Progress MS-35 (96P)

Progress MS-35 is the ninety-sixth Progress to fly to the Station — a streak that started in 1978 with the original Salyut 6 station and never paused for more than a few months. The freighter lifts off from pad 31/6 at Baikonur on a Soyuz 2.1a, reaches orbit in nine minutes, and docks to the Zvezda module two days later under fully autonomous Kurs-NA guidance. About 2,500 kg of food, water, propellant and spare parts. Russian engineering at its most matter-of-fact.

What a Progress carries

The cargo manifest breaks down into several categories:

• Dry stowage (~1,400 kg): food, clothing, medical supplies, spare parts, experiment hardware, and personal items for the crew.
• Water (~420 kg): drinking water and water for the oxygen-generation system.
• Propellant (~870 kg): unsymmetrical dimethylhydrazine and nitrogen tetroxide — the hypergolic fuels that, once docked, get fed through valves into the Station’s own propellant tanks.
• Gases: compressed oxygen and nitrogen to top up the Station’s atmosphere.

The interesting cargo on a Progress flight is rarely the food. It is the fuel. That propellant is what raises the orbit every six weeks against atmospheric drag, and what will eventually steer the Station toward its disposal over the southern Pacific at the end of its life.

How Progress works

The Progress spacecraft is derived from the Soyuz crew capsule — same basic architecture, same rocket, same launch pad — but with the crew module replaced by a cargo hold and the descent module replaced by a fuel tank. It cannot return to Earth. When its job is done, it is filled with rubbish, undocked, deorbited, and burned up over the Pacific.

The docking is fully autonomous. The Progress uses the Kurs-NA radar system to locate the Station, match its orbit, and guide itself to the docking port. The crew monitors the approach from inside the Station but does not need to intervene unless the system flags an anomaly.

The reboost role

Beyond resupply, Progress serves as the Station’s primary reboost vehicle. Periodically, mission control commands a docked Progress to fire its engines for a few minutes, raising the Station’s orbit by a few kilometres to compensate for atmospheric drag. Without these reboosts, the Station would slowly descend and eventually re-enter the atmosphere. The frequency depends on solar activity — when the Sun is active, the upper atmosphere expands and drag increases, requiring more frequent burns.

After the mission

Once unloaded, the Progress is packed with waste — broken equipment, used clothing, food packaging — sealed, undocked, and deorbited. It burns up on re-entry over an uninhabited stretch of the South Pacific. Every Progress ends the same way: a small streak of light that nobody sees.