LISA Pathfinder

LISA Pathfinder

Model of the LISA Pathfinder spacecraft
Mission type	Technology demonstrator
Operator	ESA[1]
COSPAR ID	2015-070A
SATCAT no.	41043
Mission duration	576 days
Spacecraft properties
Manufacturer	Airbus Defence and Space
Launch mass	1,910 kg (4,210 lb)[1]
BOL mass	480 kg (1,060 lb)[2]
Dry mass	810 kg (1,790 lb)
Payload mass	125 kg (276 lb)
Dimensions	2.9 m × 2.1 m (9.5 ft × 6.9 ft)
Start of mission
Launch date	3 December 2015, 04:04:00 UTC[3][4][5]
Rocket	Vega (VV06)
Launch site	Kourou ELV
Contractor	Arianespace
End of mission
Disposal	Decommissioned
Deactivated	30 June 2017
Orbital parameters
Reference system	Sun–Earth L1
Regime	Lissajous orbit
Periapsis altitude	500,000 km (310,000 mi)
Apoapsis altitude	800,000 km (500,000 mi)
Inclination	60 degrees
Epoch	Planned
Transponders
Band	X band
Bandwidth	7 kbit/s
Instruments
~36.7 cm Laser interferometer
ESA astrophysics insignia for LISA Pathfinder Horizon 2000 Plus

LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), was an ESA spacecraft that was launched on 3 December 2015 on board Vega flight VV06.^[3][4][5] The mission tested technologies needed for the Laser Interferometer Space Antenna (LISA), an ESA gravitational wave observatory planned to be launched in 2035. The scientific phase started on 8 March 2016 and lasted almost sixteen months.^[6] In April 2016 ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.

The estimated mission cost was €400 million.^[7]

Mission

LISA Pathfinder placed two test masses in a nearly perfect gravitational free-fall, and controlled and measured their relative motion with unprecedented accuracy. The laser interferometer measured the relative position and orientation of the masses to an accuracy of less than 0.01 nanometres,^[8] a technology estimated to be sensitive enough to detect gravitational waves by the follow-on mission, the Laser Interferometer Space Antenna (LISA).

The interferometer was a model of one arm of the final LISA interferometer, but reduced from millions of kilometers long to 40 cm. The reduction did not change the accuracy of the relative position measurement, nor did it affect the various technical disturbances produced by the spacecraft surrounding the experiment, whose measurement was the main goal of LISA Pathfinder. The sensitivity to gravitational waves, however, is proportional to the arm length, and this is reduced several billion-fold compared to the planned LISA experiment.

LISA Pathfinder was an ESA-led mission. It involved European space companies and research institutes from France, Germany, Italy, The Netherlands, Spain, Switzerland, UK, and the US space agency NASA.^[9]

LISA Pathfinder science

LISA Pathfinder was a proof-of-concept mission to prove that the two masses can fly through space, untouched but shielded by the spacecraft, and maintain their relative positions to the precision needed to realise a full gravitational wave observatory planned for launch in 2035. The primary objective was to measure deviations from geodesic motion. Much of the experimentation in gravitational physics requires measuring the relative acceleration between free-falling, geodesic reference test particles.^[10]

In LISA Pathfinder, precise inter-test-mass tracking by optical interferometry allowed scientists to assess the relative acceleration of the two test masses, situated about 38 cm apart in a single spacecraft. The science of LISA Pathfinder consisted of measuring and creating an experimentally-anchored physical model for all the spurious effects – including stray forces and optical measurement limits – that limit the ability to create, and measure, the perfect constellation of free-falling test particles that would be ideal for the LISA follow-up mission.^[11]

In particular, it verified:

• Drag-free attitude control of a spacecraft with two proof masses,
• The feasibility of laser interferometry in the desired frequency band (which is not possible on the surface of Earth), and
• The reliability and longevity of the various components—capacitive sensors, microthrusters, lasers and optics.

For the follow-up mission, LISA,^[12] the test masses will be pairs of 2 kg gold/platinum cubes housed in each of three separate spacecraft 2.5 million kilometers apart.^[13]

Spacecraft design

LISA Pathfinder was assembled by Airbus Defence and Space in Stevenage (UK), under contract to the European Space Agency. It carried a European "LISA Technology Package" comprising inertial sensors, interferometer and associated instrumentation as well as two drag-free control systems: a European one using cold gas micro-thrusters (similar to those used on Gaia), and a US-built "Disturbance Reduction System" using the European sensors and an electric propulsion system that uses ionised droplets of a colloid accelerated in an electric field.^[14] The colloid thruster (or "electrospray thruster") system was built by Busek and delivered to JPL for integration with the spacecraft.^[15]

LISA Pathfinder exploded view

Instrumentation

The LISA Technology Package (LTP) was integrated by Airbus Defence and Space Germany, but the instruments and components were supplied by contributing institutions across Europe. The noise rejection technical requirements on the interferometer were very stringent, which means that the physical response of the interferometer to changing environmental conditions, such as temperature, must be minimised.

Environmental influences

On the follow-up mission, eLISA, environmental factors will influence the measurements the interferometer takes. These environmental influences include stray electromagnetic fields and temperature gradients, which could be caused by the Sun heating the spacecraft unevenly, or even by warm instrumentation inside the spacecraft itself. Therefore, LISA Pathfinder was designed to find out how such environmental influences change the behaviour of the inertial sensors and the other instruments. LISA Pathfinder flew with an extensive instrument package which can measure temperature and magnetic fields at the test masses and at the optical bench. The spacecraft was even equipped to stimulate the system artificially: it carried heating elements which can warm the spacecraft's structure unevenly, causing the optical bench to distort and enabling scientists to see how the measurements change with varying temperatures.^[16]

Spacecraft operations

Mission control for LISA Pathfinder was at ESOC in Darmstadt, Germany with science and technology operations controlled from ESAC in Madrid, Spain.^[17]

Lissajous orbit

The spacecraft was first launched by Vega flight VV06 into an elliptical LEO parking orbit. From there it executed a short burn each time perigee was passed, slowly raising the apogee closer to the intended halo orbit around the Earth–Sun L₁ point.^[1][18][19]

Animation of LISA Pathfinder 's trajectory

Polar view

Equatorial view

Viewed from the Sun

   Earth ·   LISA Pathfinder

Chronology and results

The final results (red line) far exceeded from the initial requirements.

The spacecraft reached its operational location in orbit around the Lagrange point L1 on 22 January 2016, where it underwent payload commissioning.^[20] The testing started on 1 March 2016.^[21] In April 2016 ESA announced that LISA Pathfinder demonstrated that the LISA mission is feasible.^[22]

On 7 June 2016, ESA presented the first results of two months' worth of science operation showing that the technology developed for a space-based gravitational wave observatory was exceeding expectations. The two cubes at the heart of the spacecraft are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a factor of 5 better than requirements for LISA Pathfinder.^[23][24][25] In February 2017, BBC News reported that the gravity probe had exceeded its performance goals.^[26]

LISA Pathfinder was deactivated on 30 June 2017.^[27]

On 5 February 2018, ESA published the final results. Precision of measurements could be improved further, beyond current goals for the future LISA mission, due to venting of residue air molecules and better understanding of disturbances.^[28]

See also

• Einstein Telescope, a European gravitational wave detector
• GEO600, a gravitational wave detector located in Hannover, Germany
• LIGO, a gravitational wave observatory in USA
• Taiji 1, a Chinese technology demonstrator for gravitational wave observation launched in 2019
• Virgo interferometer, an interferometer located close to Pisa, Italy

References

1. "LISA Pathfinder: Operations". ESA. 8 January 2010. Retrieved 5 February 2011.
2. "LPF (LISA Pathfinder) Mission". ESA eoPortal. Archived from the original on 2015-10-17. Retrieved 2014-03-28.
3. "Launch Schedule". SpaceFlight Now. Archived from the original on 2016-12-24. Retrieved 2015-10-16.
4. "Call for Media: LISA Pathfinder launch". ESA. 23 November 2015.
5. "LISA Pathfinder enroute to gravitational wave demonstration". European Space Agency. Retrieved 3 December 2015.
6. "News: Top News - LISA Gravitational Wave Observatory". Archived from the original on 2016-04-19.
7. "LISA Pathfinder To Proceed Despite 100% Cost Growth". Space News. 22 June 2011.
8. "LISA Pathfinder Ready for Launch from Kourou" (Press release). Airbus Defence and Space. November 30, 2015 – via SpaceRef.^{[permanent dead link‍]}
9. "LISA Pathfinder international partners". eLISAscience.org. Archived from the original on 26 September 2015. Retrieved 7 September 2015.
10. science objective of LISA Pathfinder Archived 2014-10-21 at the Wayback Machine.
11. "LISA Pathfinder Science". eLISAscience.org. Archived from the original on 21 October 2014. Retrieved 9 July 2014.
12. "LISA Gravitational Wave Observatory - We will observe gravitational waves in space - New Astronomy - LISA Pathfinder".
13. Official design proposal at https://www.elisascience.org/files/publications/LISA_L3_20170120.pdf
14. Ziemer, J.K.; and Merkowitz, S.M.: “Microthrust Propulsion of the LISA Mission,” AIAA–2004–3439, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Fort Lauderdale FL, July 11–14, 2004.
15. Rovey, J. "Propulsion and Energy: Electric Propulsion (Year in Review, 2009)" (PDF). Aerospace America, December 2009, p. 44. Archived from the original (PDF) on 2015-12-08. Retrieved 2012-10-26.
16. "LISA Pathfinder Technology". eLISAscience.org. Archived from the original on 21 October 2014. Retrieved 9 July 2014.
17. "LISA Pathfinder: Fact sheet". ESA. Retrieved 20 April 2009.
18. "LISA Pathfinder: Mission home". ESA. Retrieved 5 February 2011.
19. "ESA's new vision to study the invisible universe". www.esa.int. Retrieved 26 June 2014.
20. "First locks released from LISA Pathfinder's cubes". ESA. ESA Press Release. February 3, 2016. Retrieved 2016-02-12.
21. Amos, Jonathan (1 March 2016). "Gravitational waves: Tests begin for future space observatory". BBC News. Retrieved 2016-03-01.
22. Gravitational Observatory Advisory Team, ed. (28 March 2016). The ESA–L3 Gravitational Wave Mission - Final Report (PDF). ESA–L3 Final Report. p. 4.
23. M. Armano; et al. (2016). "Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results". Physical Review Letters. 116 (23): 231101. Bibcode:2016PhRvL.116w1101A. doi:10.1103/PhysRevLett.116.231101. hdl:2117/102419. PMID 27341221.
24. "LISA Pathfinder exceeds expectations". ESA. 7 June 2016. Retrieved 7 June 2016.
25. "LISA Pathfinder exceeds expectations". Benjamin Knispel. elisascience.org. 7 June 2016. Archived from the original on 3 August 2016. Retrieved 7 June 2016.
26. "Gravity probe exceeds performance goals". Jonathan Amos, BBC Science Correspondent, Boston. 18 February 2017. Retrieved 20 February 2017.
27. "LISA Pathfinder Will Concludee Trailblazing Mission". ESA Science and Technology. ESA. 20 June 2017. Retrieved 17 August 2017.
28. "ESA creates quietest place in space". 2018-02-05. Retrieved 2018-02-07.

External links

• LISA and LISA Pathfinder's Homepage
• LISA Pathfinder mission home at ESA
• LISA Pathfinder at eoPortal Archived 2015-10-17 at the Wayback Machine
• Max Planck Institute for Gravitational Physics (Albert Einstein Institute Hannover) Archived 2016-05-17 at the Wayback Machine