Imagine yourself aboard a spacecraft pushing into interstellar space. At what point would the Sun cease to be the brightest object in your sky? We’re already looking at missions designed to study the local interstellar medium (LISM), with the goal of reaching anywhere from 300 to 400 AU, a region believed to be undisturbed by the Sun. From that range, the Sun still shows an apparent visual magnitude of -13.7, making it brighter than any other star we see from Earth (Sirius comes in at magnitude -1.46).
So it’s a long push. In fact, an early interstellar probe moving at 75 kilometers per second would have to travel six thousand years to reach the point where the Sun is no longer the brightest star. At 100,000 AU, which is 1.61 light years, our imaginary probe occupant would finally see a sky where the Sun was just another bright star.
I get this information from a fascinating paper by Mike Gruntman (USC), who was kind enough to forward links not only to it but several other papers we’ll look at in the near future. This one is a 2004 examination of the instrumentation we’ll want to have aboard the first true interstellar probe. Note the word ‘true.’ We do have two functional probes that are pushing toward interstellar space right now, but neither Voyager craft was designed from the start with an interstellar science package in mind. Nor was New Horizons, even though it may tell us much not only about Pluto/Charon but also the Edgeworth/Kuiper Belt.
No, Gruntman is talking about a specific mission indeed. You could say that such a mission is modest compared to what we hope to achieve in interstellar flight. But pushing out to 400 AU is no easy business, especially if you want to do it within the working lifetime of the people who built it. Chemical propulsion is out — even the most efficient chemical systems (specific impulse 450 seconds) demand a mass ratio of 340,000 to reach the necessary velocity. Nuclear electric propulsion and solar sail technologies, however, are in the mix, and both are not that far from the technology readiness level demanded of this mission.
The Innovative Interstellar Explorer concept shows how the idea of a dedicated interstellar probe has evolved. Gruntman is a member of this team, and much of what he brings to IIE in terms of instrumentation and methods of measurement can be seen in this paper. IIE re-examines the concept in significant ways, as can be seen from this quote on the site:
We have provided a first cut of a self-consistent design for such a mission using Radioisotope Electric Propulsion (REP), existing launch vehicle hardware, and a Jupiter gravity assist. While the final speed of the probe is not as high as might be wished, it is sufficiently high to provided new – and potentially transformational – knowledge of our surroundings in interstellar space. More importantly, the required technology advances are evolutionary such that the probe could be built – and launched – as soon as the next launch window opens in late 2014.
Whatever the final design, the thinking within NASA on mission concepts like this goes back to a 1976 conference at the Jet Propulsion Laboratory that looked at the engineering problems it would face and the scientific gains it might accomplish. Ever since then, precursor missions pointing to the current Innovative Interstellar Explorer and beyond have continued to be discussed and refined both within NASA and without.
But just what is the mission? The primary goal is to reach the true interstellar medium and examine its properties. But ponder this: The Sun moves at a velocity of 26 km/s with respect to the surrounding interstellar medium. The motion is called the ‘interstellar wind,’ and as Gruntman points out, its direction is close to the plane of the ecliptic. We consider the heliosphere the place where the Sun controls the local plasma environment, but our data on its interaction with the LISM are scarce, despite the intriguing exploits of the Voyagers. So along with studies of the LISM itself, the second goal is to put the best instruments to work on what happens as the Sun’s influence effectively ends.
Gruntman sees a dedicated probe to this area as a natural next step as we go about the lengthy process of building an interstellar future. From the paper:
The lack of the direct experimental data and the resulting uncertainties in understanding of fundamental processes of the sun–LISM interaction severely limit our ability to develop a self-consistent concept of the heliosphere. Exploring in situ the nearby galactic environment and the region of the solar system frontier is thus an essential, logical, and unavoidable step in our quest for understanding our star, its interaction with the Galaxy, and laying out the foundation for the truly interstellar ﬂight of the distant future.
The Voyagers, of course, are immensely helpful already as they push through the termination shock, and their expected lifetimes for transmitting data may see them through the heliopause, that region where the Sun’s influence becomes negligible. And as you can see, the powers that be now speak of the Voyager Interstellar Mission. But Voyager was never designed for this kind of work, and a dedicated mission to explore the heliosphere and its interactions with what lies beyond, as well as evaluating the nature of the interstellar medium itself, would clearly upgrade our fundamental knowledge.
Figuring out what kind of instrumentation such a probe will need is crucial. Interstellar plasma flows around the heliopause and is thus inaccessible to us. Large interstellar dust grains make it to the inner heliosphere but small grains are pushed out by the solar wind. Even the larger grains are heated by the Sun, with the possibility of destroying organic molecules of the kind we need to study. For these and other reasons, the local interstellar medium is largely unknown to us. We’ll talk tomorrow about how a fast moving interstellar probe will examine that medium, and what other opportunities a mission like this might open for wide-ranging study.
The paper is Gruntman, “Instrumentation for Interstellar Exploration,” Advances in Space Research 34 (2004), pp. 204-212 (available online).