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Dnepr, a conversion of the Soviet-era R-36MUTTKh missile, was a joint venture between Russia and Ukraine to leverage leftover hardware from decommissioned intercontinental ballistic missiles to place commercial payloads into orbit. Russia\u2019s ISC Kosmotras marketed the rocket, which made twenty-two flights between 1999 and 2015. Before the end of the program, several launches had been delayed by political disagreements over the drop zones for the rocket\u2019s spent stages and the Russian government had made several unrelated threats to cancel the program. Following Russia\u2019s 2014 military intervention in Crimea the partnership became untenable and launches have since stopped.<\/p>\n
The Iridium and GRACE joint launch was not the only mission that SpaceX has gained because of Dnepr\u2019s demise. In February Falcon 9 deployed Spain\u2019s Paz radar imaging satellite, which had also been booked onto Dnepr until 2016.<\/p>\n
The Gravity Recovery and Climate Experiment Follow-On (GRACE Follow-On or just GRACE-FO) consists of two identical satellites that will fly in relatively close formation. The mission will study fluctuations in Earth\u2019s gravitational field, building a highly accurate model of Earth\u2019s gravity to help studies of the planet\u2019s climate. Orbiting 220 kilometers (137 miles) apart, the spacecraft will monitor minute fluctuations in their separation distance that result from gravity perturbing their orbits.<\/p>\n
![\"\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2018\/05\/2018-05-22-113338.jpg\")
<\/p>\nGRACE-FO render by NASA.<\/p>\n
Airbus Defence and Space constructed the GRACE-FO satellites, based around the same Flexbus platform that was used for the original GRACE satellites that they will replace. Each satellite has a mass \u2013 including propellant \u2013 of 600.2 kilograms (1,323.2 lb). Each satellite has a trapezoidal prism shape \u2013 measuring 0.69 meters (2.3 feet) across at the narrowest point and 1.9 meters (6.4 feet) across at the widest \u2013 with a length of 3.1 meters (10.2 feet) and height of 0.78 meters (2.6 feet).<\/p>\n
Three sides of each satellite are covered in gallium arsenide solar cells, generating power for the mission and charging a 78 amp-hour lithium-ion battery.<\/p>\n
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The GRACE-FO mission is expected to last at least five years \u2013 but requires both satellites in healthy condition to collect useful data. Each satellite is equipped with a Microwave Ranging Instrument (MWI) \u2013 also known as K-Band Ranging (KBR) \u2013 which uses dual one-way ranging to determine the distance between the two satellites. MWI broadcasts a 24 gigahertz K-band and a 32 gigahertz Ka-band signal that are received by the other satellite. Both spacecraft comparing the phase of the signals they receive to the signals they broadcast to determine changes in their relative positioning to within a few microns.<\/p>\n
GRACE-FO also carries a Laser Ranging Inferometer (LRI), an experimental instrument that will use lasers to measure the distance between the two satellites.<\/p>\n