Vega successfully lofts LISA Pathfinder

by William Graham

The European Space Agency’s LISA Pathfinder mission launched atop a Vega rocket Thursday morning, beginning a year-long mission to demonstrate technologies that could be used for future missions to observe gravity waves. The launch – initially delayed 24 hours – took place at 04:04 UTC from the European spaceport at Kourou, French Guiana.

Vega Launch:

Conceived as a precursor mission to the now-cancelled Laser Interferometer Space Antenna (LISA) – a joint mission between the European Space Agency (ESA) and NASA, LISA Pathfinder is intended to prove a concept which scientists believe will allow gravitational waves – a phenomenon predicted by Albert Einstein’s theory of General Relativity but which has never been proven – to be observed by studying differences between perturbations in the orbit of a constellation of satellites.

LISA Pathfinder’s main experiment, the LISA Technology Package (LTP), contains two test masses, 4.6 centimetre (1.8 inch) cubes of a gold-platinum alloy, which will be allowed to float freely in an environment with minimal gravitational interference.

As the masses move within the spacecraft, LTP uses a laser interferometer to identify changes of position on the order of picometres.

NASA’s Disturbance Reduction System (DRS), a system of miniature thrusters – generating an impulse on the order of micronewtons – will be used by the spacecraft to maintain its position relative to the free-floating masses. These thrusters are able to control the spacecraft’s position to the degree of nanometres.

DRS was originally developed by NASA as part of the New Millennium program, under the designation Space Technology 7 (ST-7).

2015-12-02-213247LISA Pathfinder’s experiment is designed to validate the spacecraft can accurately measure the movements of these test masses and manoeuvre accordingly. It is a scaled-down form of the experiment that would have been flown by LISA, using two masses within the same spacecraft instead of three masses in separate spacecraft orbiting at great distances.

The European Space Agency has announced proposals to revive the concept of the LISA mission as the Evolved Laser Interferometer Space Antenna (eLISA), which is slated for launch in the mid-2030s.

LISA Pathfinder was originally to have been part of the Small Missions for Advanced Research in Technology (SMART) programme, designated SMART-2. It would have been the second such mission, after the SMART-1 mission which orbited the Moon between 2004 and 2006.

The LISA Pathfinder spacecraft is a 1,906-kilogram (4,202 lb) vehicle consisting of separable propulsion and experiment modules.

2015-12-02-185336Upon the spacecraft reaching its operational halo orbit around the Earth-Sun L1 Lagrangian point, the propulsion module will separate and manoeuvre away from the experiment module.

In a 500,000 by 800,000 kilometre (300,000 by 500,000 mile) orbit around the Lagrangian Point, the spacecraft will operate for around half a year – spending three months testing the LISA Technology Package, two months testing the Disturbance Reduction System and then finally a month operating the two systems together. A further six-month extension to the mission has been proposed.

2015-12-02-185415Airbus Defence and Space constructed LISA Pathfinder, using a custom bus for the mission. The propulsion module is loosely derived from the Eurostar 2000 bus used for geostationary communications satellites, utilising liquid propellant.

The experiment module accounts for about 480 kilograms (1,058 lb) of the spacecraft’s mass with the propulsion module accounting for the rest. Power will be provided by a panel of solar cells mounted atop the experiment module.

Designated VV06, Thursday’s launch was the sixth for Europe’s Vega rocket and success in the deployment of LISA Pathfinder has been classed as enabling the rocket to be declared operational.

The mission lifted off from the Ensemble de Lancement Vega (ELV) launch pad at the Centre Spatial Guyanais (CSG) or Guiana Space Centre, at Kourou, French Guiana.

Built on the site of a launch pad that was originally constructed for the European Launcher Development Organisation’s (ELDO’s) Europa rocket, and subsequently used in a modified form by the Ariane 1, 2 and 3 vehicles, ELV was reconstructed for Vega ahead of the vehicle’s maiden launch in February 2012.

2015-12-02-185133The four-stage Vega consists of three solid-fuelled stages, burning hydroxyl-terminated polybutadiene propellant, with a liquid propellant upper stage to provide for more precise orbit insertion.

The countdown for Thursday’s launch campaign began eight hours ahead of the planned liftoff, leading up to the start of the launch synchronised sequence four minutes before the count reaches zero.

Vega’s systems were powered up as the countdown progressed, beginning with the Multi-Function Unit (MFU) that controls the rocket’s critical systems at the five-and-a-half-hour mark.

The rocket’s inertial reference, telemetry, computer and flight safety systems were activated over the next forty minutes. Four hours before launch the rocket’s inertial guidance system underwent an initial alignment and checkout.

Two hours and forty minutes before liftoff, the pad gantry was retracted away from the rocket, a process which can take up to 45 minutes. Following the tower’s retraction a second round of alignment and checkout were conducted on the guidance system. Seventy five minutes before launch the rocket’s transponders were turned on.

2015-12-02-185258The Vega’s first stage, powered by a P80FW motor, began the ascent and burned for 110 seconds. Three seconds later the spent stage was jettisoned, and a second after that the second stage ignited to begin its own burn. Using a Zefiro-23 motor, the second stage burn edfor approximately 77 seconds before the mission entered a brief coast phase. The second stage separated 103 seconds after its engine ignited.

Twelve seconds after staging, the third stage’s Zefiro-9 motor lit to begin a 119-second burn. The vehicle’s payload fairing separated four minutes and three seconds into flight, or fourteen seconds after third stage ignition.

At the end of the third stage burn, VV06 coasted for forty two seconds before the final stage separation event. The fourth stage, the Attitude Vernier Upper Module or AVUM, ignited – for its first burn fifty nine seconds later.

2015-12-02-185210Powered by a Ukrainian RD-869 engine, originally developed for the post-boost stage of the R-36M2 missile, AVUM burns unsymmetrical dimethylhydrazine propellant oxidised by dinitrogen tetroxide.

Its first burn lasted eight minutes and 54 seconds, establishing an initial transfer orbit.

At the conclusion of the burn, the mission entered a coast phase for 84 minutes and 55 seconds, completing almost a whole revolution around the Earth before restarting for a 94-second burn to establish LISA Pathfinder’s deployment orbit.

2015-12-02-185245The LISA Pathfinder spacecraft separated from Vega, into its initial 204 by 1,504 kilometre (172 by 935 statute miles; 110 by 812 nautical miles) parking orbit at an inclination of 5.96 degrees to the equator, one hour, 45 minutes and 33 seconds after lifting off.

Using its own propulsion module, the spacecraft will subsequently depart Earth orbit to take up station at the L1 Lagrangian point between the Earth and the Sun – a point in space where the interaction of the Earth and Sun’s gravitational fields upon a body keep it locked in equilibrium between them.

Thursday’s launch was the third of the year for Vega – following February’s mission that deployed ESA’s Intermediate Experimental Vehicle (IXV) demonstrator and June’s launch of the Sentinel 2A Earth science satellite – and the eleventh of 2015 for Arianespace.

Aside from Vega, Arianespace’s previous launches this year have been flown using two Russian Soyuz rockets and six Ariane 5 vehicles. The company has one further launch scheduled for 2015, with a Soyuz slated to loft a pair of Galileo navigation satellites on 17 December.

The next Vega launch is expected to occur no later than July, with PeruSat-1 as its primary payload. Another launch towards the end of the year will carry Turkey’s Göktürk 1.

(Images via ESA and Arianespace).

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