Why Undertake an Asteroid Redirect Mission?

In Kerbal Space Program, asteroid redirect missions are among the most complex and rewarding undertakings. Beyond the pure challenge, they serve practical purposes: captured asteroids can be mined for ore, used as orbital fuel depots, or even as mobile science labs. Successfully redirecting an asteroid into a stable Kerbin orbit also unlocks substantial reputation and science gains, making the effort worthwhile for any seasoned space agency. This guide walks you through every phase, from mission planning through final orbit insertion, with detailed tips to ensure your Kerbonauts return safely.

Understanding Mission Objectives and Constraints

The core objective is simple: locate, intercept, capture, and redirect an asteroid into a stable orbit around Kerbin. However, the devil lies in the details. You must manage delta-v budgets, account for the asteroid's mass and rotation, and plan intercept windows that may only open every few weeks. The first step is to familiarize yourself with the classes of asteroids your tracking station can detect: Class A (small, ~10–20 tons), Class B (medium, ~20–60 tons), Class C (large, ~60–150 tons), and Class D/E (very large, 150+ tons). For a first attempt, target a Class A or B asteroid; they are easier to capture and redirect with a reasonably-sized spacecraft.

Tracking and Selecting Your Target

Use the Tracking Station to filter for asteroids. Click on each detected object to view its orbit, closest approach to Kerbin, and its class. Prioritize asteroids that already pass close to Kerbin (within 1-2 million kilometers) as they require less energy to intercept. Also note the asteroid's orbital inclination; a low-inclination object near the equatorial plane is far easier to match than one tilted 40 degrees. Keep a spreadsheet or use mods like Kerbal Alarm Clock to mark optimal intercept windows.

Step 1: Planning Your Mission – The Delta-v Budget

Every kilogram of spacecraft and asteroid mass requires fuel for maneuvers. Use a delta-v map of the Kerbin system to estimate the total Δv needed: roughly 4,500 m/s to reach low Kerbin orbit, plus another 1,000–2,500 m/s to intercept and capture a Class B asteroid, depending on its flyby trajectory. Add 300–500 m/s for orbital insertion of the captured combination. Most importantly, overestimate by at least 20% to account for piloting errors and the added mass of the captured asteroid.

Spacecraft Design Principles

Your redirect vessel must be built for efficiency and stability. Follow these guidelines:

  • Propulsion: For large asteroids, a cluster of powerful chemical engines (like the S3 KS-25 "Vector" or the LV-N "Nerv" nuclear engine) provides high thrust and reasonable efficiency. For smaller asteroids, ion engines are extremely fuel-efficient but require patience for long burns.
  • Fuel Tanks: Use inline tanks with a high fuel-to-dry-mass ratio. Avoid flimsy radial tanks that can break under torque when the asteroid pulls on the craft.
  • Structure: Attach multiple docking ports in a claw arrangement, or use the Advanced Grabbing Unit (Klaw) for a secure hold. Ensure your craft's center of mass shifts as little as possible after capture; place reaction wheels and RCS thrusters symmetrically.
  • Power & Control: Cover the craft with solar panels (retractable if close to the Sun) and include a battery bank. A probe core with built-in SAS is mandatory; add reaction wheels for extra torque.

For a Class B asteroid, a 30-40 ton spacecraft with 6,000 m/s total Δv works well. Use a "pusher" design where the main engines are aligned with the craft's long axis. Avoid "tugboat" designs that pull from a single weak point; the Klaw at the front of a long truss works if reinforced with struts. Test your design in the VAB by adding a heavy dummy payload (e.g., a full ore tank of 100 tons) to simulate the asteroid's mass and verify TWR > 0.3 once captured.

Step 2: Executing the Interception

Launch your spacecraft into a low parking orbit (80-100 km) around Kerbin. Then plan your transfer burn to the asteroid's intercept point. Use a maneuver node on the ascending or descending node if the asteroid has high inclination to save fuel by plane-matching first. Ideally, you want the asteroid's closest approach to Kerbin to be near your own orbit—time your launch so that the asteroid arrives at that point when you do.

Matching Velocity and Approach

Once you're in a transfer orbit, create a maneuver node at the asteroid's orbital path. Adjust prograde/retrograde to get a close encounter (within 0.1 km is ideal but 1-2 km is acceptable). After executing the burn, use RCS and fine-tune your approach. When within 50 meters, match the asteroid's velocity as closely as possible—your relative velocity should be under 5 m/s. Firing engines while too fast will send you careening past. Use your ship's target-relative velocity readout (on the navball) to slow down gradually.

Capturing the Asteroid with the Klaw

The Advanced Grabbing Unit (Klaw) is the most reliable capture method. Approach the asteroid with the Klaw facing it. When you're within 5 meters and relative velocity < 2 m/s, right-click the Klaw and select "Extend" then "Grab." The Klaw will sink into the asteroid's surface. Immediately turn off SAS to prevent reaction wheels from fighting the asteroid's rotation. Then enable the asteroid's rotation to match your craft—you can do this by using the "Control from Here" option on the Klaw and then locking the asteroid's rotation relative to the ship. This stabilizes the stack.

Step 3: Redirecting the Asteroid

With the asteroid attached, your craft is now much heavier. The center of mass has shifted toward the asteroid. Recalibrate your vessel—disable any autostruts that may cause phantom forces. Now plan a series of burns to lower the asteroid's orbit into a stable Kerbin orbit. You cannot simply burn straight toward Kerbin; you must perform a capture burn when you reach periapsis of your current transfer orbit.

The Capture Burn Strategy

Your transfer orbit will take you back out toward the asteroid's original path. The most efficient time to capture is at periapsis—the point where you will swing closest to Kerbin. Place a maneuver node there and adjust retrograde until the orbit becomes elliptical with an apoapsis well within Kerbin's sphere of influence (say, 200-300 km). Burn at full throttle but watch your fuel; a long burn may cause your trajectory to drift. Use multiple small or medium burns to fine-tune the result.

Orbit Insertion and Circularization

After you complete the capture burn, you will be in a highly elliptical orbit. Lower the apoapsis by burning retrograde at periapsis again. Ideally, you want a circular orbit between 100 and 200 km altitude. Higher orbits require less Δv but are harder to service later. Lower orbits are useful for future mining but risk atmospheric drag if below 70 km. Aim for 150 km as a safe compromise. Once circular, verify the orbital period and inclination. You can now detach the Klaw if desired, but keeping it attached allows you to later mine the asteroid with drills.

Advanced Techniques and Troubleshooting

Even with careful planning, things can go wrong. Here are common issues and solutions:

  • Spinning out of control: Disable all reaction wheels except one on the craft. Use the "Lock Rotation" option on the Klaw to let the asteroid rotate freely while the ship maintains orientation. Then apply gentle RCS corrections.
  • Insufficient fuel: If you run low before capture, consider using a "slingshot" maneuver: fly past the Mun or Minmus to slow your orbit relative to Kerbin. This can save several hundred m/s of Δv.
  • Structural failure: Reinforce the connection between the Klaw and your craft with struts in the VAB. If you already launched, use EVA construction (if enabled) to add struts.
  • Incorrect orbit: If your captured orbit is too high, you can use multiple aerobraking passes through Kerbin's upper atmosphere (60-70 km altitude) to lower the apoapsis. Be careful not to go too deep or the asteroid may explode due to heat.

Using Gravity Assists

For asteroids on a distant trajectory, you can use a Mun gravity assist to bend your orbit inward. Perform the capture burn with the Mun instead of Kerbin. This requires precise timing; use mods like Trajectories to predict the effect. The Mun's gravitational pull can reduce total Δv cost by 20-30% for high-energy intercepts.

Rewards and Utilization of Captured Asteroids

Once your asteroid is safely in orbit, reap the benefits. Attach a drill and ore tank to the asteroid to mine unlimited ore, which can be converted into fuel using an ISRU converter. This effectively creates a refueling station. Alternatively, use the asteroid as a science platform: experiments performed while attached to an asteroid yield bonus science points. You can also push the asteroid to a different orbit (e.g., a Minmus orbit) or even land it on a planetary body, though that requires immense Δv.

Contracts and Career Mode

Many career-mode contracts specifically ask for asteroid redirects. Completing these yields massive funds and reputation. Note that contracts often require a specific asteroid class or destination orbit. Always read the fine print before accepting. If you mod the game, mods like SCANsat can help map asteroid compositions before committing to a redirect.

Final Thoughts: Practice Makes Perfect

Your first asteroid redirect will likely be messy—you may run out of fuel, lose your asteroid to a missed grab, or end up in a wildly eccentric orbit. That's normal. Each failure teaches you about thrust-to-weight ratios, center-of-mass shifts, and the importance of RCS placement. With a few attempts, you'll be able to redirect Class C asteroids with confidence. Remember to quicksave (F5) frequently before critical maneuvers. The sense of accomplishment when you watch a once-wild rock settle into a neat orbit around Kerbin is one of KSP's greatest joys.

For further reading, see the official KSP wiki page on asteroids and the community guide "How to Catch an Asteroid". Good luck, and happy redirecting!