Our recent discussions about Claudio Maccone’s FOCAL mission to the Sun’s gravitational focus, and the ongoing work at the Jet Propulsion Laboratory for NASA’s Innovative Advanced Concepts office, have had Alex Tolley thinking about alternative scenarios. Yes, a spacecraft moving along the focal line extending from the solar gravitational lens (SGL) would be capable of extraordinary imaging, and could serve as a communications relay for interstellar probes, but that tricky Sundiver maneuver suggested by Slava Turyshev and team in their ‘string of pearls’ concept puts huge demands on sail materials. Moreover, we’d ideally like to be able to slow the craft as it moves along the focus, to allow maximum time for observations. To achieve both fast transit and maneuverability at the gravitational focus, Alex advocates beamed propulsion, a method whose advantages and consequences are discussed below. Synergies with the ongoing Breakthrough Starshot effort are apparent.
by Alex Tolley
The huge increase in discovered exoplanets, many in the habitable zones (HZ) of their stars, has increased the push to determine if life exists on any of these worlds. With even our best telescopes, light from these worlds is just a single pixel in extent, which allows spectrographic analysis for biosignatures and techno-signatures. However, a 2D image of an exoplanet would answer many more questions about these worlds. The image makes a lot of difference both to scientists and the public. The astronomy books of my youth showed the moons of the gas giants as points of light, and Pluto was just a star-like object. As late as 2010, the Hubble telescope could only manage a crude blurry image of Pluto showing some light and dark regions. The New Horizon probe flyby with high-resolution images changed our view of the Pluto-Charon binary.
Image: Hubble 1200×1200 pixel image of Pluto in February 2010.
To acquire a megapixel image of even a nearby exoplanet would require a telescope of tens of kilometers in diameter to collect the light of the planet while masking out the far more intense light of its star. A possible solution was proposed by Claudio Maccone, with his proposed FOCAL mission to use our sun’s gravity to focus distant light rays on a telescope . While there are technical issues still to be resolved in how to form a low noise 2D image of 10,000 (100 x 100) to 1 million (1000 x 1000) pixels, the big question is:
“How do we get there in a short enough time for a project , and how do we best collect the data to make the image?”
The Sundiver Problem
The solar gravitational focus (SGF) is a line extending from about 550 AU to infinity. The focus starts out in interstellar space. For comparison, Voyager 1 launched over 40 years ago and has just passed through the heliopause of our sun and into interstellar space. It is only ¼ of the way to the SGF. It will not reach the focus for another 120 years. Clearly, we need a much faster way to reach the focus so that scientists and engineers can reasonably engage in a realistic mission, rather than taking the generations that were required to build a cathedral.
At this point, we can rule out most propulsion technologies, even using gravity assists. The most promising is a solar sail, and as I will argue, a sail augmented with a beam.
The authors of the recent NIAC II report [1, 6] opt for a pure solar sail. They propose a mission that could reach the SGF within a reasonable 25 years using a very advanced solar sail and leveraging a ‘sundiver’ trajectory. This sail could achieve a velocity of 25 AU/yr (about 125 km/s), of which most of that velocity is achieved quickly as the sail leaves perihelion. When I say ‘advanced,’ the mission needs a sail that has an areal density of less than 10 g/m2, and possibly 1 g/m2. Note that this density is not just the sail material, but must include the probe structure, payload, and any auxiliary equipment, such as communications, maneuvering thrusters, etc.
To put this in perspective, the Planetary Society’s CubeSat LightSail has an areal density of about 143 g/m2, a similar density as the upcoming NEA Scout probe. The target areal density of the Breakthrough Starshot beamed sail is 1.4 g/m2, achieved by having just a chip-sized payload on the sail of just 1 m2 .
Without such low areal densities, 2 to 4 orders of magnitude lower than currently achieved, there is no hope of reaching the needed velocities. The sail craft must also do a sundiver maneuver to gain maximum thrust at perihelion. Maximum velocity is achieved by the orbit of Saturn. How close the sail can approach the sun depends on the sail materials. To get both the very low sail mass to achieve the needed areal density, and approach the sun to within 0.1 AU (15 million km) requires a strong, low-density material with a high melting point, such as a ceramic.
If such a solar sail is achievable, then the craft with its telescope payload will continue on into interstellar space along the focal line.
While the focal line continues to infinity, is there an optimum “sweet spot” to image the target? Maccone states that although complex, the best position might be fairly close to the 550 AU start of the SGF . If that is correct, it is suboptimal to allow the telescope to continue traveling rapidly away from that position.
The NIAC authors solve this rather cleverly: launch a series of sails, a year apart, forming a “string of pearls” separated by 25 AU, so that each probe stays within 25 AU of the start of the SGF at 550 AU before handing off the data collection to the next probe, or even contributing to the collection of other data as it journeys on into interstellar space.
This solution has several disadvantages, not the least of which is the need to ensure that each sail can align independently and correctly with the target.
Therefore, ideally, one would want the sailcraft and its telescope to effectively stop at the best distance just beyond 550 AU. This has several advantages:
- The focus remains unchanged – reducing issues with image deconvolution
- The required coronagraph to isolate the Einstein ring of light is fixed in dimension and position.
- The tracking of both the target star’s position and the exoplanet in its orbit is simplified as the distance to track relative motion increases as the distance from the sun increases.
- Only one imaging telescope is needed, as well as the auxiliary equipment. The scale economies of building and flying multiple sails can be used to image multiple targets instead of just one.
- Data collection can be as long as desired, not fixed for the number of sails sent, allowing better images to be produced, as well as longer-term observations of the target for other purposes. If the payload includes a receiver for a communication bridge to a probe orbiting an exoplanet, the communication link can be established for as long as needed, perhaps many decades.
Using sail material that is off-the-shelf, how could we achieve such a mission?
The Benefits of Beamed Propulsion
My proposed solution is to use a beamed sail. The beam would most likely be a phased laser array located on Earth with the capability of pushing a sail in all but the highest latitudes. This is the type of power source that is being studied by the Breakthrough StarShot team. Because the sail need not achieve the fractional c velocities needed for interstellar flight, the size of the sail, payload, and beam intensity on the sail can be matched to the materials and payload requirements. Ideally, the sail velocity would exceed that of 25 AU/yr to shorten the flight time, but no longer restricted to the high-performance sundiver mission needed to reach that velocity. There is also no need to tolerate high temperatures at perihelion, although this will also depend on the laser power and duration.
Moreover, there is a way to stop a beamed sail, which was suggested by Robert Forward . Figure 1 below shows a schematic of the concept for this deceleration maneuver.
The sail would separate into 2 parts with the larger part focussing the beam on the smaller sail to decelerate it. This was proposed by Forward as a way to decelerate a sail and its payload on arriving at its desired destination. The same approach could be used to decelerate a beamed sail when it reaches the 550 AU minimum focus position. When “stopped”, the sail would deploy its imager/receiver.
Image: Schematic of the round-trip interstellar lightsail concept proposed by Forward (not to scale), shown during the deceleration phase. Credit: Geoffrey Landis .
The relative sizes of the two sails will depend on the achievable laser strength on the sail over the deceleration distance that ends around 550 AU, and on how well the reflected beam can be targeted to the sail with the payload. The aim is to slow the smaller sail with the telescope payload down towards a dead stop, although any low velocity, like that of Voyager 1, is adequate to ensure a long data collection period and minimal changes to the required maneuvering to track the star and planet. Pointing the laser at the sail will require relatively simple position prediction up to about 160 hours in the future based on the most recent time-stamped location received from the craft. [160 hours is 2x the light travel time to 550 AU; signal sent from the craft that reaches earth 80 hours later and the next 80 hours for the beam to reach the sail.]
The slow movement of the stars relative to the sun will still require some traversal across the focus to track this motion and ensure the craft stays within the best position to receive the communication or visual image. For example, Proxima moves about 3.85 arcseconds per year across the heavens. At 550 AU, this implies that to keep the sun in line with Proxima, the craft must travel a modest 50 m/s to maintain position.
One issue not so far mentioned is the position of the stellar target in the sky. Unlike most of our planetary missions to date, where the planets fairly closely aligned with the ecliptic, the target stars for an SGL mission are spread out over the celestial sphere. For example, Alpha Centauri is in the southern sky with a declination of over -62 degrees. The TRAPPIST 1 planetary system, about 40 ly away, with some possibly very interesting exoplanets in close proximity for imaging, is about -5 degrees of declination when observed from Earth. The sail craft must travel in the opposite direction so that the target is directly behind the sun, so that for Proxima the telescope must be positioned with a declination of over +62 degrees and the appropriate 180 degrees offset to right ascension.
Each sail craft can only image one target star at a time, although if that star has several planets, all these planets may be imaged over time. The stars are far too separated over the celestial sphere for any reasonable time to navigate between them. For the 30 pc (100 ly) volume, the 15,500+ FGK stars would be separated by about 60 AU on average. Therefore each sailcraft could only image one star system. This would therefore require considerable care in selection. However, given the laser infrastructure and the scale economy of manufacturing the sail craft, it might well make sense to send many craft out to their SGF positions.
The benefit of a FOCAL mission to acquire relatively high resolution images of exoplanets far beyond any single telescope we can envision today is offset by the demands of reaching the gravitational focal line starting at 550 AU. Once there, ideally, the craft should not continue its outbound journey. To achieve this, a beamed sail that can decelerate its payload is proposed.
1. Gilster, P. (2020) JPL Work on a Gravitational Lensing Mission, https://www.centauri-dreams.org/2020/12/16/jpl-work-on-a-gravitational-lensing-mission/
2. Landis, G (1989). Optics and Materials Considerations for a Laser-propelled Lightsail” http://www.geoffreylandis.com/lightsail/Lightsail89.html
3. Vulpetti, G., Johnson, L., & Matloff, G. L. (2008). Solar sails: A novel approach to interplanetary travel. New York: Copernicus Books.
4. Montgomery, E, Johnson, L, (2017) Solar Sail Propulsion: A Roadmap from Today’s Technology to Interstellar Sailships. Presentation to Foundations of Interstellar Studies Workshop, New York City College of Technology, Brooklyn, NY
5. Maccone, C. (2009) Deep Space Flight And Communications: Exploiting The Sun As A Gravitational Lens (Springer Praxis Books / Astronautical Engineering)
6. Turyshev et al. (2020), “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission.” Final Report. NASA Innovative Advanced Concepts Phase II
7. Forward, R (1984) “Roundtrip interstellar travel using laser-pushed lightsails,” American Institute of Aeronautics and Astronautics, v21 No.2, pp 187-195