We can turn Earth into a giant telescope. According to a recent study titled “The ‘Terrascope’: On the Possibility of Using the Earth as an Atmospheric Lens”, published by David Kipping of Columbia University, our planet offers an opportunity for pronounced lensing.
Building a Terrascope
The Earth’s mass is way too low to build a “terrascope” using “gravitational lensing” (see gravitational lensing below). But, luckily, distant starlight passing through the Earth’s atmosphere is refracted by an angle of just over one degree near the surface. The focus of this “lens” is located just beyond the orbit of the Moon. Placing an orbiting detector between Earth and one Hill radius Notes 1 could exploit this refractive lens – practically turning Earth into a telescope.
According to the study, a 1-meter Hill radius “terrascope” is calculated to produce an amplification of ~45,000 for a lensing timescale of ~20 hours.
But, In practice, the amplification is likely halved in order to avoid daylight scattering i.e. 22,500 for a 1-meter terrascope, or equivalent to a 150-meter optical/infrared telescope.
Sun into a telescope?
If we can turn Earth into a giant telescope, why not turn the Sun into a telescope even more giant?
Kipping says: “The Solar gravitational lens, first devised by Von Eshleman in 1979, is an old idea that inspired this work. It has some major drawbacks, such as requiring over a century to fly to the focus point. The Terrascope focus is on our doorstep located just beyond the orbit of the Moon.”
There’s even a proposed space telescope that would use the Sun as a gravity lens, the “Fast Outgoing Cyclopean Astronomical Lens (FOCAL)”. In order to use the Sun as a gravity lens, it would be necessary to send the telescope to a minimum distance of 550 astronomical units away from the Sun, which should take more than 100 years with today’s technology.
To turn Sun into a telescope, you’d need to observe gravitational lensing. First, you need a distant object which emits light, like a star. Second, the observer, in this case, a spacecraft. Last, you need something between the observer and the distant light source that’s massive enough to act as a lens – the Sun.
Unfortunately, even our Sun’s mass is way too low to build a telescope using gravitational lensing of Sun in the near future.
The gravitational lens effect was first derived by Albert Einstein, and the concept of a mission to the solar gravitational lens was first suggested by Radio Scientist, Planetary Explorer, and Electrical Engineering Professor Von Eshleman (1924-2017), and analyzed further by the Italian SETI astronomer, space scientist, and mathematician Claudio Maccone and others.
According to Albert Einstein’s general relativity, the light follows the curvature of spacetime, hence when light passes around a massive object, it is bent.
When a mass bends the light from a distant source as the light travels towards the observer, this effect is known as gravitational lensing.
It was not until 1979 that this effect was confirmed by observation of the so-called “Twin QSO” (a double quasar which is actually one quasar but has two images in the sky) SBS 0957+561. The double image is a result of gravitational lensing caused by the galaxy YGKOW G1 that is located in the line of sight between the quasar and our planet.
Why it appears like a twin quasar? You can think of normal glass lenses: they can make things look bigger and clearer, but when they are out of focus, you start seeing double.
The light from the quasar passing through the gravitational lens (galaxy YGKOW G1) does not converge perfectly from the other side. Instead of coming back to create one, nicely focused image, it creates two separate images of the quasar.
Because we can’t change the galaxy YGKOW G1’s distance from the observer, the Earth. But, while building a terrascope, we can fine-tune the “lens” by putting the orbiting spacecraft (the observer) into the right distance from our planet.
If you put the observer into the right distance from the Earth, you can build a terrascope. Please note that the terrascope won’t use gravitational lensing, because the Earth’s mass is way too low for that. It will use the effect of distant starlight passing through the Earth’s atmosphere refracted by an angle of just over one degree near the surface.
David Kipping is an Assistant Professor of Astronomy at Columbia University, where he researches extrasolar planets and moons. He leads the Cool Worlds Lab at Columbia.
Kipping is most well-known for his work on exomoons but his research interests also include the study and characterization of transiting exoplanets, the development of novel detection and characterization techniques, exoplanet atmospheres, Bayesian inference, population statistics and understanding stellar hosts.
He is the Principal Investigator (PI) of The Hunt for Exomoons with Kepler (HEK) project. Kipping also enjoys publicizing science and run a YouTube channel named “Cool Worlds” discussing his group’s research and related science.
1. Hill Radius (Hill Sphere)
Hill radius is the radius of a Hill Sphere. The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. The outer shell of that region constitutes a zero-velocity surface. To be retained by a planet, a moon must have an orbit that lies within the planet’s Hill sphere.
Every mass has a Hill sphere of its own.
The Hill sphere for Earth extends out to about 1.5 million kilometers / 932,000 miles (0.01 AU – Astronomical Unit, the distance between the Sun and Earth). The Moon’s orbit, at a distance of 0.384 million kilometers (239,000 miles) from Earth, is comfortably within the gravitational sphere of influence of Earth and it is therefore not at risk of being pulled into an independent orbit around the Sun.
Study: The “Terrascope”: On the Possibility of Using the Earth as an Atmospheric Lens by David Kipping on arxiv.org (Cornell University archive)
The gravitational lens on Wikipedia
Hill Sphere on Wikipedia
Fast Outgoing Cyclopean Astronomical Lens (FOCAL) on Wikipedia