Ralph McNutt’s recent update on the progress of the Innovative Interstellar Explorer concept elicited plenty of comments, enough that Dr. McNutt wanted to answer them in a new post. Now at Johns Hopkins University Applied Physics Laboratory, McNutt is Project Scientist and a Co-Investigator on NASA’s MESSENGER mission to Mercury, Co-Investigator on NASA’s Solar Probe Plus mission to the solar corona, Principal Investigator on the PEPSSI investigation on the New Horizons mission to Pluto, a Co-Investigator for the Voyager PLS and LECP instruments, and a Member of the Ion Neutral Mass Spectrometer Team on the Cassini Orbiter spacecraft. With all that on his plate, it’s hard to see how he has time for anything else, but McNutt also continues his work as a consultant on the Project Icarus interstellar design study. His Innovative Interstellar Explorer is a precursor mission designed to push our technologies hard.

by Ralph McNutt

I typically do not get involved with commenting on comments just because of the time constraints of protracted discussions, but some of the questions raised by your readers are, I think, very good and deserving of a response. [The original post is Update on Innovative Interstellar Explorer — readers may want to skim through the comments there to get up to speed — PG].

Let me try to take the comments, for the most part, in order. At one point we did take a look at Sedna and the other large trans-Neptunian Objects (TNOs). The orbit of Sedna (can be found here) will move through ~60° of arc and through its perihelion between now and 2100 (just prior to the aphelion of Pluto) — this is out of an orbital period of ~12,600 years. All of this motion is within 90 AU of the Sun, the orbital inclination is ~12° and is certainly accessible with the appropriate “tweak” at a Jupiter gravity assist. Such an aim point also puts constraints on exactly where with respect to the direction of the incoming interstellar wind one is aiming. To exit the solar system rapidly, one wants a speed as high as possible. Traveling “only” ~17 km/s (about the flyby speed of Voyager 1 past Titan and faster than the speed of New Horizons past Pluto of ~13 km/s), close imaging is problematic (with a radius of 1500 km, this is an object radius travelled in ~100s). Several months of high resolution imaging are possible with a large camera such as LORRI on New Horizons but not with a cell phone camera (which would die rapidly in the space radiation environment shortly after launch anyway).

Eris, with an orbital inclination approaching 45° is currently about 97 AU from the Sun (orbit here) and is inbound to perihelion, crossing the plane of the ecliptic at ~90 AU in the early 2070’s, but still outside of 83 AU in 2100. Makemake (orbit) is currently well above the plane of the ecliptic, passing through the plane of the ecliptic just after the end of this century and just inside of 50 AU; its orbit has a relatively small eccentricity of ~0.16 and an inclination of ~29, etc. The real problem is that doing a flyby of a TNO really is a different mission.

It is perhaps also worth noting that nuclear electric propulsion has been looked at – and in some detail under NASA’s Project Prometheus. The problem is that the power system needs to have a specific mass no greater than ~30 kg/kW (something noted by Ernst Stuhlinger back in the 1960’s — Stuhlinger literally wrote the book on ion propulsion) to have an advantage in speed delivered by nuclear electric propulsion (NEP). But that has to include the mass of the system for dumping the waste heat of the reactor (from the second law of thermodynamics) as well as its mechanical supports. The Prometheus architecture came in at over twice that, and that is the problem. To date all NEP designs come in underpowered when engineering closure on the system as a whole is examined. Think of Hiram Maxim’s steam-powered airplane versus the gasoline-powered airplane of the Wright Brothers. This is ultimately the problem with VASIMIR as well – a more mass-efficient means of providing the wall-plug electricity is needed, if it is to ever become a real system.

The spacecraft mass question is a good one as well. We tried pushing that on the precursor to IIE that was funded by NIAC – an “all the stops pulled out” approach that reduces the spacecraft mass to ~150 kg including a payload. Again the problem is engineering closure. Even if I miniaturize the electronics to microminiaturized solid state items, I need communications, guidance and control, power, thermal control, and a payload. The payload sensors have to be a finite size just to collect the data if all I am fighting is Poisson statistics – which can be traded against integration time (but it makes no sense to spend 10 years to make one measurement). Even with an iPad or equivalent that is radiation hardened, one cannot reduce the mass arbitrarily and then still make the measurements that are the raison d’etre for the effort in the first place. Ultimately, one runs into physical limits set by the properties of the materials from which one constructs components.

Part of this is manufacturing and part is the physics of the material itself. Practicalities are also involved. For the NIAC effort, we looked at the idea (and not a new one) of using ultra-low power (ULP) electronics running at liquid nitrogen temperatures. But now I have a real problem in testing such devices, as the coefficients of thermal expansion of the materials as well as the Johnson noise can preclude operation at room temperature. I could fix that with a lab and facilities on the Moon, but now that infrastructure is required, and the technicians would have to work in space suits – and I have a scenario that does not close economically (and may not technically either). Everyone in the deep-space robotic business has mass reduction as a primary goal – on everything. One can build *something* for less than ~250 kg, but the indications are that to build the desired functionality, that type of mass limit will be “sporty.”

Image: IIE Initial Concept Closeup. Credit: JHU/APL.

With respect to launch vehicles, the use of “really, big” vehicles for robotic missions has always been problematic because of the cost. There was a Voyager Mars Program in the late 1960’s which envisioned using a Saturn V to send to large rovers to Mars. Similarly we looked at implementing IIE with an Ares V combined with either a Centaur or NERVA upper stage. While flyout times are reduced, the decrease is not a factor of two.

With respect to communications, in the NIAC work we looked at an IR optical communications systems running at about 890 nm (see this paper). That was not the problem. The problem was holding spacecraft pointing well enough to keep the laser spot on the Earth from 1000 AU (the requirement for that more aggressive mission). One can certainly do the pointing with a sufficiently capable guidance system – but that drove the mass even more. We found that the best trade was a high gain antenna (HGA) of just under 3 meters diameter (about what is on Pioneer 10 and 11 and New Horizons). One driver is holding tolerances during manufacture and another is holding them under the vibration environment imposed by the launch. Materials are not infinitely stiff (which is good, because then they would break), but that means corrections and feedbacks as required. The other interesting thing about a laser com system running from ~5 light days out is that closed-loop operation is not credible, and the beam is sufficiently small and the distance sufficiently large that to minimize power, I need a clock with an ephemeris that can be used to point the transmitter to where the Earth will be when the modulated laser carry arrives there.