Interstellar Probe: Science, Engineering, Logistic, Economic, and Social Factors

I have read the materials you gave me. The text was very interesting and I mostly agree with it. In my opinion, it is a mistake not to assemble the ship in orbit, as it could be divided into two parts, the propulsion and the probe. This would allow the drive unit to have more fuel, allowing the probe to reach a higher speed and/or carry more attachments on board. Mounting in orbit would also allow sending a better power supply system, for example, a nuclear reactor instead of an X-ray generator, which would extend the probe’s operating time and increase the distance it will fly. I know that this will increase the costs of the mission, which is already expensive, but the additional funding will allow you to get more scientific data, and this is worth its weight in gold because it is not known when such a mission will repeat. I have thought this through, and I have the more convinced about my idea. In my opinion, a reactor would be better than a radioisotope generator for several reasons. The reactor produces much more energy, for example, the “kilopower” reactor (Figure 1) is currently being developed – its output power is 1–10 kW, so you can use electric drives such as plasma and ion drives. Besides, the reactor itself (from what I was able to find out) can work 12 years on a single fuel rod, I am sure there could be a technology to replace worn-out fuel rods.

Figure 1.

As for the Space Launch System (SLS) rocket, I am not sure if this is the best vehicle for launching the probe (especially in terms of price). In my opinion, you could also consider reusable rockets, for example, (Figure 2) Falcon Heavy (FH; capable of reaching 60 t per orbit) and Starship (SpaceX rocket currently being developed, which can reach over 100 t) or New Glenn (Blue Origin rocket also under development, capable of reaching 45 t). The use of these rockets could reduce the costs of the mission and also allow the probe to be assembled in orbit.

Figure 2.

I am in favor of dividing the probe into segments, for example, propulsion with fuel and engine, engine room with a nuclear reactor, heat sinks, command room with scientific instruments, communication antenna, and an on-board computer (Figure 3). The probe will also fly past other scientifically interesting places. One could think of cubesats that could be fired in the right place. In my opinion, this mission should be used as much as possible because as I mentioned, it is not known when this opportunity will repeat itself. A segmented probe, electric or nuclear propulsion could pave a new path for future missions. Another advantage of the modular design is the possibility of modernizing – as technology advances – only selected segments; may be in the future, it will even enable a trip to another star.

Figure 3.

The list of devices for recording individual data (devices with large space heritage) is interesting, but the question is whether and how they will be modified in relation to their predecessors. From the perspective of the radiofrequency (RF) research, the receiving circuit working up to 300 kHz (and, more specifically, its accuracy) shall be important. One of the articles mentioned frequencies in the 3-30 kHz range – this is pertinent, however especially with the prospect of visiting small trans-Neptunian objects it must be extended to 300 kHz at least (as shown in Figure 4). The plan of not activating devices close to Earth and during a possible gravitational assistance of Jupiter is highly objectionable – this will significantly and unnecessarily reduce the amount of data for analysis (more so as the quality of the downlink shall be satisfactory due to the distance). Other spacecraft at that time and in such distances were operating successfully and provided valuable data (e.g., Voyager and Cassini).

Figure 4.

If the IP velocity is reached as it was mentioned there, that is, 3× the Voyagers’ velocity, then presumably, a better solution (due to the registration of RF signals) would be to continuously record selected parts of the spectrum, not to sweep the entire spectrum under consideration every time with a given small-bandwidth step. This feature would be crucial, especially during a flyby in the near-earth environment (30-35, 162, 171, 225, 540 kHz; see Figure 5) and near gas planets. It seems that the use of the Sun’s gravitational assistance, despite its advantages, may result in deep and relatively unnecessary in the long-term modification of the probe (thermal shields, etc.) and imposing additional, unimportant from the point of view of distant parts of the solar system, thermal resistance and heat transfer requirements for onboard equipment.

Figure 5.

The developed power budget is reasonable. However, to extend the operation time of individual instruments and eliminate the gradually degrading component in the form of thermocouples, a promising solution could be the introduction of at least partial diversification of power sources in the form of small ionization cells (alphavoltaic, semiconductor or liquid), which, by design, do not any emit thermal radiation into space, do not use thermocouples and are powered by, for example, Am-241 (low energies of gamma quanta) and do not significantly increase the mass of the component. Small sensor / data recording systems on intermediate memory registers (limited to a single instrument) could be successfully powered in this way, functioning in longer periods in comparison to the RTG (Radioisotope Thermal Generator)-based power system. These devices could be qualified and tested to successfully operate in the space environment without any major problems. This type of device (the alphavoltaic cell/reactor) was the subject of my master thesis (awarded in the competition for the best works on nucleonics – Polish Nuclear Society. At one time, a large study on this type of cells (but of a different, semiconductor-type) was developed by NASA itself.

In both publications, the proposed objects and phenomena that can be observed during the mission are presented in a very general way. They describe various exit scenarios for the Interstellar Probe (IP), which is equipped with research instruments, power supply, and the outline of the structure. The only thing that puzzled me was whether it was necessary to waste time and energy to explore as many things “along the way” as possible, namely, Kuiper belt objects, dwarf planets, Pluto, Neptune, their moons, and other things. Both publications discuss what can be done before the probe reaches the limits of the heliosphere. Wouldn’t it be easier, better, cheaper, and more reliable to launch the probe, hibernate it until it reaches the limits of the heliosphere, and then begin its proper mission? Then, it would be possible to take only the equipment needed to study what we want to find in the heliosphere – equipment made only for this specific application. On the other hand, scientific goals of IP published originally by McNutt clearly indicate the desire of the scientific community to also explore the interior of the heliosphere, as well as heliosheath, during the IP mission.

Figure 6.

My point is to focus on how to launch the IP into space and give it the speed necessary to escape. Both articles mention SLS 1B+ as the first stage, without considering any alternatives. While the load capacity and dimensions of the cargo hold are indeed (now and in the near future) unbeatable, taking into account the costs, a different mission configuration can be proposed.

SLS 1B+ has a predicted load capacity to Low Earth Orbit (LEO) up to 105 t, with a cargo space measuring 8.3 × 19 m (diameter × height). This allows you to accommodate the third- and fourth-stage (in the form of two of the shown Castor/Centaur/Interim Cryogenic Propulsion Stage [ICPS]) ships with a total weight of about 70 t and the IP itself (0.5 t), as shown in Figure 7, but the planned cost of a single SLS flight is 800–1600 M$.

Figure 7.

Alternatively, the FH is able to launch the LEO 63 t with a 5 × 13 m hold. This allows you to plan a mission with two FH rockets. In one of them, there would be a third stage, while in the second one, there would be a fourth stage together with IP, as shown in Figure 8. The joining of the last stages would take place at the apogee of the high-elliptical orbit: after the lifting of two approx. 35–40 t charges in both second FH stages, there will still be fuel left. Next, the mission is analogous to using SLS. The argument for the presented plan is, of course, the price: currently, the offered cost of launching a single FH rocket is 90–150 M$, which gives a total of 180–300 M$.

Figure 8.

From what I was able to determine, the budget of the IP program is currently unknown, but by comparing the presented values, for example, with the budget of the Parker Solar Probe mission (1.5 B$), it is possible to estimate the impact of potentially shifted funds toward the construction of more complex scientific instruments, as shown in Figure 9.

Figure 9.

I have read the articles and although many comments came to my mind while reading, practically all of them were resolved in the following parts of the articles. Particularly, for example, I was interested in the selection of the probe’s trajectory – which planets/other interesting objects will be passed along the way from a distance, allowing for even fleeting studies, and in which area of the heliopause (HP) it is worth aiming the probe. This, however, was described in some detail in chapters 3.2 and 4 of the article (Brandt et al.).

The second issue of interest to me was the probe’s target measurement capabilities, but this topic was also well covered. In turn, the issue of mass optimization, power consumption, and the use of data transfer by probe components is described quite briefly, as if there was no clear concept of what devices should fly. I assume that this issue will be dealt with in more detail as the project progresses.