September 19, 2019
As part of NASA’s continued effort to ensure a steady supply of astrophysics and astronomy missions, the agency is undertaking the Astro2020: Decadal Survey on Astronomy and Astrophysics. Currently, in the “Concept Study” phase, the survey includes proposals for four large-scale space telescopes – including the Habitable Exoplanet Observatory (HabEx).
HabEx would facilitate direct observation of exoplanets, carry a primary focus on imaging Earth-like planets around Sun-like stars, and be able to detect biomarkers or signs of possible life in those exoplanets’ atmospheres via spectroscopic observations.
HabEx – taking exoplanet research to the next level:
For millennia, the question of whether or not humanity is alone in the universe has captivated the minds of explorers and scientists.
But until recently, an important part of that equation remained elusive: exactly how many exoplanets exist in our galaxy and the universe?
The first confirmed detection of an exoplanet occurred in the early 1990s, with subsequent observations confirming the earliest detection actually occurred in the 1980s. But ground-based observations were slow and far between.
To help solve the question once and for all, NASA launched the Kepler Space Telescope in 2009 – a telescope tasked solely with searching for exoplanets and determining how common they are.
Kepler shattered all expectations of the number of exoplanets near Earth, revealing over the course of its multi-year mission that not only are exoplanets common throughout all regions within the visible space surrounding Earth but that almost every single star hosts at least one planet.
What’s more, Kepler revealed an astonishing number of exoplanets that orbit within the so-called habitable zone of their parent stars – the zone in which liquid water can exist on the surface of a terrestrial planet.
As of 1 September 2019, there are 4,109 confirmed exoplanets in 3,059 systems, with 667 systems having more than one planet.
With that discovery, the desire to create better telescopes capable of directly imaging exoplanets and sampling their atmospheres catapulted to the top of the astrophysics mission wish lists.
But so far only very large planets, many times larger than Jupiter and far away from their host stars have been imaged directly. Spectroscopic observations of exoplanet atmospheres are possible using telescopes and technology is possible but rare and very limited. The holy grail of directly imaging Earth-like planets around Sun-like stars is currently not possible.
Thus, the holy grail of directly imaging Earth-like planets around Sun-like stars is currently not possible.
HabEx with its starshade performing operations NASA/JPLK-Caltech
Enter HabEx. This proposed mission carries the stated goals of:
Seeking out nearby worlds and exploring their habitability
Mapping out nearby planetary systems and understanding the diversity of the worlds they contain, and
Enabling new explorations of astrophysical systems from our solar system to galaxies and the universe by extending our reach in the ultraviolet, optical, and near-infrared spectrum.
How will HabEx work?
When directly observing exoplanets, the biggest problem to overcome is the glare of the host star, which is billions of times brighter than the exoplanet.
Two of HabEx’s four instruments are designed to do exactly that.
The first instrument is a star shade, which is actually a second spacecraft that would fly in formation at an average distance of 124,000 km in front of HabEx and block most of the host star’s light but not the light of planets in orbit of the host star.
A dedicated instrument on HabEx would then pick up the light of these exoplanets and measure their spectrum.
With enough observation time, HabEx’s instruments would be able to measure the concentration of water vapor, oxygen, ozone, and dust through Rayleigh scattering.
SETI Institute “Next-Generation NASA Space Telescopes” 1:13:30
The telescope would also be able to detect carbon dioxide and methane in an exoplanet’s atmosphere if they were present in higher concentrations than on Earth.
The observation campaign would theoretically involve nine nearby solar systems at a distance of 10 to 20 light years.
They would be observed three times each with an accumulated observation time of three months within the first five years of the telescope’s operation.
Due to the nature of formation flying, pointing the star shade on a different target would be a slow, time-consuming process. Therefore, during times when the star shade would be repositioned on another star system, HabEx would use its other instruments, including a vector vortex coronagraph, to create family portraits of around 110 exosolar systems.
The coronagraph, a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star’s bright glare – can be resolved, would block the light of the host star but not the light of the exoplanets in the system.
Within the first 5 years, the coronagraph would be used for 3.5 years to create family images of 110 exosolar systems and detect dust, asteroid belts, and Kuiper Belt-like regions of exosolar systems. From these observed star systems, the ones with rocky planets in their respective habitable zones would be scheduled with star shade observations.
Exoplanet observation with HaBEx and its star shade. NASA/JPL-Caltech
Together, the star shade and coronagraph would take about 75% of the first 5 years of observation time. The remaining 25% would be dedicated to the scientific community, which would submit observation proposals via a similar selection process as used today for the Hubble Space Telescope.
In addition to the coronagraph and star shade, HabEx is proposed to contain two other important instruments: the HabEx Workhorse Camera (HWC) and the UV Spectrograph (UVS).
The UVS intended for HabEx would provide 10 times larger area coverage compared to Hubble’s equivalent: the Cosmic Origins Spectrograph. With the Hubble Space Telescope close to the end of its life, the astronomic community will lose its only UV spectrograph with HST. UVS would fill that gap.
Additionally, HabEx’s Workhorse Camera (HWC) would be an evolutionary step from Hubble’s Wide-Field Camera 3 and would provide imaging and multi-slit spectroscopy for two channels ranging from the near UV to the near IR.
When executing exoplanet observations, both HWC and UVS could also be used in parallel with the star shade and coronagraph.
What’s in a star shade?:
The star shade for the HabEx Observatory would have a diameter of 72 m, consists of several thin sheets of material, and would be scaled relative to its operational distance from HabEx so that the telescope would be able to observe Earth-like planets around sun-like stars at a distance between 10 and 20 light years.
A breakdown of HabEX. NASA/JPL-Caltech
It would have a 40 m diameter disk and 24 petals, each 16 m long and 5.25 m wide at its base for a structure tip-to-tip of 72 m. The total mass of the star shade is currently estimated at 2,520 kg with an additional 500 kg for the deployment mechanism.
The material used to create the shade would be made of multiple layers of carbon-impregnated black Kapton. A gap between the individual layers would minimize the risk of a micrometeorite hit inducing a direct line of sight path between the target star and the telescope.
The edges of each petal would also be chemically etched to produce a very sharp and smooth edge that minimizes light scattering.
The star shade would be attached to its control hub, which is currently projected to weigh in at 6,394 kg.
The hub would consist of propellant and control systems, including 12 hydrazine thrusters for station keeping with HabEx. These thrusters would use 1,407 kg of liquid bipropellant.
Additionally, the hub would be equipped with six xenon Solar Electric Propulsion (SEP) thrusters for retargeting. This would require 5,600 kg of xenon gas.
The amount of propellant planned would be enough for 100 individual pointings with an initial mission design of 18 pointings for the first 5 years.
Using a coronagraph on HabEx:
Coronagraphs are already in use for solar observations as well as in various ground-based telescopes and upcoming space missions such as the James Webb Space Telescope (JWST) and WFIRST.
Like these telescopes, HabEx’s coronagraph could only work well if the light path through the telescope is extremely stable and matches its design exactly. Any deformation due to thermal gradients, vibration in the spacecraft, polarization, and other effects would diminish its functionality.
The quality of optical surfaces must also be very high, which is why the coronagraph is the design driving element for many aspects of the HabEx telescope.
To limit the vibration of HabEx, the telescope would not employ reaction wheels for pointing. Instead, microthrusters would be used, as demonstrated by NASA’s Gravity Probe B and ESA’s (European Space Agency’s) Gaia and LISA Pathfinder missions.
The microthrusters would induce far less vibration to the system and would not be prone to failures as reaction wheels are.
To limit the thermal stress on the primary mirror, the instruments are housed on the side of the telescope.
Images: Hubble left, HabEX right
The diameter of HabEx main mirror is proposed at 4 m and designed to be made of 0-expansion glass ZERODUR, which would be heavier than other options but can be handled by the usual manufacturers without major hassle contrary to the Beryllium mirrors of JWST.
Moreover, the coronagraph would drive the focal length of the optical design (i.e.: the length of the telescope) to a long telescope.
How to launch HabEx:
Should HabEx be approved as a mission, the immediate question would become how to launch it.
In all, an integrated launch of HabEx and its star shade would place the launch mass at a little less than 35,000 kg with the launch needing to inject HabEx into the Earth-Sun L2 Lagrangian Point 1.5 million km from Earth.
In short, there aren’t many options.
NASA’s SLS Block 1B would be capable of launching the telescope. SpaceX’s Starship vehicle, while still in development with fluid performance numbers, is in a similar class as SLS 1B, and is thus another potential option
However, neither of those rockets exist operationally at this point, and even when/if they do, there are questions as to what they will ultimately be capable of doing.
SLS’s Block 1B future is precarious at best with an unknown funding situation of the crucial Exploration Upper Stage – which has already been delayed multiple years and has forced NASA to switch several early SLS missions to the Block 1 configuration – as well as an “at any cost” lunar landing objective by 2024 for which the Block 1B is in no way required and would divert funds and attention away from.
If SLS Block 1B does come to fruition and is used to launch HabEx, the telescope would benefit from the rocket’s capacity to throw more than 36,000 kg to the Earth-Sun L2 point.
The Earth-Sun L2 point would be the primary operation location for HabEx given the area’s flat gravitational gradient and an undisturbed thermal environment.
It would also allow for relatively easy servicing of HabEx as now mandated by the U.S. Congress in 2010 that all large spacecraft be serviceable.
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