Proposed First Gravity Lens Mission by 2028 that Could Spot Large Islands on Exoplanets by 2050

A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Earth diameter is 12724 kilometers. Ten-kilometer resolution would be better than a one-megapixel image.

Above : Left: original RGB color image with a (1024´1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~1000 per color channel, or overall SNR of 3000; right: the result of image deconvolution

The resolution used against the Earth would let an observer identify Java, the main island of Indonesia and island of Cuba.

Executive Summary: Innovations and Advanced Concepts Enabled
Direct multipixel imaging of exoplanets requires significant light amplification and very high angular
resolution. With optical telescopes and interferometers, we face the sobering reality: i) to capture even a single-pixel image of an “Earth 2.0” at 30 parsec (pc), a ~90 kilometer (km) telescope aperture is needed (for the wavelength of l = 1 µm); ii) interferometers with telescopes (~30 meter) and baselines (~1 kilometer) will require integration times of ~10,000 years to achieve a signal-to-noise ratio, SNR=7 against the exozodiacal background. These scenarios involving the classical optical instruments are impractical, giving us no hope to spatially resolve and characterize exolife features.

To overcome these challenges, in a NIAC Phase II study they examined the solar gravitational lens (SGL) as the means to produce direct high-resolution, multipixel images of exoplanets. The SGL results from the diffraction of light by the solar gravitational field, which acts as a lens by focusing incident light at distances >548 AU behind the sun (Figure 1). The properties of the SGL
are remarkable: it offers maximum light amplification of ~100 billion and angular resolution of ~10 billionth of ab arcsec, for l = 1 µm. A probe with a 1-meter telescope in the SGL focal region (SGLF), namely, in its strong interference region, can build an image of an exoplanet at 30 pc [100 light years] with 10-km scale resolution of its surface, which is not possible with any known classical optical instruments. This resolution is sufficient to observe seasonal changes, oceans, continents and surface topography.

They reached and exceeded all objectives set for our Phase II study:
* They developed a new waveoptical approach to study the imaging of exoplanets while treating them as extended, resolved, faint sources at large but finite distances.
* They designed coronagraph and spectrograph instruments needed to work with the SGL.
* They properly accounted for the solar corona brightness.
* They developed deconvolution algorithms and demonstrated the feasibility of high-quality image reconstruction.
* They identified the most effective observing scenarios and integration times.

As a result, they are now able to estimate the SNR for light from realistic sources in the presence of the solar corona. They have proven that multipixel imaging and spectroscopy of exoplanets up to 30 pc [100 light years] are feasible. By doing so, they were able to move the idea of applications of the SGL from a domain of theoretical physics to the practical mainstream of astronomy and astrophysics. Under a Phase II NIAC program, they confirmed the feasibility of the SGL-based technique for direct imaging and spectroscopy of an exoplanet, yielding technology readiness level (TRL) of TRL 3.

They have developed a new mission concept that delivers an array of optical telescopes to the SGL focal region and then flies along the focal line to produce high resolution, multispectral images of
a potentially habitable exoplanet. Our multisatellite architecture is designed to perform concurrent observations of multiple planets and moons in a target exoplanetary system. It allows for a reduction in integration time, to account for target’s temporal variability, to “remove the cloud cover”.

In this Report, they describe the mission architecture and the relevant technology steps, which they can begin today, that would allow the launch of a Solar Gravity Lens Focus mission by 2028-2030.

About Six Times Faster than Voyager Less Than 25 Year Travel Time – Maybe 2050 Arrival

The new architecture developed in this study uses smallsats (Less than 100 kg) with solar sails to fly a trajectory spiraling inward toward a solar perihelion of 0.1-0.25 AU and then out of the solar system on a nearly radial-out trajectory at 15-25 AU/year. Our design goal is 25 AU/year, to permit reaching the SGLF region in less than 25 years (maybe 2050 arrival). A long time, but less than it took Voyager to reach the heliopause at less than a fifth of the distance of our goal in the interstellar medium (ISM). We would reach the heliopause and enter the ISM in ~7 years, compared to the ~40 years of Voyager.
Today we are technologically ready to seize the unprecedented opportunity of using the SGL with a mission transit time only ~2.5´ longer than the transit time of New Horizons to Pluto.

The SGLF CONOPS uses multiple small satellites in an innovative “string-of-pearls” (SoP) architecture where a pearl consisting of an ensemble of smallsats is periodically launched. As a series of such pearls are launched (to form the “string”) they provide the needed comm relays, observational redundancy and data management needed to perform the mission. For example, if pearls are launched annually, then they will fly outward towards and then along the SGLF at 20 AU intervals.

By employing smallsats using AI technologies to operate interdependently, we build in mission flexibility, reduce risk, and drive down mission cost. This makes possible concurrent investigations of multiple exosolar systems by launching strings towards multiple exoplanet candidates.

We concluded that most of the technologies for SGLF mission either already exist (rideshare/cluster launch, sailcraft, RF/optical comm, all at TRL9), or are at intermediate levels of readiness: Sail materials (TRL 2-3), thermal management in solar proximity (TRL7), swarm operations (TRL5), terabit onboard processing (either FPGA or GPU, TRL 9/7), CONOPS (TRL7).

What is missing is the system approach to assemble all these technologies for autonomous operations in deep space (TRL3).

There is a clear path on how to close this gap, maturing the SGLF concept to TRL 4-5.

This affordable architecture design reduces cost in many ways:
1) It cuts the cost of each participant by enabling multiple participants (space agencies, commercial firms, universities, etc.) broad choices of funding, building, deploying, operating, and analyzing system elements.
2) It delivers economies of scale in an open architecture designed for mass production to minimize recurring costs.
3) It drives down the total mass (and thereby both NRE and recurring costs) by using smallsats.
4) It uses solar arrays of realistic size (~16 vanes of 1000 square meters) to achieve high velocity at perihelion (~150 km/s).
5) It applies maturing AI technologies to allow virtually autonomous mission
execution, eliminating the need for operator-intensive mission management,
(6) It reduces launch costs by relying on “rideshare” opportunities to launch the smallsats, avoiding the costs of large dedicated launchers, and
7) the SoP approach makes possible concurrent and affordable investigations of multiple exosolar systems by launching strings towards multiple exoplanet candidates.

The SGLF mission concept proposes three innovations:
i) a new way to enable exoplanet imaging,
ii) use of smallsat solar sails to go further and faster at lower cost into the interstellar medium, and
iii) an open architecture to take advantage of swarm technology in the future. It enables entirely
new missions, providing a great leap in capabilities for NASA and the greater aerospace community.

It lays the foundation for fast transit (over 20 AU/yr) and exploration of our solar system and beyond (outer planets, moons, Keiper Belt Objects (KBOs), and interstellar objects/comets).

Treatment of extended sources

The entire image of an Earth-like planet at 30 pc is compressed by the SGL into a cylinder with a diameter of ~1.3 km in the vicinity of the focal line. The telescope, acting as a single-pixel detector while traversing this region, can build an image of the exoplanet with kilometer-scale resolution of its surface.

NOTE- Everything along the path to the exoplanet can be observed with 100 billion time amplification. You can look beyond at stars and galaxies.

Assuming that the planet is positioned at z0 = 30 pc away from the Sun, we estimated the signal from the planet as Qplanet = 8.01´104 (d/1 m)2 (650 AU/z)
1/2(30 pc/z0) (l/ 1 µm) photons/s.

This estimate translates to a flux of 240,000 photon/s for an exoplanet at z0 = 10 pc and 1.85 million photon/s for at z0 =1.3 pc. Figure 11 summarizes the photon fluxes and the relevant signal-limited
SNR (i.e., no noise) as a function of heliocentric distance.

Using these estimates, we compared the performance of a conventional telescope against one aided by the SGL. The angular resolution needed to resolve features of size D in the source plane requires
a telescope with aperture dD~1.22 (l/D) z0 ~1.19´105 km = 18.60 RÅ, which is not realistic. The photon flux of a d=1m telescope from such a small area on the exoplanet yields the value of 1.97
´ 10-8 photons/s, which is extremely small. Comparing this flux with Qplanet received with the SGL, we see that the SGL, used in conjunction with a d = 1 m telescope, amplifies the light from the
directly imaged region (i.e., an unresolved source) by a factor of ~3.38´109 (d/1m)(650 AU/z)1.5(z0/30 pc)2 . This estimate justifies using the SGL for imaging of faint sources.

Realistic Signal to Noise

They accounted for the zodiacal background, solar corona brightness, spacecraft jitter, realistic losses, etc. We assumed a coronagraph suppression of one million. With these assumptions, they estimate that a 1-m telescope, operating beyond 650 AU would allow reaching a post-deconvolution SNR of 7 in ~1 year, yielding an image of this target with (100´100)-pixel resolution. Ten thousand pixels.

Creating a megapixel image requires one million separate measurements. For a typical photograph, each detector pixel within the camera is performing a separate measurement. This is not the case for the
SGL. Only the pixels in the telescope detector that image the Einstein ring measure the exoplanet, and the ring contains information from the entire exoplanet, due to the blur of the SGL and also to
the relative distribution of different regions of the exoplanet to different azimuths of the ring.

What is encouraging is that temporal variability in the cloud cover helps the deconvolution. Assuming N~50 observations of every pixel, clouds “disappear” after ~10 observations. If spectroscopic data is also used, we can reduce this issue and “see through” the clouds. The deconvolution of the data from several s/c allows to see the surface of the Earth in a few months of data.

By studying direct deconvolution, we have shown that with a 1-m telescope we would need ~1 year to build a (100´100)-pixel image with SNR~7. Two factors that can reduce the integration time by a factor of up to 100 times are i) the number of image pixels, N, and ii) the telescope diameter.

A larger image of 1000X1000 pixels of an exoplanet at 25 pc may be produced in ~6 years with a 2-meter telescope. This time may be reduced if there are time-varying features of predictable periodicity on the planet’s surface or in its atmosphere. Also, the integration time is reduced by a factor of ~1/n if we fly n imaging spacecraft. Six 2-meter scopes would let the megapixel image be produced in one year. Seventy 2-meter scopes could let us make a megapixel image in one month.

Accounting for the motion of the Sun in the solar system BCRF, consider the plate scale. The SGL reduces the size of the image of the exoplanet at 30 pc by a factor of ~10,000 at 650 AU. An orbital radius of 1 AU becomes ~150000 km and an orbital velocity of 30 km/s translates into 3 meters per second. Solar gravity accelerates the Earth at 6 mm/s2. Consequently, the imager s/c needs to accelerate at ~1 micron per second squared to move in a curved line mimicking the motion of the exoplanet.

For a given number of desirable pixels, more distant (fainter) planets will require longer integration times resulting in a longer imaging mission phase. In fact, image quality improves with time allowing for more repeat scanning of the same pixel and also as spacecraft moves to further heliocentric ranges the SNR increases. These factors allow for an improved image reconstruction. It is desirable for the duration of the imaging mission phase to be on the order of 10 years. This would translate into much-increased image quality and temporal resolution of the atmospheric and surface processes occurring on the target exoplanet.

Mass Production of Smallsats

However, the SGL mission requires radioisotope power. It will easily cost billions for one first mission. Technology and methods can be drastically improved to try to get each missions costs down to the tens of millions each. However, it would be very beneficial to drive the costs down so more solar systems could be observed.

SpaceX is mass-producing Starlink satellites of comparable size for less than $1 million. It could be possible to bring the costs for these types of satellites down to $100,000. Mass producing a million would cost about $100 billion. About $200 billion was spent on the international space station. We could have eighty satellites observing each of over ten thousand solar systems within 100 light-years.

Test Flight

Interplanetary smallsats are still to be developed – the recent success of MarCO brings them perhaps to TRL 7. Solar sails have now flown – IKAROS and LightSail-2 already mentioned, and NASA is preparing to fly NEA-Scout. Scaling sails to be thinner and using materials to withstand higher temperatures near the Sun remains to be done. As mentioned above, we propose to do this in a technology test flight to the aforementioned 0.3 AU with an exit velocity ~6 AU/year. This would still be the fastest spacecraft ever flown. They have roughly estimated could be done within three years at a cost less than $40 million – and using a rideshare launch to approximately GEO.

Getting There- Existing and Nearterm Technology

Interplanetary smallsats are still to be developed – the recent success of MarCO brings them perhaps to TRL 7. Solar sails have now flown – IKAROS and LightSail-2 already mentioned, and NASA is preparing to fly NEA-Scout. Scaling sails to be thinner and using materials to withstand higher temperatures near the Sun remains to be done. As mentioned above, we propose to do this in a technology test flight to the aforementioned 0.3 AU with an exit velocity ~6 AU/year. This would still be the fastest spacecraft ever flown.

Analysis based on current materials show that solar orbit injection with a perihelion distance of 10 solar radii (approximate orbit of the NASA’s Parker Solar Probe) generates exit velocities of 15–18 AU/yr for A/M ratio of 100–200 m2/kg, and 25 to 30 AU/yr for A/M of 400–600 m2/kg.

Key technologies being developed to drive down weight risk and cost include solar sail materials, solar sail propulsion control, higher speed computers and rad-hard computers. Relevant developments in the next 10 years anticipate battery density (J/kg) to increase by factor of 2 to 4, removing about 5-7 kg of mass per SGL s/c. Onboard clocks can foresee a factor of 100 improvement in chip scale atomic clocks and 33 years for 1 Hz drift (0.1 ppm). Star trackers with high resolution data from Gaia mission should reach 1 microarcsecond angular resolution.

Millimeter-wave D-band RF antenna arrays could provide for efficient RF crosslink and lower SWaP (e.g. NuvoTronics). NASA’s Inter-spacecraft omnidirectional optical communicator (ISOC), or Honeywell’s Optical Pointing and Tracking Relay Assembly (OPTRAC) 10Gps optical could enable intersatellite communication. And for large-volume data transfer downlink communication NASA’s Terabyte Infrared Delivery (TBIRD) Program could be utilized.

Coronagraph

A design for the SGL coronagraph (Zhou, 2018) rejects sunlight with a contrast ratio of about ten million. At this level of rejection, light from the solar disk is completely blocked to the level
comparable to the brightness of the solar corona. Taking a further step, we consider two possible coronagraph concepts: A conventional coronagraph (which we call a “disk coronagraph”) that blocks light only from the solar disk and the solar corona up to the inner boundary, b-, of the l/d annulus centered on the Einstein ring, and a coronagraph that also blocks light outside the outer boundary, b+, of the l/d-annulus centered at the Einstein ring (the “annular coronagraph”).


Figure 12 describes the relevant sizes and observing configuration.


FIG. 9: The annular coronagraph concept. The coronagraph blocks light from both within and outside the Einstein ring.
The thickness of the exposed area is determined by the diffraction limit of the optical telescope at its typical observational
wavelength.

Solar coronagraphy was invented by Lyot to study the solar corona by blocking out the Sun and reproducing solar eclipses artificially. Coronagraphs are also considered to block out light from point sources, such as the host star of an exoplanet imaged with conventional telescope. The SGL coronagraph is different, as it needs to block the light from the Sun and the solar corona, leaving visible only those areas where the Einstein ring appears.

SOURCE – Arxiv – Final Report Phase 2 – DIRECT MULTIPIXEL IMAGING & SPECTROSCOPY OF AN EXOPLANET WITH A SOLAR GRAVITATIONAL LENS MISSION, Turyshev
Written By Brian Wang, Nextbigfuture.com

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