ESA’s Automated Transfer Vehicle Johannes Kepler is more than just a cargo carrier for the International Space Station, it is also an on-orbit refueling station and orbit booster. On May 17-19, 2011 the Kepler ATV is scheduled to conduct its first refueling of the ISS, as it will transfer about 850.6 liters (225 gallons) of propellant for the station’s own thrusters for future boosts in orbit.
Preparations for the ISS refueling began on March 22 with a leak test of the propellant transfer lines, to ensure the connections between the ISS and ATV-2 were completely sealed; the test was a success, meaning that as of now, everything is go for the station’s refueling.
In mid-March, the ATV increased the ISS’s orbit with a 882-second (14 and a half minutes) burn, giving the ISS an extra push of about 2.1 m/s. In all, Kepler brought nearly 10,000 pounds (4,500 kilograms) of propellant that has been used by its thrusters to boost the space station to a new altitude of 400 kilometers (248 miles) above the Earth. This will be the new “normal” for the station’s orbit. Previously, the ISS orbited about 350 km (220 miles) up.
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The main benefit of raising the station’s altitude is to cut the amount of fuel needed to keep it there by more than half. This also means that visiting vehicles will not be able to carry as much cargo as they could if they were launching to the station at a lower altitude since they will need more fuel to reach the station, but it also means that not as much of that cargo needs to be propellant.
The orbit of the ISS degrades because Earth’s atmosphere — though tenuous at those altitudes – expands and contracts through the Sun’s influence, and there are enough molecules that contact the surfaces of its large solar array panels, the large truss structure, and pressurized modules to change its speed, or velocity, which is about 28,000 kilometers an hour (17,500 mph).
At the ISS’s old altitude, the space station uses about 19,000 pounds of propellant a year to maintain a consistent orbit. At the new, slightly higher altitude, the station is expected to expend about 8,000 pounds of propellant a year. And that will translate to a significant amount of food, water, clothing, research instruments and samples, and spare parts that can be flown on the cargo vehicles that will keep the station operational until 2020 and beyond.
Kepler also sent a breath of fresh air to the station by transferring about 8kg of oxygen to the ISS in March, which was the first re-pressurization of the ISS’s internal atmosphere conducted by Kepler.