The Sun as a Gravitational Lens Options

[JRehling]

I titled this topic thusly to avoid the connotation of "solar" as it is often used, speaking of the Sun's radiation as an asset.

This mission concept, which is almost shockingly radical, is on firm scientific grounds, but would deliver a tremendous capability at the cost of some considerable engineering resources.

The concept is: At a distance of >550 AU, the Sun could be used as a gravitational lens to magnify a given target located on the other side of the Sun. The magnification that could be achieved is in principle (here's a word you don't encounter often in the engineering realm) infinite. A recent study by Landis delves into the issues far better than I can here:

https://arxiv.org/abs/1604.06351

In a nutshell, a potential mission would fly a telescope out to >550 AU (four times Voyager 1's current distance) and would look back towards the Sun, which would be hidden behind an occulting screen, so that a target antipodal to the Sun would be magnified far beyond what any existing telescope can achieve. The image of, say, an exoplanet located tens of light years away would actually be too large (~10 km at the focal plane) for a telescope to collect the whole image at once. It would, in effect, see one "pixel" at a time, and to image an entire exoplanet, it would have to scan back and forth across the image, either actively or passively, either scanning one strip across the exoplanet or making some effort to gather a 2D image.

Because of the orbital dynamics, the craft would be essentially an interstellar craft rather than sun-orbiting as we normally think of it. It could not effectively observe multiple different targets because it would have to travel considerable distances to aim at a new target, although observing multiple planets in the same exoplanet system seems to be achievable. If we wanted to observe five different exoplanet systems, we would have to launch five different copies of this mission.

The Trappist-1 system introduces a case where a mission like this might have a respectable ROI. One telescope launched to Trappist-1's focal location from the Sun would perhaps be able to scan several or all of the planets in the system, either in 1D or 2D mode, perhaps repeatedly – the details depend upon the resources for propulsion to make the scanning work. The focal distance for Trappist-1, however, would be significantly farther than 550 AU. Landis' paper suggests that something more like 2500 AU would be required. That is very far, but not nearly as far as the stars are. It seems like the most painful requirement would be to get a telescope with a good amount of propulsive capability out to that distance before the mission planners die of old age.

The requirements are fantastic, but it seems like a good possibility that this would be cheaper than the seeming alternatives.

I'm not sure that I've ever read about a mission concept so radically different than anything I'd read before. It seems feasible – just hard.

[fredk]

Lensing by star- or planet-sized objects actually forms the basis of microlensing, which has been used to put strong constraints on models of dark matter, for example. In that case you're getting information about the distribution of the lenses, not about the sources.

In a cosmological context things are very different, since timescales are very long and we're seeing essentially a snapshot. There are many strong galaxy lenses known, including Einstein rings, which are useful for various reasons, such as measuring the expansion rate of the Universe.

[JRehling]

There are many strong galaxy lenses known, including Einstein rings

Indeed, my target for tonight's astrophotography is the Twin Quasar, which is one quasar seen twice because of gravitational lensing caused by a galaxy in fortuitous alignment between us and the quasar. I've imaged it before, but at just about the threshold of detection, not a very impressive picture. In that case, the quasar is about 9 billion years of light travel time away and the galaxy is roughly half as far. The fact that any such cases exist makes it seem like there must be a tremendous number of additional cases to be discovered. Still, any useful case for exoplanet imaging is probably very temporary, because a planet moves many, many times its radius in a relatively short time.