Sentinel-5p launches on Russia’s Rokot launch system

by William Graham

The European Space Agency’s Sentinel 5 Precursor satellite launched Friday aboard what could be the penultimate flight of Russia’s Rokot carrier rocket. Liftoff, from the Plesetsk Cosmodrome in Northern Russia, was on schedule at 12:27 Moscow Time (09:27 UTC).

Rokot Launch:

Sentinel 5 Precursor, or Sentinel-5p, is a small scientific satellite that will contribute to the Europe’s Copernicus programme, dedicated to Earth science and observation. Sentinel-5p will focus on the planet’s atmosphere, detecting and monitoring trace gases in the troposphere – the lowest region of Earth’s atmosphere.

The satellite will serve as a gap-filler, restoring capabilities lost when the Envisat spacecraft ceased operating in 2012 before a new generation of instruments begins launching in the early 2020s.

Copernicus, formerly the Global Monitoring for Environment and Safety (GMES), is an ambitious project being undertaken in partnership between the European Commission and the European Space Agency (ESA) to deploy a fleet of satellites and instruments in space which will gather data on Earth and its environment. The European Commission manages the project, while ESA is responsible for its space component – which it is executing through its series of Sentinel missions.

Sentinel consists of six different types of satellites or instruments, each providing different insights on an aspect of the Earth. Sentinel-1 satellites carry synthetic aperture radar (SAR) payloads providing all-weather radar imagery of the surface.

Sentinel-2 satellites carry high-resolution multispectral optical imaging payloads to take images of the planet at various wavelengths. The Sentinel-3 spacecraft study the oceans with a suite of instruments.

Sentinel-4 and Sentinel-5 will be hosted payloads aboard next-generation weather satellites that EUMETSAT will launch in the early 2020s. Sentinel-4 will be carried aboard Meteosat Third Generation (MTG) satellites in geostationary orbit, while Sentinel-5 will operate in low Earth orbit aboard MetOp Second Generation (MetOp-SG) spacecraft.

Both will monitor atmospheric composition, with a focus on detecting trace gases. With their launches still several years away, the Sentinel-5p mission will restore ESA’s ability to collect composition data at wavelengths it has not been able to monitor since Envisat malfunctioned.

Sentinel-6 spacecraft, also known as Jason Continuity of Service (Jason-CS) will use radar altimetry to study variations in height of the surface of the Earth’s seas. Successors to the three Jason ocean research satellites, the Sentinel-6 satellites are expected to begin launching in 2020.

The Copernicus programme builds on research performed by ESA’s Envisat mission. The largest dedicated Earth science satellite ever launched, Envisat carried a suite of instruments to perform the same research for which Sentinel now employs a fleet of satellites. The eight-tonne (18,000 lb), ?2.3 billion satellite was launched aboard an Ariane 5G rocket in March 2002 to begin a planned five-year mission.

After exceeding its design life, the satellite abruptly stopped communicating with its ground stations in early April 2012 and was declared lost a month later.

Sentinel-5p is the sixth Sentinel satellite to launch. Two Sentinel-1 satellites are currently in orbit, having been deployed by Soyuz-2-1a rockets in April 2014 and April 2016, while a pair of Sentinel-2 satellites were launched by Vega rockets in June 2015 and March this year. Sentinel-3A was deployed in February 2016 by a Rokot.

Unlike the planned Sentinel-5 missions, which will be integrated with MetOp-SG spacecraft, Sentinel-5p is a free-flying satellite. Constructed by Airbus Defence and Space – formerly EADS Astrium – the satellite is based on the AstroBus-L 250M platform.

Weighing-in at 820 kilograms (1,800 lb) at launch, the spacecraft is expected to operate for at least seven years. Sentinel-5p’s three solar arrays will provide at least 1,500 watts of power, with the satellite’s average consumption expected to be around 430 watts.

Sentinel-5p’s PM-22 propulsion system provides maneuvering capabilities, with a tenth of the satellite’s liftoff mass consisting of hydrazine propellant. The satellite is three-axis stabilized, with an attitude and orbit control system (AOCS) using star trackers, GPS receivers, magnetometers and an Earth sensor to determine its position and orientation – while reaction wheels and magnetotorquers will deliver attitude control.

Scientific data will be collected aboard the satellite, via the Payload Handling and Transmission subsystem (PDHT) which has 480 gigabits (equivalent to 52.5 gigabytes) of onboard storage capacity. When passing over a ground station, data will be downloaded via an X-band transmitter with a data rate of up to 310 megabits per second. The satellite also carries two S-band transponders for telemetry and control.

The Tropospheric Monitoring Instrument, or Tropomi, is the only scientific instrument aboard Sentinel-5p. A multispectral imaging spectrometer, ESA describe it as the most advanced instrument of its type to have been flown to date. Tropomi operates in three different bands of the electromagnetic spectrum: 270-500 nanometres, 675-775 nanometers and 2,305-2,385 nanometers.

The first of these bands covers parts of the near-ultraviolet and the blue end of the visible spectrum, while the other bands are in the infrared – covering the near infrared and short-wave frequencies respectively.

Tropomi will measure the spectra of light that has been reflected from the atmosphere and of sunlight. Comparing these spectra will allow it to highlight absorption lines, caused by particular wavelengths of light being absorbed by chemicals in the atmosphere. Different chemicals have unique signatures, so by studying which absorption lines are present scientists will be able to identify which gases are present in the atmosphere.

At the wavelengths Tropomi will observe, it is expected to be able to detect absorption lines caused by bromine oxide, carbon monoxide, chlorine dioxide, formaldehyde, glyoxal, iodine oxide, methane, sulphur dioxide, nitrogen dioxide, atomic and molecular oxygen, ozone, water – including semi-heavy molecules with a single deuterium atom – and some aerosol compounds.

Developed by ESA in conjunction with the Netherlands Space Office, Tropomi has a mass of 220 kilograms (485 lb). The instrument will help Sentinel-5p to study pollution in the troposphere, providing high-resolution data on the presence of chemicals that play a major role in pollution. Its observations will also contribute to monitoring of volcanic ash in the atmosphere and forecasting ultraviolet radiation levels.

Tropomi will compliment other space-based spectrometers, such as the Global Ozone Monitoring Experiment 2 (GOME-2) aboard EUMETSAT’s MetOp satellites and the Ozone Monitoring Instrument (OMI) aboard NASA’s Aura.

Neither of these instruments shares Tropomi’s short-wave infrared capabilities, which cover wavelengths that were previously monitored by Envisat’s Scanning Imaging Absorption Spectrometer for Atmospheric Chartography [sic], or SCIAMACHY spectrometer. Absorption lines at these wavelengths are indicative of the presence of carbon monoxide and methane.

Sentinel-5p will operate in a circular sun-synchronous orbit, at an altitude of 824 kilometers (512 miles, 445 nautical miles), an inclination of 98.7 degrees and a local time of ascending node of just after 13:30. In this orbit, the satellite will complete sixteen revolutions per day and revisit the same point on the surface every sixteen days, or 227 orbits.

This orbit has been chosen to coordinate observations with Suomi, a US weather satellite that launched in 2011. Sentinel-5p will follow closely behind Suomi, using the US satellite’s cloud cover observations to calibrate its observations. Suomi is expected to be replaced by the JPSS-1 satellite, due to launch next month.

Russia’s Rokot/Briz-KM rocket launched Sentinel-5p into orbit, in a launch contracted by Germany’s Eurockot Launch Services. Owned by Europe’s ArianeGroup and Russia’s Khrunichev State Research and Production Space Centre, Eurockot markets launches of the Rokot vehicle – which can also be spelled Rockot under a schema used to transliterate Russian into some European languages – to the European Space Agency and commercial customers.

Rokot itself – its name meaning Rumble – consists of the first two stages of a decommissioned UR-100N missile, which is topped by a Briz upper stage.

Operational Rokot launches have used the Briz-KM upper stage, powered by an S5.92 engine. It is related to the larger Briz-M, which uses the same propulsion module but with a toroidal drop tank that allows it to carry additional propellant for launches to higher orbits, boosted by Proton or Angara rockets. When flying with Briz-KM, the rocket is known as Rokot/Briz-KM, or simply Rokot-KM.

Rokot was originally developed to support the Soviet Union’s development of Naryad, an anti-satellite system, leveraging surplus ICBM hardware. The UR-100N missile, also known by the NATO reporting name SS-19 Stiletto, was developed in the 1970s as an upgraded version of Vladimir Chelomei’s UR-100 missile.

The Rokot programme uses missiles that were withdrawn in favor of the upgraded UR-100NUTTKh version, or withdrawn under the terms of the START-1 arms reduction treaty.

Rokot first flew in November 1990, making a suborbital launch from the Baikonur Cosmodrome in an early silo-launched configuration that used a Briz-K upper stage instead of the later Briz-KM. The type’s third launch, in 1994, was the first orbital mission, successfully deploying the Radiosputnik 15, or Radio-ROSTO, satellite.

It was the final Rokot launch from Baikonur’s silos before operations moved to a surface pad at the Plesetsk Cosmodrome. This was also the final launch to use the Briz-K upper stage, after a fourth launch – to have taken place from Plesetsk with the RVSN-40 payload – was canceled after the vehicle’s payload fairing separated while it was still on the launch pad.

Rokot’s first launch from Plesetsk, and the first to use a Briz-KM, occurred in May 2000 carrying SimSat – a pair of dummy payloads representing the mass of two Iridium communications satellites.

Friday’s launch was Rokot’s twenty-seventh mission with a Briz-KM upper stage, and thirtieth overall excluding the canceled RVSN-40 launch attempt. Prior to the development of the European Vega rocket, Rokot was regularly used by the European Space Agency to launch its small scientific payloads, while the type has also carried payloads for commercial operators and the Russian government.

Of the twenty-nine Rokot launches to date, twenty-six have been completed successfully, with two failures and one partial failure. In addition to the failures, on one of the successful missions the Briz-KM failed to perform a planned deorbit burn after spacecraft separation.

Because of its missile heritage, Rokot is fuelled by storable hypergolic propellant: unsymmetrical dimethylhydrazine (UDMH) oxidized by dinitrogen tetroxide. The first stage is powered by an RD-0233 engine, while the second features an RD-0253 with four RD-0236 vernier motors.

Rokot’s launch took place from Site 133/3 at the Plesetsk Cosmodrome in Northern Russia, which was used by the Soviet Union’s Kosmos-2I and Kosmos-3M rockets – derived from the R-12 and R-14 missiles respectively, from 1967 to 1994.

After lifting off, the rocket climbed away from Plesetsk, encountering max-Q, the area of maximum dynamic pressure, about a minute into the flight. The first stage shut down and separated about 120 seconds after liftoff, with the second stage igniting to continue the climb into orbit. Rokot’s payload fairing separated a little under three minutes into the mission.

Rokot’s course took it north from Plesetsk, over the Arctic. The second stage burned for about three minutes, delivering Briz-KM to a suborbital trajectory. Shortly after the spent second stage separated, Briz ignited to inject itself and the payload into an initial parking orbit. The mission then entered a coast phase, with Briz-KM waiting until it reaches apogee to restart for a brief circularisation burn, attaining the planned circular sun-synchronous orbit.

With the second burn complete, Sentinel-5p was released from the Briz-KM via separation of the CASA CRSS-937 clamp band that keeps it attached to the rocket. Following spacecraft separation, Briz-KM made a third burn to lower its orbit – putting distance between itself and the payload and either hastening its decay, or deorbiting itself to a safe, destructive reentry.

Friday’s launch may have been Rokot’s penultimate flight. Russia is retiring Rokot in favor of newer rockets such as the Soyuz-2-1v and Angara-1.2.

Early last year, Russia’s TASS news agency reported that the type would be used for two more Russian government launches, with Gonets-M communications satellites, but earlier this year TASS reported that the final launch would occur in late 2017 or early 2018 with ESA’s Sentinel-3B satellite. That launch is expected to occur in the next few months.

(Images via ESA and Eurockot)

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