LISA Pathfinder will conduct in-flight testing of the concept of low-frequency gravitational wave detection: it will put two test masses in a near-perfect gravitational free-fall, and control and measure their motion with unprecedented accuracy. To do this it will use inertial sensors, a laser metrology system, a drag-free control system and an ultra-precise micro-propulsion system.
The mission aims to demonstrate, in a space environment, that free-falling bodies follow geodesics in space-time, by more than two orders of magnitude better than any past, present, or planned mission. Specifically, it must:
• Demonstrate drag-free flight and attitude control in a spacecraft containing two free proof masses
• Test feasibility of laser interferometry with low frequency picometre resolution
• Test the endurance of the instruments and hardware in the harsh space environment
In other words, LISA Pathfinder will test – in spaceflight – the technology needed to detect gravitational waves in space for the very first time.
Gravitational waves are ripples in space-time predicted by Albert Einstein’s Theory of General Relativity. Detecting gravitational waves would greatly enhance our knowledge of General Relativity and allow scientists to detect the impact of astronomical events which are thought to cause a miniscule distortion on the fabric of space itself.
|ROLE||Space observatory (technology proof)|
|LAUNCH DATE||Autumn 2015|
|ORBIT||Halo orbit about the Lagrangian point L1 (1.5 million km from Earth)|
|NOMINAL MISSION||6 months; may be extended up to 12 months|
|+ Preparing the technology to test gravitational-wave detection in space +|
The LISA Pathfinder Mission Operations Centre (MOC) is located at ESOC, Darmstadt, Germany.
The Flight Control Team
The Flight Control Team will operate from a Dedicated Control Room located at ESOC.
Ian Harrison joined LISA Pathfinder as Spacecraft Operations Manager (SOM) in 2006 and is now working to develop the overall architecture for the mission operations system and the ground segment. He will also oversee creation of FCT and begin team training in the 2007-08 period.
In addition to the mission-dedicated Flight Control Team, other experts from ESOC will play a daily role in managing and controlling flight operations, including engineers from Flight Dynamics, Ground Facilities, Navigation and Mission Data Systems.
Mission operations overview
LISA Pathfinder is due to be launched in 2015 by a Vega rocket from the European Spaceport in Kourou, French Guiana.
For a Vega launch, the upper stage will inject the LISA Pathfinder spacecraft into an elliptical orbit inclined at 5° to the equator, with apogee near 1600 km and a perigee at 200 km. For a Rockot launch, the upper stage, named Breeze KM, will inject the LISA Pathfinder spacecraft into an elliptical orbit inclined at 63° to the equator, with apogee near 800 km and a perigee at 200 km. The critical Launch and Early Orbit Phase (LEOP) will be controlled from ESOC.
The LISA Pathfinder propulsion module, loaded with 1100 kg of bipropellant fuel, will be used to raise the apogee and then finally escape the Earth orbit to transfer the spacecraft to the operational halo orbit at L1. The transfer is targeted such that LISA Pathfinder will drift directly into a pseudo-stable orbit around L1 without any injection burn being necessary.
Transfer to operational orbit about L1
The propulsion module separates prior to the arrival at L1. Very accurate navigation and very careful timing of the separation is necessary to allow the successful transfer into the operational orbit.
After separation from the propulsion module, the LISA Pathfinder spacecraft will be stabilised using the micro-Newton thrusters, entering a Lissajous orbit around the first Sun-Earth Lagrange point (L1).
The L1 orbit has been chosen because it is an intrinsically "quiet" place in space, far away from massive bodies, which induce tidal forces on the spacecraft, has constant illumination from the Sun, and has a quasi-constant Earth distance for communication. This orbit fulfils the stringent requirements of LISA Pathfinder concerning thermal and gravitational stability.
Nominal 180-day mission
Following the initial on-orbit check-out and instrument calibration, the in-flight demonstration of the LISA technology will take place in the second half of 2010. The nominal lifetime of the mission is 180 days, including periods for LTP (LISA Technology Package) instrument operations and DRS (Disturbance Reduction System, supplied by NASA) instrument operations. These instruments are being flown because their technology can not be demonstrated on Earth. As such they will be in a constant state of commissioning during the entire mission period, with the very large learning curve this entails.
The challenge of the Flight Control Team is the intense activities of 3-week orbit raising and escape operations, then maintaining the inherently unstable orbit about L1 and finally maintaining the science module safety while commissioning a highly complex technology demonstration payload with a limited mission duration.
The ground station - Cebreros
Communications to the spacecraft will be performed in X-band through a network of ground stations, including Kourou, Maspalomas and Perth, during the Launch and Early Operations Phase; for this the ground station infrastructure has been upgraded to include X-Band Acquisition aids mounted on the LEOP stations.
In routine operations, communications will be maintained through DSA 2, ESA's 35m deep-space antenna located in Cebreros, Spain. Using Cebreros station, the communications link will be established for six to eight hours per day. During each pass the data stored on board will be retrieved, spacecraft tracking will be performed and the command timeline will be uplinked.
Ground segment & mission control system
The LISA Pathfinder ground segment at ESOC will use the SCOS-2000 mission control system Release 5.0. The LISA Pathfinder ground segment comprises two operational centres, both provided by ESA:
- The MOC is responsible for LEOP, the transfer phase, and all operations during the routine phase and is in contact with the spacecraft for eight hours per day through the ground station(s)
- STOC is the point of interface to the scientific community, and is responsible for the payload scheduling (both long and short-term), quick-look data analysis, data processing and archiving
The design of the mission provides a high level of autonomy. This minimises real-time intervention to mission operation commands sent from ground during LEOP, the transfer and in-orbit operation phases.
A command schedule covering three days of operation will be periodically uploaded from ground and executed autonomously on board. The data generated on board (satellite housekeeping and technology data) will be stored in a mass memory that will be downloaded to ground daily. Such autonomy allows ground operations to be limited to eight hours a day.
The major task of the STOC is to plan and validate the utilisation and related configuration of the scientific instruments on board the spacecraft, derive from the mission products the science and technology results, and feed back these results into later planned instrument activities.
The platform and payload
The spacecraft has an octagonal shape and is 2.9m high with propulsion module attached and 2.1m in diameter; the science module once separated is 0.85m high with a 2.1m diameter. The platform comprises all the necessary systems for communications, on-board computing and data management, propulsion, attitude and orbit control, thermal control and power generation necessary to support the instrument.
In order to attain its scientific objectives, the spacecraft is equipped with two innovative flight control systems: the Drag-Free and Attitude Control System, and the Drag-Free Propulsion System.
LISA Pathfinder will carry two test packages: the LISA Technology Package (LTP), provided by European institutes and industry, and the Disturbance Reduction System (DRS), provided by NASA.
LISA Technology Package
The LTP represents one arm of the (future) LISA interferometer, in which the distance between the two proof masses is reduced from 5 million kilometres to 35 centimetres. As in LISA, the proof masses fulfil a double role: they serve as mirrors for the interferometer and as inertial references for the drag-free control system.
Disturbance Reduction System
The Disturbance Reduction System (DRS) is a NASA-supplied system, which contributes to the LISA Pathfinder mission goals and uses the European LTP. The DRS consists of two clusters of colloidal thrusters that use ionised droplets of a colloidal solution accelerated in an electric field to provide micro-propulsion, and drag-free control software residing on a dedicated computer. The DRS will use the sensor information of the LTP (test masses position and attitude) to control the spacecraft attitude with independent drag-free software and will use the colloidal thrusters as actuators.
Last update: 31 August 2015