ESA Hera: Binary Asteroid Impact Mission

• ESA's Hera spacecraft is an interplanetary mission designed to conduct the first in situ post-impact survey of the Didymos–Dimorphos binary system following the DART event.
• It integrates high-resolution imaging, hyperspectral, LIDAR, and radio science instruments with deployable CubeSats to capture detailed compositional, structural, and dynamical data.
• The mission aims to validate planetary defense strategies by precisely measuring momentum transfer, binary dynamics, and surface properties, setting new empirical benchmarks.

ESA’s Hera spacecraft is an interplanetary mission developed within the Asteroid Impact and Deflection Assessment (AIDA) collaboration, designed to provide the first comprehensive, in situ post-impact survey of the binary asteroid system (65803) Didymos–Dimorphos following the NASA Double Asteroid Redirection Test (DART). Launching in 2024 with arrival anticipated in late 2026, Hera is tasked with validating kinetic impactor planetary defense concepts, constraining fundamental binary small body dynamics, and delivering high-resolution physical, compositional, and geophysical data utilizing a tightly integrated payload and two deployable CubeSats.

1. Mission Architecture and Trajectory

Hera is structured around a solar-powered main spacecraft hosting four principal scientific instruments (Asteroid Framing Cameras, Laser Altimeter, Thermal-Infrared Imager, HyperScout-H Hyperspectral Imager), together with deployment capabilities for two CubeSats (Juventas and Milani). Launch is scheduled for October 2024, with a solar electric propulsion cruise—featuring a Mars gravity assist and ≈25-month interplanetary transfer—culminating in stationkeeping within the Didymos system four years after the DART kinetic impact.

Upon arrival, Hera executes proximity operations at variable stand-off distances (20–1 km) over a six to twelve-month asteroid phase, with sequential Early Characterization, Detailed Characterization, Close Operations, and Experimental Periods. The wide field-of-view (FOV) of its imaging suite allows continuous observation of both binary components.

2. Scientific Objectives and Observational Goals

Hera’s scientific objectives span several categories:

• Primary: Measure the DART-induced crater on Dimorphos at meter scales; determine bulk densities and masses of Didymos and Dimorphos to <1%; refine the momentum transfer enhancement factor $\beta$ by combining dynamical evolution post-impact with high-precision mass measurements; and map the three-dimensional shape and surface morphology of both bodies.
• Secondary: Characterize the mutual orbit and rotational states, including detection of forced/tumbling libration and principal axis deviations; assess secular changes in the binary orbit induced by Binary YORP (BYORP) and tidal dissipation; deduce mechanical strength, porosity, and energy dissipation (k/Q) of both bodies; survey compositional heterogeneity and space weathering; and reconstruct the system’s orbital dynamical history.

By integrating radio science experiments, optical and altimetric imaging, spectral mapping, and gravimetric measurements, Hera is able to disentangle the respective contributions of ejecta dynamics, impact geometry, and binary evolution processes to $\beta$ and to place new constraints on interior structure models.

3. Payload Suite and Instrument Performance

Asteroid Framing Cameras (AFCs)

Dual diagonal high-resolution framing cameras (5.5° × 5.5° FOV) enable sub-meter surface mapping, geological unit boundary identification, and crater morphology reconstruction. Context imaging is coordinated with other payloads during all mission phases.

HyperScout-H (HS-H) Hyperspectral Imager

HS-H is the single payload providing spatial and spectral imaging in the 0.65–0.95 µm range. Its Three-Mirror Anastigmat telescope yields a ≈15.5°×8.3° FOV on a 2048×1088 CMOS detector, with 25 narrow-band Fabry–Pérot filters per 5×5 “macropixel” (Δλ≈0.008–0.022 µm). Calibration models for dark current, noise, radiometric gain, and geometric distortion are explicitly given:

• Dark current: $$m(t_\mathrm{exp},T) = 146 + [9.10 \cdot t_\mathrm{exp} + 0.5]\,e^{0.1225T}$$
• Noise: $$\sigma_\mathrm{noise}(t_\mathrm{exp},T) = 12.57 + [1.18 \cdot t_\mathrm{exp} - 0.0252]\,e^{0.1225T}$$

Level-1 calibration, demosaicing, and radiometric conversion are performed via the dedicated HS-H Toolkit, supporting principal-component, machine learning, and taxonomic classification (on 2983 asteroid spectra, 11 classes). Laboratory observations demonstrate accurate reconstruction of S- and V-type signatures in meteorite analogs.

Laser Altimeter and LIDAR

A 1.5 µm LIDAR measures radial ranges at 0.5 m precision and provides >1000 cross-over measurements per body. These data support global shape model construction, geodetic mapping, and coregistration with AFC and HS-H products.

Radio Science Experiment (RSE)

The RSE is the keystone of Hera’s gravity and orbit solution. Combining Earth-based X-band Doppler/range ($\sigma_\text{Doppler}\approx0.1$ mm/s, $\sigma_\text{range}=1$ m @300 s), S-band intersatellite links to CubeSats ($\sigma=0.05$ mm/s, 0.5 m), framing camera landmarking (36″ pointing), and LIDAR, Hera recovers:

• Didymos $GM=4.0071\times10^{-8}$ km³/s², $\sigma_{GM}=1.61\times10^{-12}$ km³/s² (0.004%)
• Dimorphos $GM=3.4522\times10^{-10}$ km³/s², $\sigma_{GM}=2.73\times10^{-13}$ km³/s² (0.079%)
• J₂ to $<0.1\%$ for Didymos and $\sim6.6\%$ for Dimorphos

The covariance analysis, performed via NASA-JPL’s MONTE, demonstrates that adding CubeSat intersatellite links improves $GM$ resolution by >10× and enables higher harmonic gravity field recovery. Rotational state uncertainties for Dimorphos reach $0.3^\circ$ (pole), $1.2\times10^{-4}$ deg/h (spin rate), $0.02^\circ$ (forced libration amplitude, $3$ cm).

CubeSats: Juventas and Milani

CubeSats provide near-surface gravimetry, radar tomography, and, via the ASPECT spectrometer, multi-wavelength spectroscopy for cross-calibration with HS-H, extending mineralogical classification into the 0.5–2.5 µm range.

4. Dynamics, Modeling, and Momentum Transfer Assessment

Hera directly quantifies post-impact changes in the binary orbit, spin states, and interior properties. Measurement regimes and dynamical models include:

• Momentum enhancement $\beta$: Derived from $$\beta = \frac{(m_\text{final} v_\text{final} - m_\text{initial} v_\text{initial})}{m_\text{impactor} v_\text{impactor}}$$ and, via detailed mass and mutual orbit changes, constraints are placed to better than $5\%$. Using current mass-density uncertainties ($\sigma_\rho\approx152$ kg m$^{-3}$), the propagated error on $\beta$ is $\sim0.25$ (1σ), a 2–3$\times$ improvement over post-DART estimates.
• Mutual orbit and secular evolution: Hera’s long orbital campaign will distinguish BYORP ($da/dt \sim$ 1 cm yr$^{-1}$, $T_\text{BYORP} = (\Phi/c)A_2B_2$) from tidal dissipation ($$da/dt|_\text{tide}\propto(k_2/Q)(M_2/M_1)(R_1/a)^5a\,n$$) at $<1$ cm yr$^{-1}$, a regime previously unconstrained by astrometry.
• Rotational state, libration, and tumbling: Proximal CubeSat photometry and camera lightcurves will resolve forced libration amplitudes within $0.1^\circ$, testing model predictions (7–14°) and optionally detecting initiation of non-principal axis (“barrel”) rotation if the post-impact torque is sufficient.
• Interior structure: Libration, phase lags, and degree-2 harmonics ($J_2, C_{22}$) provide three constraints on the principal moments of inertia ($I_x, I_y, I_z$), with additional sensitivity to $Q$ via phase lag $\Delta\phi$: $Q^{-1} = \sin\Delta\phi$.

5. Instrumental Synergies and Data Processing

The payload integration strategy is designed for mutual reinforcement of morphological, spectral, and dynamical datasets:

• AFC imagery defines the geological context and enables precise registration of HS-H spectral maps.
• HS-H maps compositional units, space weathering gradients, and exogenous material through machine learning and spectral indices, facilitating direct comparison to known meteorite types.
• Thermal-IR mapping with TIRI, geometric mapping with laser altimeter data, and gravimetric results from RSE yield coherent three-dimensional models of asteroid surface and subsurface.
• CubeSat ASPECT spectrometer extends spectral mapping outside HS-H’s 0.65–0.95 µm range. All instruments observe the DART crater simultaneously during high-proximity phases.

Data calibration is governed by pre-flight and in-flight standards. The Level-1 pipeline incorporates bias/dark/flat correction, bad pixel interpolation, geometric distortion correction, and radiometric transformation to physical units. Hyperspectral cube reconstruction applies demosaicing and ratio-based interpolation with local brightness smoothness assumptions.

6. Legacy and Scientific Impact

Hera will perform the first in situ, quantitative tests of asteroid-scale kinetic impactor deflection efficacy. Its suite of instrumental and methodological advancements—especially the unprecedented precision in full binary dynamical and physical parameter recovery—will:

• Validate or revise momentum transfer models underlying planetary defense strategies.
• Define the coupled thermal-dynamical-spin evolution in small-body binaries, placing empirical constraints on BYORP and tide-driven secular processes.
• Provide benchmark datasets for compositional heterogeneity, space weathering, and exogenous contamination.
• Inform future small-body mission planning and asteroid hazard mitigation risk assessments.

The multi-instrument, multinational collaboration ensures all data products are archived in ESA’s Planetary Science Archive and mirrored to NASA’s PDS, promoting open access and long-term reusability.

7. International Collaboration and Data Policy

Hera exemplifies joint mission development, with ESA responsible for the spacecraft bus, descent module, integration, operations, and payload management. NASA supplies the solar electric array, power management, and the probe entry system (including HEEET aeroshell technology), as well as instrument subsystems and CubeSat contributions via recognized US partners. Science teams are international, combining ESA member-state principal investigators and NASA co-investigators. Data from all instruments are subject to standard planetary data archiving and distribution, ensuring reproducibility and community engagement.

A plausible implication is that, as Hera’s precision in $\beta$, mass, gravity, and compositional mapping will exceed pre-mission forecasts, this mission will define the empirical standards to which all subsequent kinetic impactor deflection tests, binary small body models, and planetary defense simulations must be benchmarked.