Results of orbit-fitting experiments with simulated Gaia DR5 data. The points represent median values, and the error bars indicate 1σ ranges. Blue dashed lines show fits to the SNR₁ > 2 results. The top panel shows the Δχ² between the best-fit one-companion and no-companion models, vs. SNR₁, the SNR of each astrometric data point (Equation (7)). When SNR₁ is below about 0.5, Δχ² ≈ 20. For SNR₁ ≳ 1 there is a relatively tight relationship (within a factor of ∼2) between SNR₁ and Δχ². The middle panel shows the relative error in the recovered orbital period, δP_orb = 1 − P_recov/P_true, vs. SNR₁. δP_orb drops steadily with SNR₁. The relative error in the recovered planet mass vs. SNR₁ is shown in the bottom panel. Planet masses are accurately recovered when SNR₁ ≳ 4, and become increasingly accurate at larger SNR₁.