Extend the reach of a science payload

Development of M-3SII began in 1981. It retained the M-3S first stage while replacing the other stages with new designs. An optional fourth kick stage helped launch Sakigake and Suisei in 1985 toward encounters with Halley’s comet. A family relationship does not mean every mission used the same complete stack.

A kick stage adds propulsion late in the launch sequence. Reaching an Earth orbit and leaving Earth on a useful interplanetary path are different delivery tasks. A spacecraft may also perform later maneuvers of its own. The launch contract must state where the launcher’s work ends and the spacecraft’s work begins.

In an original simplified exercise, a launch allocation permits a 700 kg assembly for a specified trajectory. A science spacecraft is 520 kg, its adapter is 80 kg and required separation equipment adds 30 kg. The assembly is 630 kg, leaving 70 kg under that stated limit. The adapter is useful hardware even though it does not collect science data.

This is a bookkeeping example, not M-3SII performance data. A real analysis also checks dimensions, structural loads, vibration, electrical connections and separation behavior. An unused mass allowance does not guarantee that another instrument or passenger will physically fit or survive launch.

Budget for the entire payload assembly

ISAS records seven successful M-3SII launches in eight attempts. Its successful-mission table includes comet probes, Hiten’s lunar work and several observatories. That variety shows why a launch program can support different scientific questions without the launcher itself becoming the scientific instrument.

For any proposed payload, use documentation for its specific configuration and target trajectory. An optional stage may change the delivered energy, mass allocation and interfaces. A capacity quoted for low Earth orbit cannot simply be reused as the allowable mass for an interplanetary mission.