Epsilon Eridani has always intrigued me because in astronomical terms, it’s not all that far from the Sun. I can remember as a kid noting which stars were closest to us – the Centauri trio, Tau Ceti and Barnard’s Star – wondering which of these would be the first to be visited by a probe from Earth. Later, I thought we would have quick confirmation of planets around Epsilon Eridani, since it’s a scant (!) 10.5 light years out, but despite decades of radial velocity data, astronomers have only found one gas giant, and even that confirmation was slowed by noise-filled datasets.
Even so, Epsilon Eridani b is confirmed. Also known as Ægir (named for a figure in Old Norse mythology), it’s in a 3.5 AU orbit, circling the star every 7.4 years, with a mass somewhere between 0.6 and 1.5 times that of Jupiter. But there is more: We also get two asteroid belts in this system, as Gerald Jackson points out in his new paper on using antimatter for deceleration into nearby star systems, as well as another planet candidate.
Image: This artist’s conception shows what is known about the planetary system at Epsilon Eridani. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Epsilon Eridani is located about 10 light-years away in the constellation Eridanus. It is visible in the night skies with the naked eye. The system’s inner asteroid belt appears as the yellowish ring around the star, while the outer asteroid belt is in the foreground. The outermost comet ring is too far out to be seen in this view, but comets originating from it are shown in the upper right corner. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
This is a young system, estimated at less than one billion years. For both Epsilon Eridani and Proxima Centauri, deceleration is crucial for entering the planetary system and establishing orbit around a planet. The amount of antimatter available will determine our deceleration options. Assuming a separate method of reaching Proxima Centauri in 97 years (perhaps beamed propulsion getting the payload up to 0.05c), we need 120 grams of antiproton mass to brake into the system. A 250 year mission to Epsilon Eridani at this velocity would require the same 120 grams.
Thus we consider the twin poles of difficulty when it comes to antimatter, the first being how to produce enough of it (current production levels are measured in nanograms per year), the second how to store it. Jackson, who has long championed the feasibility of upping our antimatter production, thinks we need to reach 20 grams per year before we can start thinking seriously about flying one of these missions. But as both he and Bob Forward have pointed out, there are reasons why we produce so little now, and reasons for optimism about moving to a dedicated production scenario.
Past antiproton production was constrained by the need to produce antiproton beams for high energy physics experiments, requiring strict longitudinal and transverse beam characteristics. Their solution was to target a 120 GeV proton beam into a nickel target  followed by a complex lithium lens . The world record for the production of antimatter is held by the Fermilab. Antiproton production started in 1986 and ended in 2011, achieving an average production rate of approximately 2 ng/year . The record instantaneous production rate was 3.6 ng/year . In all, Fermilab produced and stored 17 ng of antiprotons, over 90% of the total planetary production.
Those are sobering numbers. Can we cast antimatter production in a different light? Jackson suggests using our accelerators in a novel way, colliding two proton beams in an asymmetric collider scenario, in which one beam is given more energy than the other. The result will be a coherent antiproton beam that, moving downstream in the collider, is subject to further manipulation. This colliding beam architecture makes for a less expensive accelerator infrastructure and sharply reduces the costs of operation.
The theoretical costs for producing 20 grams of antimatter per year are calculated under the assumption that the antimatter production facility is powered by a square solar array 7 km x 7 km in size that would be sufficient to supply all of the needed 7.6 GW of facility power. Using present-day costs for solar panels, the capital cost for this power plant comes in at $8 billion (i.e., the cost of 2 SLS rocket launches). $80 million per year covers operation and maintenance. Here’s Jackson on the cost:
…3.3% of the proton-proton collisions yields a useable antiproton, a number based on detailed particle physics calculations . This means that all of the kinetic energy invested in 66 protons goes into each antiproton. As a result, the 20 g/yr facility would theoretically consume 6.7 GW of electrical power (assuming 100% conversion efficiencies). Operating 24/7 this power level corresponds to an energy usage of 67 billion kW-hrs per year. At a cost of $0.01 per kW-hr the annual operating cost of the facility would be $670 million. Note that a single Gerald R. Ford–class aircraft carrier costs $13 billion! The cost of the Apollo program adjusted for 2020 dollars was $194 billion.
Science Along the Way
Launching missions that take decades, and in some cases centuries, to reach their destination calls for good science return wherever possible, and Jackson argues that an interstellar mission will determine a great deal about its target star just by aiming for it. Whereas past missions like New Horizons could count on the position of targets like Pluto and Arrokoth being programmed into the spacecraft computers, the preliminary positioning information uploaded to the craft came from Earth observation. Our interstellar craft will need more advanced tools. It will have to be capable of making its own astrometrical observations, sending its calculations to the propulsion system for deceleration into the target system and orbital insertion, thus refining exoplanet parameters on the fly.
Remember that what we are considering is a hybrid mission, using one form of propulsion to attain interstellar cruise velocity, and antimatter as the method for deceleration. You might recall, for example, the starship ISV Venture Star in the film Avatar, which uses both antimatter engines and a photon sail. What Jackson has added to the mix is a deep dive into the possibilities of antimatter for turning what would have been a flyby mission into a long-lasting planet orbiter.
Let’s consider what happens along the line of flight as a spacecraft designed with these methods makes its way out of the Solar System. If we take a velocity of 0.02c, our spacecraft passes the outgoing Voyager and Pioneer spacecraft in two years, and within three more years it passes into the gravitational lensing regions of the Sun beginning at 550 AU. A mere five years has taken the vehicle through the Kuiper Belt and moved it out toward the inner Oort Cloud, where little is currently known about such things as the actual density distribution of Oort objects as a function of radius from the Sun. We can also expect to gain data on any comparable cometary clouds around Proxima Centauri or Epsilon Eridani as the spacecraft continues its journey.
By Jackson’s calculations, when we’re into the seventh year of such a mission, we are encountering Oort Cloud objects at a pretty good clip, with an estimated 450 Oort objects within 0.1 AU of its trajectory based on current assumptions. Moving at 1 AU every 5.6 hours, we can extrapolate an encounter rate of one object per month over a period of three decades as the craft transits this region. Jackson also notes that data on the interstellar medium, including the Local Interstellar Cloud, will be prolific, including particle spectra, galactic cosmic ray spectra, dust density distributions, and interstellar magnetic field strength and direction.
Image: This is Figure 7 from the paper. Caption: Potential early science return milestones for a spacecraft undergoing a 10-year acceleration burn with a cruise velocity of 0.02c. Credit: Gerald Jackson.
It’s interesting to compare science return over time with what we’ve achieved with the Voyager missions. Voyager 2 reached Jupiter about two years after launch in 1977, and passed Saturn in four. It would take twice that time to reach Uranus (8.4 years into the mission), while Neptune was reached after 12. Voyager 2 entered the heliopause after 41.2 years of flight, and as we all know, both Voyagers are still returning data. For purposes of comparison, the Voyager 2 mission cost $865 million in 1973 dollars.
Thus, while funding missions demands early return on investment, there should be abundant opportunity for science in the decades of interstellar flight between the Sun and Proxima Centauri, with surprises along the way, just as the Voyagers occasionally throw us a curveball – consider the twists and wrinkles detected in the Sun’s magnetic field as lines of magnetic force criss-cross, and reconnect, producing a kind of ‘foam’ of magnetic bubbles, all this detected over a decade ago in Voyager data. The long-term return on investment is considerable, as it includes years of up-close exoplanet data, with orbital operations around, for example, Proxima Centauri b.
It will be interesting to see Jackson’s final NIAC report, which he tells me will be complete within a week or so. As to the future, a glimpse at one aspect of it is available in the current paper, which refers to what the original NIAC project description referred to as “a powerful LIDAR system…to illuminate, identify and track flyby candidates” in the Oort Cloud. But as the paper notes, this now seems impractical:
One preliminary conclusion is that active interrogation methods for locating 10 km diameter objects, for example with the communication laser, are not feasible even with megawatts of available electrical power.
We’ll also find out in the NIAC report whether or not Jackson’s idea of using gram-scale chipcraft for closer examination of, say, objects in the Oort has stood up to scrutiny in the subsequent work. This hybrid mission concept using antimatter is rapidly evolving, and what lies ahead, he tells me in a recent email, is a series of papers expanding on antimatter production and storage, and further examining both the electrostatic trap and electrostatic nozzle. As both drastically increasing antimatter production, as well as learning how to maximize small amounts, are critical for our hopes to someday create antimatter propulsion, I’ll be tracking this report closely.