Starshade

Source: Adapted from Genszler et al. (2026) "Surveying orbits in cislunar space for telescope-starshade observatories"

Site: https://cislunarspace.cn

Definition

A Starshade is a starlight suppression system consisting of an external occulter with petal-shaped edges that flies in formation with a space telescope along the telescope's line of sight to a target star. By blocking on-axis starlight while allowing off-axis light from an exoplanet to reach the telescope, the Starshade enables direct imaging spectroscopy of exoplanets. Key mission concepts using Starshades include HabEx (Habitable Exoplanet Observatory) and LUVOIR (Large Ultraviolet Optical Infrared Surveyor).

Operating Principle

The Starshade and telescope maintain a formation flying configuration along the Line of Sight (LOS) to the target star:

where $s$ is the separation distance between Starshade and telescope, with a typical value of $120 \times 10^3$ km (120,000 km).

Petal-Shaped Design

The Starshade typically employs a flower-like design — a circular central disk with petal extensions around the edge. This geometry minimizes diffraction by concentrating the Airy disk energy within the central occultation zone, significantly reducing target star light leakage.

Inner Working Angle

The Inner Working Angle (IWA) characterizes the Starshade's blocking capability:

where $R$ is the Starshade radius and $s$ is the separation distance. For a 72 m diameter Starshade at 120,000 km separation, the IWA is approximately 0.06 arcseconds.

Key Technology Challenges

Starshade technology has not yet reached mission-ready status (TRL 9). Primary challenges include:

NASA's S5 program has addressed starlight suppression, formation sensing and control, and deployment accuracy, advancing Starshades toward TRL 5. However, significant development remains before operational deployment.

Relative Motion with Telescope

In the synodic frame, the Starshade does not follow a closed periodic orbit. Each observation slew requires two impulsive maneuvers:

1. First burn: Initiates transition from current target star to next target
2. Second burn: Matches telescope velocity at the end of the maneuver to resume observation tracking

This involves precise control of differential acceleration between the Starshade and telescope — a core challenge in mission design.

Cislunar Space Applications

Genszler et al. (2026) investigated the feasibility of a Starshade technology demonstration mission using orbits in cislunar space, considering three orbit families:

• Distant Retrograde Orbits (DRO): Stable multi-year periodic orbits suitable for repeated observation slews
• EML1 Halo Orbits: Periods of ~8–10 days, with short transfer times from Earth
• EML2 Halo Orbits: Periods of ~6–10 days, with favorable observation geometry

Key finding: With a $\Delta v$ budget of 20 m/s, the separation distance must decrease to approximately 10,000 km for feasible slew maneuvers — far smaller than the 120,000 km separation used in SEL2 missions, indicating that cislunar Starshade demonstrations require significantly scaled-down systems.

Related Concepts

• Distant Retrograde Orbit (DRO)
• Near-Rectilinear Halo Orbit (NRHO)
• Earth-Moon L1/L2 Halo Orbit (EML1/EML2 Halo)
• Circular Restricted Three-Body Problem (CR3BP)
• Direct Imaging of Exoplanets
• Formation Flying

References

• Genszler G, Savransky D, Soto G J. Surveying orbits in cislunar space for telescope-starshade observatories[J]. 2026.
• Vanderbei R J, Cady E, Kasdin N J. Optimal occulter design for finding extrasolar planets[J]. Astrophysical Journal, 2007, 665(1): 794.
• Morgan R, Savransky D, Turmon M, et al. An exploration of expected number of exoplanets for a 6 m class direct imaging observatory[C]. SPIE, 2022, 12180: 761-775.
• Willems P, Lisman D. NASA's starshade technology development activity[J]. JATIS, 2021, 7(2): 021203.