Title: MAUVE: An Ultraviolet Astrophysics Probe Mission Concept

Authors: Mayura Balakrishnan, Rory Bowens, Fernando Cruz Aguirre, Kaeli Hughes, Rahul Jayaraman, Emily Kuhn, Emma Louden, Dana R. Louie, Keith McBride, Casey McGrath, Jacob Payne, Tyler Presser, Joshua S. Reding, Emily Rickman, Rachel Scrandis, Teresa Symons, Lindsey Wiser, Keith Jahoda, Tiffany Kataria, Alfred Nash, and Team X

First Author’s Institution: Department of Astronomy, The University of Michigan, Ann Arbor, MI, USA

Status:  Published in Publications of the Astronomical Society of the Pacific, Volume 136, Number 10 [open access]

Welcome back avid readers to the astrobitedome (fans of Mad Max Beyond Thunderdome or the Thunderdome music festival this one’s for you). Today’s bite is another thrilling installment in what is, frankly, becoming a mini-series on the theoretical mission MAUVE (Mission to Analyze the UltraViolet universE). MAUVE was designed during the inaugural NASA Astrophysics Mission Design School (AMDS) held at the Jet Propulsion Laboratory in 2023 and the resulting paper is a gold mine for learning how to design a mission concept. Here are the bites so far in our mini-series and what they’ve covered:

1. An Exercise in Satellite Mission Design: The technical motivation for MUAVE (the need for a new UV space-based mission), science cases and their connection to the Astro2020 Decadal survey (making progress towards answering the field’s most pressing questions), and we briefly outlined the proposed instrumentation–a low resolution imaging spectrograph named THISTLE.
2. An Exercise in Satellite Mission Design: Getting Specific about Science Objectives: Defining physical and observable parameters (what’s the difference?), coming up with a physical parameter and almost more importantly–its precision, and outlining observables in a way that ensures we will measure the physical parameter to its required precision; all using an example science objective from MAUVE.

If you haven’t read our previous bites on MAUVE we highly recommend taking a look before diving into today’s bite! Today we will cover how the information from physical parameters and requirements on observables flow down into the instrument requirements.

What is a requirement anyway?

We know the definition of requirement–something that must be done–but a requirement has additional nuance in mission design. Requirements ensure that a mission achieves its science goals exactly as stated at minimum. Minimum is a key word here–designing and fabricating, and launching a mission that goes to space is expensive, so NASA sets strict limits on mission budgets. Even then, throughout the lifespan of a mission design and fabrication, unexpected costs pop up and having clear engineering requirements linked to science capability makes it easier to navigate what science capabilities a mission might have to cut to stay in budget. For example, if a particular piece of hardware for an instrument becomes too expensive to manufacture, a mission team can think about what science goals the mission will no longer reach if that hardware is not implemented and weigh if that science is mission critical–is it worth spending the money on this mission at all if the mission does not have these science capabilities?

This is why the structure of the Science Traceability Matrix (STM; Table 1 above) is so important: Questions the Astro2020 Decadal Survey Deemed Most Important→ Science Objectives → Physical Parameters → Observables → Instrument Requirements → Mission Requirements. As cost forces a mission to cut science capabilities, the STM makes it easy to see things like: does this mission seek to answer multiple Decadal questions, does the mission answer enough science questions to be worth the cost, or is the instrument(s) capability unique when compared to existing space-based instruments? 

There are several different types of requirements when it comes to mission design, but today we are focusing on the instrument and mission requirements–specifically for the same objective we highlighted in in our last bite, O1:

Science Objective: To determine whether sub-Neptune atmospheric escape is caused by photoevaporation or core-powered mass loss. (If you want some more background on why we care about how exoplanets lose their atmosphere, check out this Astrobite from Clarissa.)

• Physical Parameter: Ejection velocities between 0-20 km/s with a 1-2 km/s precision.
• Observables: 
  • Atmospheric transmission spectra of the Lyman-alpha line with transit depths up to 50% absorption with 1% resolution
  • Stellar flux from the host star specifically between 50-91.2 nm.

Informing Instrument Requirements

Now that we know what observable information we need to answer this science objective, we can think about what type of instrument design can enable these observables. For this objective, these are the three instrument requirements the MAUVE team came up with including some of my thoughts on the requirements:

1. Spectrograph spanning 120-123 nm with spectral resolution, R, ~ 1,000.
   • Spectrograph wavelength range is just wide enough to include the Lyman-alpha region. A spectral resolution of 1000 will give you good insight into the Lyman-alpha line shape morphology (seen in Figure 1) while also still detecting the line. Exoplanet atmospheres are faint, so if you disperse the light too much, it’s difficult to see the spectral signal from them at all!
2. Spectrograph with spatial resolution of 7 arcseconds.
   • Spatial resolution defines how close together two objects can be before they start to blend into one object. It’s not very transparent where this requirement comes from. I think it’s likely that for the target list they had in mind, the closest companion to any of the target stars is greater than 7 arcseconds.
3. Spectrograph spanning 50-92 nm with R = 42.
   • Wavelength range spans the hydrogen ionizing stellar flux region and the resolution is low, which I interpret as a narrow band filter for imaging rather than how we traditionally think of resolution for spectroscopy. For an example of what I mean you can see Table 1 for the NIRCam filters, specifically the “Narrow” filters.

These requirements alone point to an EUV/FUV imaging-spectrograph imager type of instrument design, and the instrument for MAUVE, THISTLE, includes an EUV arm specifically for this objective!

You might be wondering, what happens when requirements for the instrument from two different objectives are contradictory? For example, if MAUVE theoretically had another objective that required much higher spectral resolution, say R ~ 25,000, for the same wavelength range as outlined in O1 (120-123 nm). We can think about something like this in a few different ways:

1. Is it possible to change the instrument design of THISTLE to allow for a higher resolution EUV channel? Luckily, the designers of MAUVE thought about this for the FUV/NUV channel, see Figure 1, so maybe we could get away with doing this for the EUV channel too.
   • Ultimately, the ability to make a change like this depends on how much the weight and cost of the mission would change. If this instrument design puts the mission over budget then maybe we can consider option 2 (below).
2. If only high or low resolution is cost feasible for the EUV channel, is it possible to bin the higher resolution spectra for the O1 objective down to lower resolution and still reach the required sensitivity for the objective?
   • Higher resolution means that for a particular wavelength range we sample the flux per wavelength more frequently. If we don’t want higher resolution we can take the flux from several wavelengths in a row and combine them, assigning them to a new wavelength (the average of the several wavelengths we combined), which reduces the overall spectral resolution. If you’re still a little confused, Answer 1 from this question on StackExchange might help with visualization of binning!
   • To answer if this is feasible, we would have to be sure that we could reach the required transit depth precision (outlined in our last bite about MAUVE) from O1 while binning the data. If we cannot reach that requirement with binning, either O1 or this theoretical objective would have to become malleable or dropped.
3. If binning is possible, but high resolution is too costly.
   •  It’s possible this theoretical objective would be fully “down-selected” (removed, but remembered) from the STM entirely since we wouldn’t be able to answer the science question without the high-resolution capabilities.

Summary

Armed with our specific physical parameters and observables, we can outline the requirements for an instrument in a way that is traceable and transparent! Having instrument requirements that flow directly from science objectives makes it easy to see what science capabilities lose support when considering hardware descopes, often due to rising mission costs. In the next installment of this mission design mini-series we will discuss how science requirements and instrument requirements flow down into mission requirements!

Astrobite edited by Kaz Gary.

Featured image credit: My sick twisted mind and Pixlr Advanced Photo Editor.