Gravity Tractor: Slow-Push Asteroid Deflection

Mission Parameters

Spacecraft mass 2,000 kg

Hover standoff distance 150 m

Mission (tractoring) duration 3.0 yr

Warning time before encounter 10.0 yr

Deflection Status

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Tractor accel.

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Accumulated Δv

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Predicted miss dist.

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Required (2× R⊕)

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How it works

A gravity tractor never touches the asteroid. It hovers at a fixed standoff distance d and uses angled ion thrusters to hold position against the asteroid's own pull on the spacecraft. The reaction to that station-keeping thrust is aimed so its exhaust never hits the rock — but the spacecraft's own gravity keeps tugging the asteroid toward it the entire time, for as long as the mission runs.

a = G·m_sc / d²                (continuous acceleration on the asteroid)
Δv = a · t_mission               (velocity change after the tractoring period)
miss ≈ Δv · (t_warning − t_mission)   (position shift accumulated over the remaining lead time)

Because Δv only has to build up over years, a gravity tractor works on rubble-pile asteroids that a kinetic impactor might shatter unpredictably — the push is gentle and precisely tunable, at the cost of needing a much longer warning time than an impact mission.

Spacecraft mass — heavier spacecraft pull harder (a scales linearly with m_sc).
Standoff distance — closer hovering pulls much harder (a scales as 1/d²), but risks the plume disturbing loose surface material.
Mission duration — longer tractoring accumulates more Δv, but eats into the warning time left to let the deflection compound.
Warning time — how many years before the asteroid would otherwise reach Earth; the leftover lead time (after the mission ends) is what turns Δv into an actual position shift.

The required miss distance here is set to twice Earth's radius (≈12,742 km) as a safety margin — real deflection campaigns target several Earth radii of clearance to absorb trajectory uncertainty.

Original path → Earth Deflected path Tractor thrust axis

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