Unpacking the proposed exo-planet imaging telescope HabEx

by Roland Winkler

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. (Credit: NASA/JPL)

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. 

Potential resolution difference between Hubble (left) and HabEx (right). (Credit: NASA/JPL)

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.

An early overview of Starship capabilities. (Credit: SpaceX)

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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