After all this time, I’m still trying to wrap my head around the idea of massive objects in space as lenses, their distortion of spacetime offering the ability to see distant objects at huge magnification. On Friday we saw how the lensing effect caused by galactic clusters can be used to study dark energy. And consider the early results from the Herschel-ATLAS project, conducted by ESA’s Herschel Space Observatory. Herschel is scanning large areas of the sky in far-infrared and sub-millimeter light. Many of its brightest sources turn out to be magnified by gravitational lenses, where light from a very distant object passes a galaxy much closer to the Earth, bending that light so that the image of the more distant galaxy is magnified and distorted.
Because Herschel has only covered one-thirtieth of the entire Herschel-ATLAS survey area, it’s likely that the project will uncover hundreds of gravitational lenses, offering astronomers the chance to probe galaxies in the early universe that would otherwise be hidden. Thus we learn about the evolution of galaxies from a time when the universe was only a few billion years old, not to mention the possibilities of studying dark matter and its effect on galactic lensing.
Lensing on the Small Scale
Closer to home, we’ve talked a lot in these pages about using the Sun’s gravitational lens for studies of everything from the cosmic microwave background (CMB) to exoplanets around nearby stars. The Sun’s lens is at 550 AU, but the focal line extends to infinity, meaning the spacecraft keeps moving outward while continuing its observation program. In fact, getting beyond 550 AU is a good idea, because it progressively diminishes the problem of solar coronal distortions.
The Tau Zero Foundation is continuing to advocate Claudio Maccone’s FOCAL mission, which would be the first attempt to get a spacecraft to our own Sun’s gravitational lens. In a recent visit, Maccone and I discussed the paper on FOCAL he had delivered at the International Astronautical Congress in Prague. Just as EPOXI’s second cometary pass has shown us how much a spacecraft’s mission can be extended by intelligent marshaling of its resources, so a mission to 550 AU offers up an entirely new set of observations as the vehicle continues to move outward from the Sun. These observations would be progressively more difficult, but they are worth examining for a potential mission trajectory into interstellar space.
For Maccone realized that even as observations of the Sun’s gravitational focus proceeded, a successor to the FOCAL spacecraft could, as it pushed ever deeper into space, tap the lenses of individual planets. The question of planetary gravitational lenses has come up on Centauri Dreams before, and Maccone has now gone into the specifics. If we must reach a minimum of 550 AU to make use of the Sun’s lens, how far do we travel to tap the lenses of the planets?
A Widening Series of Focal Spheres
Jupiter is the most massive planet, and we find that its focal sphere is about 1.1 light months out, or 6100 AU. That’s a useful number to remember, because it’s always possible that the Sun’s coronal effects may distort what we’re trying to look at on the other side of the Sun. If that is the case, we still have a lens at 6100 AU, and that becomes an obvious next target. Beyond this, Neptune’s focal sphere appears at 13,525 AU (2.6 light months). The fact that Neptune is next in line is due to the surprisingly high ratio of the square of its radius to its mass — Maccone demonstrates that the ratio of radius squared to mass is the key factor in this analysis.
Thus Saturn’s lens effect actually comes into play beyond Neptune’s, at 14,425 AU despite the difference in planetary size. As you see, we are now deep into the Oort Cloud, at a distance from the Sun farther than that of Proxima Centauri’s distance from Centauri A and B. Remarkably, the focal sphere of the Earth is found at 15,375 AU, closer than the focal sphere of Uranus, the point being that Earth is the body with the highest density (ratio of mass to volume) in the entire Solar System. Getting to the Earth’s gravitational lens would be useful because we know the composition of our planet’s atmosphere and surface better than that of any other planet. We would thus have maximum data for using its lens for observations.
Image: The complete BELT of focal spheres between 550 and 17,000 AU from the Sun, as created by the gravitational lensing effect of the sun and all planets, here shown to scale. The discovery of this belt of focal spheres is the main result put forward in this paper, together with the computation of the relevant antenna gains. Credit: C. Maccone.
A good part of Maccone’s presentation on the matter goes into the question of effective ‘gain’ — Maccone calculates numerical values for the gains at five selected frequencies, from the hydrogen line (1.420 GHz) to the CMB peak at 160 GHz, and evaluates each for planetary gravitational lenses as well as the Sun’s. Clearly, the Sun emerges as our primary target given the poor gain afforded by planets like the Earth, but if future antenna technologies emerge that make it possible to study the weak signatures of the latter, a number of advantages emerge.
The Beauty of Movable Lenses
Obviously, a FOCAL mission that could reach these distances would also qualify as a cometary observer, a spacecraft that would cross the inner Oort Cloud, and that has advantages of its own. But if we can develop the technologies for such a mission, we’ll also have an interesting new take on lensing. For if we start thinking in terms not of a single gravitational lens (the Sun’s) but a series of focal spheres between 550 and 17,000 AU — a series that the spacecraft would cross as it departs our system — then we can take advantage of the fact that we now have a selection of moving targets that paint the background sky with a broader brush.
From the paper:
> …while the Sun does not move in the Sun-centered reference frame of the Solar system, all the planets do move. This means that they actually sweep a certain area of the sky, as seen from the spacecraft, so that a spacecraft enjoys a sort of moving magnifying lens. How many extrasolar planets would fall inside this moving magnifying lens? Well, we don’t know nowadays, of course, but the over 400 exoplanets found to date are a neat promise that many more such exoplanets could be detected anew by a suitably equipped spacecraft crossing the distances between 550 and 17,000 AU from the Sun thanks to the gravitational lenses of the planets.
A moving, magnifying series of lenses that we study as the planets sweep out their orbits, on a mission that offers not only observations of distant astronomical phenomena but direct exploration of the Kuiper Belt and the Oort Cloud along the way. Maccone adds:
> …looking back to the work done thus far about the possibilities of a truly interstellar flight, it seem fair to say that all planners of the Alpha Centauri missions, in their efforts to reach 277,000 AU, have missed what was at hand at just 17,000 AU. Or, in terms of light time, in order to get all the way to 4.37 light years in a single shot, they have missed what was just three light months away, like the Earth’s focal sphere.
Pushing FOCAL to Its Limits
This is not the FOCAL mission we have discussed in these pages before. That mission is designed to be our first exploration of the Sun’s gravitational lens, one that will demand new developments in propulsion to accomplish its task within a fifty-year timeframe, but one that in the broader scheme of things is reasonably near-term. Think of the ‘moving lens’ mission as a follow-on, a more futuristic concept, one we can consider as a motivator to develop still faster technologies and the hugely sensitive antennae needed to pull down the data, not to mention the sophisticated communications demanded to relay the information back to Earth.
These are ambitious mission ideas, but it’s by thinking about what the universe offers us by way of observation and analysis that we set our goals. From the discovery of new exoplanets to the close study of galactic and extragalactic objects, the crossing of the space between 550 AU to 17,000 AU would be profitable in ways we have not before considered. Evaluate nearby space in terms of lensing opportunities and you begin to see the Solar System and neighboring stars in a different light, one that may even have SETI implications, as we’ll see in an upcoming story.
The paper is Maccone, “A New Belt Beyond Kuiper’s: A Belt of Focal Spheres Between 550 and 17,000 AU for SETI and Science.” I’ll update this with the complete reference when the paper is published.