-for-planetary-science-in-the-decade-2013-2022

Use the free download option unless you want to make a contribution to the federal government.

Here is the statement on the Ice Giant study:

Decadal Findings: Vision and Voyages outlined nine prioritized science objectives for an ice giant mission including an orbiter and atmospheric probe. In a mission concept study performed in support of the decadal survey, all of these objectives were addressed to some extent using a scientifically broadly based complement of small, high-heritage instruments based on successful, previously flown instrumentation.

Assessment: In 2015 NASA commissioned an Ice Giants Predecadal Study Report (delivered in 2017), to take a fresh look at science priorities and concepts for missions to the Uranus or Neptune system. The study team developed 12 science objectives, of which 2 were identified as higher priority. Compared to Vision and Voyages,

• Internal convection was identified as a new, high-priority objective;
• Study of the large and small moons was subdivided into several objectives, placing greater emphasis on characterization of the structure, origin, and composition of smaller moons, mass transport between moons, and the origin and evolution of organic compounds on the moons; and
• Investigation of clouds as a function of depth was dropped.

The core recommended payload complement meeting a $2 billion cost cap was nearly the same mass as with the Vision and Voyages ice giants mission study (50 vs. 55 kg), but was reduced to only two remote instruments (a Doppler imager for seismological measurements of interior structure and a camera), and a magnetometer, plus a probe with a mass spectrometer and an atmospheric structure instrument. The 2017 recommended payload did not include a nephelometer that was part of the Vision and Voyages strawman. A Doppler imager for investigation of giant planet structure, which accounts for half the payload mass, has not been flown previously on any spacecraft. The notional instruments were significantly more massive than assumed in the previous study, possibly due to assumed additional radiation shielding appropriate to a Jovian environment.

The mission concept described in the 2017 Ice Giants Predecadal Study report is ambitious in investigating the interior structure of an ice giant using a Doppler imager, analogous to undertaking helioseismological investigations of the Sun. This approach at any giant planet, including ice giants, is currently unproven theoretically or experimentally. Indeed, the study itself states: “There are several significant risks associated with a Doppler imager-type instrument, however, which must be assessed before selecting it for any actual ice-giant flight opportunity. The one easily addressed is the TRL level (currently 6), which—while a common level for a proposal—is the lowest for any instrument considered for the main spacecraft. More problematic is that while oscillations have likely been detected on Jupiter (Gaulme et al., 2011) and Saturn (Hedman and Nicholson, 2013), we do not know if the oscillation amplitudes on Uranus or Neptune will be detectable, and their excitation mechanism is not well-enough understood to even make an accurate prediction from what we see on the gas giants.”

In order to accommodate this large instrument, the proposed payload is reduced in scientific scope. Loss of several instruments from the Vision and Voyages strawman payload compromises some of the principal objectives for an Ice Giant mission. Without data from plasma spectrometers, studies of the structure and dynamics of the magnetosphere and effects of solar wind on the magnetosphere will be incomplete, and characterization of the internal field may be degraded. Without a UV imaging spectrograph, a visible/near-IR imaging spectrometer, and a thermal IR radiometer, important investigations of the composition and regolith structure at both large and small satellites will be incomplete, and compositional information on the ice giant atmosphere away from the probe site will not be obtained. In the event that the Doppler imager does not perform successfully, a large part of the Vision and Voyages science objectives would be degraded or lost. The reduction of the magnetospheric objectives of an ice giant flagship mission appears premature. The magnetospheres of these planets represent numerous unique physical situations that will enable a broader and more general understanding of planetary magnetospheric physics. The magnetic field of both Uranus and Neptune may be crucially important in understanding their internal structure.

Finding: Exoplanet discoveries further enhance the importance of an ice giants mission, already recognized as a high priority in Vision and Voyages. (See Chapter 2 for further explanation.)

Finding: The notional ice giants mission described in Vision and Voyages would address a broad range of ice giant science objectives using mature instrumentation.

Finding: The objectives of the mission concept described in the 2017 ice giants predecadal study have been changed significantly from the original Vision and Voyages science objectives. The scientific payload carries significant risk of failing to make the measurements proposed in Vision and Voyages.

Furthermore, if the Doppler imager were not successful scientifically, a large part of the revised science objectives would be degraded or lost.

Recommendation: NASA should perform a new mission study based on the original ice giants science objectives identified in Vision and Voyages to determine if a more broad-based set of science objectives can be met within a $2 billion cost cap.

[TripleSeven]

the folks who built the LM and a great stretch of Naval fighters including my old ride...are long gone

This is TRW legacy.  Northrop nor Grumman have little to do with this.  Grumman's space experience was gone by the end of the 80's.  Northrop had little in the first place.

and then there was a sunshield...

The sunshields on Galileo had nothing to do with the antenna.

if it had been on Cassini it would have changed the data time enormously

Wrong, it would not have.  Galileo's antenna was no larger than Cassini's.  The only "advantage" of Galileo's antenna over Cassini's was weight.

Cassini was a 4 meter antenna the TDRSS was is  4.5  it makes a difference.  particularly as one goes up the frequency range.  it changes enormously the max power radiation areas of a dish.

[TripleSeven]

Aerocapture has not been done for any planet (although EDL people have told me that it's not a very hard thing to do--and plenty of spacecraft have reentered Earth's atmosphere and landed on Earth, which is, well, a form of aerocapture).

The problem with doing it for Neptune is that--as I understand it--the atmosphere is rather slushy, and it's not easy to predict the density of any altitude or region ahead of time. That requires that the spacecraft do a lot more sensing and steering as it is flying through the atmosphere. It has to be pretty smart. Aerocapture at other bodies, like Titan, is more predictable and therefore not as demanding on your control system.

so being able to model the atmosphere correctly...doesnt matter all that much?    I dont know...asking

[Jim]

Cassini was a 4 meter antenna the TDRSS was is  4.5  it makes a difference.  particularly as one goes up the frequency range.  it changes enormously the max power radiation areas of a dish.

Nah, .5m is in the noise.  Cassini could have been 4.5m if it was really needed, there was room for it in the fairing.
But it was not a factor.  Max planned data rate for Galileo was 134kbps and Cassini was more at 166kps (even with the extra distance)

[TripleSeven]

Cassini was a 4 meter antenna the TDRSS was is  4.5  it makes a difference.  particularly as one goes up the frequency range.  it changes enormously the max power radiation areas of a dish.

Nah, .5m is in the noise.  Cassini could have been 4.5m if it was really needed, there was room for it in the fairing.

no its not...at the higher frequencies it makes a lot of difference in the main lobe. "dishes" are funny that way...where most of the power is pushed... the surface area is "larger" as the dish gets more radius...and that makes a big difference in the main lobe strength .....

  they did not do it on Cassini because they did not have the mass the dish would have been a lot heavier.  good night up early

[Blackstar]

so being able to model the atmosphere correctly...doesnt matter all that much?    I dont know...asking

I think that when it comes to aerocapture at Neptune, you wouldn't be able to model the atmosphere to enough fidelity to matter. You still have to design a spacecraft that can sense and react and do that with confidence. That's apparently easier at other planets. Titan, for instance, has a nice thick atmosphere that makes EDL experts drool, because it's simple.

[Jim]

no its not...at the higher frequencies it makes a lot of difference in the main lobe. "dishes" are funny that way...where most of the power is pushed... the surface area is "larger" as the dish gets more radius...and that makes a big difference in the main lobe strength .....

  they did not do it on Cassini because they did not have the mass the dish would have been a lot heavier.  good night up early

No, they didn't do it for Cassini because it was not needed.  Didn't need to waste the mass on it.
 Max planned data rate for Galileo was 134kbps and Cassini was more at 166kps (even with the extra distance)

Hence in the noise.

[TripleSeven]

so being able to model the atmosphere correctly...doesnt matter all that much?    I dont know...asking

I think that when it comes to aerocapture at Neptune, you wouldn't be able to model the atmosphere to enough fidelity to matter. You still have to design a spacecraft that can sense and react and do that with confidence. That's apparently easier at other planets. Titan, for instance, has a nice thick atmosphere that makes EDL experts drool, because it's simple.

makes sense...when they lowered I think it was magellan at Venus by kind of aerobraking with the solar arrays...they nibbled at it very carefully. it was not only the atmosphere but the temps on the array...as I recall...

 I'll ask a friend who is in "wing development" my old company loaned her out to do that a long time ago.  she is now designing the 77X folding wing.  She had a good time with the scene on 2010... she talks about that a lot

good night and thanks