One of the least studied and understood parts of the Solar System is the outer heliosphere: the realm of the Sun’s magnetic influence that extends well beyond the orbit of Neptune, composed of the steady stream of charged particles that is released by the Sun’s upper atmosphere at speeds up to 800 km/second, known as the solar wind. Even though NASA’s Voyager 1 spacecraft has provided scientists with the first in-situ measurements of the heliosphere’s outer limits during its historic passage into interstellar space in 2012, the end of the heliosphere (the heliopause) remains a largely unexplored region. Acknowledging this reality the National Academy of Science’s 2012 Heliophysics Decadal Survey underscored, among other things, the need for the development of advanced propulsion systems that could propel space science missions toward the heliopause and beyond within the timeframe of a single decade.
Enter the Heliopause Electrostatic Rapid Transit System, or HERTS. Based on the E-Sail concept that was first developed and patented by Dr. Pekka Janhunen at the Finnish Meteorological Institute back in 2006, HERTS constitutes a propellant-less propulsion concept that utilises an electric sail (E-Sail) in order to send a miniaturised space probe to the outer limits of the heliosphere much faster than by using other more conventional means of propulsion like chemical or ion thrusters. At face value electric sails look like the more familiar solar sails, but even though they both use the same principles for producing thrust without propellants, their operation is significantly different. For instance, solar sails use the subtle pressure that is exerted by sunlight in order to propel themselves, whereas electric sails use the pressure of the electrically charged particles of the solar wind.
More specifically, the electric sail design consists of a series of 10-20 km-long, ultra-thin slowly rotating tethers that are positively charged. As the positively charged protons of the solar wind collide with the rotating tethers they would be repelled and this exchange of momentum would produce a slight push that would add up with time to a considerable acceleration. Since the solar wind consists of negatively charged electrons as well, the electric sail concept uses an electron gun in order to discharge them and keep the tethers’ positive charge intact.
One additional key difference in the operation of solar and electric sails is that the pressure of the solar wind is significantly smaller than that of solar radiation pressure—at the Earth’s distance from the Sun (1 Astronomical Unit away), it is approximately 5,000 times smaller. Nevertheless, the E-Sail’s concept envisions that the tethers’ effective area after their full deployment in space would be several square kilometres wide. Despite the solar wind’s significantly smaller pressure compared to that of photons, its net pressure would result in a peak acceleration for the electric sail of 10 AU per year. This acceleration would remain consistent far beyond the distance of 5 AU away from the Sun where the solar photon pressure becomes zero. This way, an electric sail could reach the end of the heliosphere in about a decade, whereas NASA’s Voyager 1 spacecraft even though it was helped by various gravity assists with the outer planets spent 35 years to cover the same distance.
Led by Bruce Wiegmann, an engineer at NASA’s Marshall Space Flight Center in Huntsville, Ala. HERTS was selected for further study as part of the space agency’s 2014 Phase I Innovative Advanced Concepts program. The goal of the NIAC program is to foster development of advanced concepts that could potentially lead to breakthroughs in various aerospace fields. Having gone through NIAC’s initial nine-month-long Phase I period which was devoted to the concept’s preliminary investigation, HERTS was later selected for the two-year-long Phase II in 2015. During that time, the project’s team will conduct various tests and feasibility studies, which, if successful, would show NASA that HERTS is a viable mission concept design that could be used on future missions. To that end, HERTS has recently began its test phase, as announced by NASA, during which the project’s team will examine the rate of charge particle collisions with a positively charged wire inside a control plasma chamber at the Marshall Space Flight Center. These tests will provide engineers with valuable data with which they can improve their computer models of HERTS’s overall behavior and operation in the hostile conditions of the space environment.
AmericaSpace had the chance to talk with Bruce Wiegmann regarding the details of the much-promising HERTS E-Sail concept that could potentially revolutionise the way that interplanetary missions are conducted in the not too distant future. What follows is an excerpt of this talk with Mr. Wiegmann. A part of the discussion has been edited for clarity.
AmericaSpace: Good afternoon, Mr. Wiegmann, and thank you for taking the time to answer some of our questions. Would you like to tell us what is the Heliopause Electrostatic Rapid Transit System and give us an overview of the project’s history?
Wiegmann: HERTS is a revolutionary propulsion system that is investigating the E-Sail propulsion system which is patented by Dr. Pekka Janhunen of the Finnish Meteorological Institute that will enable spacecraft to travel to the edge of the solar system in 10 to 12 years of travel time. It takes 35 plus years for a chemical propulsion system to make the same trip and that is with multiple planetary gravitational assists from Jupiter, Saturn, etc. Our funded activities will: 1) Develop the fundamental spacecraft to solar particle plasma physics model which will be used to evaluate such a spacecraft propulsion system, 2) Develop notional tether deployer concepts, and 3) Define potential Technology Demonstration Mission spacecraft designs so that an Electric Sail propulsion system can be tested within the next decade.
AmericaSpace: What are the main differences between HERTS and the E-Sail concept that was developed by Dr. Janhunen?
Wiegmann: We are working together on these investigations. In Dr. Janhunen’s past designs at the end of each tether was a ~2 pound mass element whose function was to control the tether at various spin rates. But it is our belief that these elements at the end of each tether make the system much more complex and through our team member – Dr. Rob Hoyt the President of Tethers Unlimited, Incorporated – we are actively investigating long tethers with only a small 30 to 50 gram mass at the end of each tether. The tether simulations that our team must develop will show the dynamics of the deployment of these tethers. That analysis will not be available until 2017 as we are currently focused on testing of a charged wire to understand the space plasma physics phenomenon.
AmericaSpace: The E-Sail concept is based on our current understanding of solar wind physics and its interaction with the interplanetary environment. Could we expect any surprises in this regard that could have a negative impact in the operation of an E-sail?
Wiegmann: We believe that the laws of space plasma physics govern the overall positive nature of this propulsion system, but many of the complex codes used to this date have simplified some of the assumptions involved. We are striving to develop a better computer model that does not over simplify the physics. Only after the development and running of the engineering particle in cell model (PIC for short) will we be able to answer this question. This PIC model will not be fully developed until January 2017.
AmericaSpace: Conventional solar sails that use photons in order to accelerate can be utilised out to a distance of 5 A.U. from the Sun, beyond which the energy of the solar photons as well as the sails’ effective area decrease substantially. In contrast, an E-Sail’s effective area would continue to increase by that point. What is the reason for this antithesis?
Wiegmann: The number of solar photons or photonic flux decreases by 1/r2 [where r is the radial distance from the Sun] so the further the solar sail is away from the sun the number of photons that are reflected from the solar sail would actual decay at a rate of 1/r2. O At a distance of ~5 AU the solar sail spacecraft stops accelerating so it has reached its top speed. The E-Sails rate of decay is governed by the expression of 1/r7/6. So this is more a linear decay rate. But the E-Sails effective area is composed mainly of each wire’s Debye distance and by the number of wires of a spacecraft in the particular environmental conditions. [The Debye distance is the distance over which a single charge’s electric field extends inside a plasma environment like the heliosphere. The electron Debye distance of the tethers decreases as the electric sail moves further away from the Sun causing the tethers’ positive electic field to grow, resulting in a continuous acceleration from the pressure of the solar wind out to a distance of 30 AU away].
AmericaSpace: What are the main technological challenges of deploying and operating in space an array of electrically charged wires that are 10-20 km long and what have your studies shown you about the feasibility of the overall concept so far?
Wiegmann: Our team (including Tethers Unlimited Inc.) did a feasibility study that investigated a number of approaches of deploying these tethers and some of the more promising approaches with more rigorous analysis will be released within the next year, but at this time preliminary work is still ongoing.
AmericaSpace: It has been suggested that the amount of power needed to drive an E-Sail’s electron gun would be prohibitive for the logistics of a space science mission. Does that represent a serious hurdle for the development of the concept?
Wiegmann: Determining the rate that the electrons will be collected onto the charged wires is one of the reasons we are doing space plasma physic experiments at the Marshall Space Flight Center. The amount of electrons collected is key to designing the overall E-Sail system. The experimentally collected data on electron capture rates will be scaled to represent the number of electrons collected by a full size spacecraft via the particle in cell (PIC for short) model being developed by the University of Alabama in Huntsville. Our team will only be able to size the required spacecraft electron gun and determine its electrical power needs only after our models have been developed.
AmericaSpace: The original goal for HERTS has been for the concept to be used for a mission toward the edge of the heliosphere. What other types of space science missions could benefit from utilising an E-Sail architecture?
Wiegmann: We could equally sail towards the inner planets. One mission we are investigating is sailing outside of the Solar System’s ecliptic plane. Also missions to the outer planets (Pluto in 5 years, Jupiter in 2 years) could also be done.
AmericaSpace: The E-Sail is currently envisioned for unmanned science missions. Could the concept be man-rated in some point in the future for crewed missions within our Solar System?
Wiegmann: At this time we are only investigating unmanned missions and our work to date has investigated a spacecraft with a mass of 500 kg. Any system that travels away from the Earth in space needs acceleration in an outward direction. And we know that Force = Mass x Acceleration. The amount of thrust force generated by a 500 kg spacecraft whose E-Sail propulsion system is 120 kg is very small and the characteristic acceleration created at 1 AU is equal to 1 mm/sec2. So if a manned mission would have a mass of 30 tons and would be propelled by an E-Sail, the E-Sail wires would have to get many times longer than 20 km to lengths that may be unrealistic. Also manned planetary missions are required to brake to be captured at a planetary orbit. This maneuver may not be doable with an Electric Sail.
AmericaSpace: NASA has selected HERTS for Phase II of its Innovative Advanced Concepts Program. What has been the agency’s overall reception of the HERTS concept?
Wiegmann: Positive, they have funded us, and are primarily interested in the results of this year’s plasma chamber testing coupled with the development of the engineering PIC model as this model will enable what is actually achievable since no models exist to date. Note, the NIAC office routinely funds projects that are 10+ years distant and if a projects results show negativity, then that is a lesson learned as well. This year they are funding 7 Phase II NIACs and I believe they have a positive view of each of the NIACs as we are still investigating each of our potentials.
AmericaSpace: As part of Phase II, HERTS will go through a two-year period of rigorous testing. What will these tests include and what are you hoping the results will be?
Wiegmann: Our testing most likely will be concluded by January 2017. Some members of our team have in-depth knowledge of space plasma physics and have had experience with plasma experiments from the Tether Satellite System that was deployed with the Space Shuttle. They believe that the possible thrust force that could be generated by the E-Sail could be 2 or 3 times as much as what Dr. Pekka Janhunen has theorized and this is based upon previous plasma testing at Marshall. So once our team finalizes the testing and an engineering PIC model has been developed, we will be able to ascertain if this is a valid assumption. I only hope the results do substantiate what Dr. Nobie Stone [mission scientist for the Tether Satellite System] has speculated – as this would once again prove that an experienced seasoned scientist can make correct calculations based upon the previous collected and generated analyses and applied to this new problem.
AmericaSpace: Mr. Wiegmann, how far into the future do you envision the first space application of the HERTS concept?
Wiegmann: 2021 to 2025 for Technology Demo Flights and 2025 to 2030 for first large case space applications.
AmericaSpace: Earlier this month Russian entrepreneur Yuri Milner and British astrophysicist Stephen Hawking announced the Breakthrough Starshot project with the goal of developing within a generation a solar sail capable of reaching Alpha Centauri at 20 percent the speed of light. How close do you think we are at realising such an ambitious goal?
Wiegmann: I cannot comment on a project that I have no understanding of all of its constraints, assumptions, programmatics, etc. I am optimistic in the fact that citizens of the world are now empowered and striving to accomplish what once civilization felt was not a reality. By continuing to press ahead these individuals may make the dreams of today the reality of tomorrow’s generation.
Video Credit: NASA/MSFC
The author would like to thank Ms. Tracy McMahan and Kimberly Newton at the Marshall Space Flight Center for their kind assistane in arranging this interview.