LISA Frequently Asked Questions (FAQs)
February 2, 2011
If you are interested in some questions specific to ESA, please visit their website or FAQ. For more information on LISA science, please visit the LISA Science Team website and FAQ
4. LISA Pathfinder (LPF)
1.1 Can LISA science be done from the ground?
1.2 How can we be sure that LISA will see gravitational wave sources?
- No. Both ground motion and time variations in gravity from spurious mass motions on the Earth prevent observations below about 1 Hz on the ground. It is necessary to make measurements in space in order to observe many of the important astrophysical sources throughout the Universe.
1.3 Why should we launch LISA before LIGO makes a direct detection?
- We have guaranteed verification binaries known from conventional electromagnetic astronomy (stacks.iop.org/CQG/23/S809), and will have ten thousand or more additional white dwarf binaries. The black hole merger and EMRI rates are somewhat uncertain, but as the Beyond Einstein Program Assessment Committee (BEPAC) confirmed, even the most pessimistic rate estimates allow LISA a few exciting sources.
- LISA can see merging black holes to z>20 if they exist. We see black holes at the center of galaxies and the fossil remains of galaxy mergers, so mergers must be happening. Even the most pessimistic rate estimates yield several detections of massive black hole binaries with LISA. Observation of such a low rate would mean that we would need to rethink models of hierarchical structure formation.
1.4 Since LISA is sensitive to all of the sources at the same time, won't there be source confusion?
- LISA and LIGO are complementary. However, LIGO observations are made in an entirely different frequency range and won't tell us anything useful for planning and building LISA. LISA is sensitive in the band from 0.03 to 100 mHz, and LIGO is sensitive from 0.01-1.5 kHz. They look at entirely different sources, and entirely different astrophysics.
- LISA has guaranteed sources (stacks.iop.org/CQG/23/S809) and many more expected sources in general. Many of these sources are long-lived and at frequencies in the LISA band tend to have relatively strong signals that can be seen with SNRs of 100 or more.
- Observations by LIGO will provide useful information on mergers of neutron star binaries and roughly 10 solar mass black holes. However, only LISA will be able to provide new information about the initial formation and growth of the much more massive black holes that have been found in galactic centers, and about the interaction of such massive black holes with the galaxy formation process out to large redshifts.
1.5 Is LISA more than just a niche mission for relativists?
- We've already demonstrated we can separate sources effectively. See for example the Mock Lisa Data Challenge results (for example http://astrogravs.nasa.gov/docs/mldc/ and http://arxiv.org/pdf/0711.2667v1). Thirteen collaborations submitted 22 entries demonstrating the extraction of ~ 20,000 Galactic binaries, accurate estimation of Massive Black Hole inspiral parameters, and positive detection of Extreme Mass Ratio Inspiral (EMRI) signals.
- LISA is similar to a radio telescope in that it measures both signal magnitude and phase. Much like a radio receiver, LISA can be "tuned" (at the time of data analysis, without touching the instrument) to zero in to a specific source, silencing all others. It will be possible to separate many sources this way. It is true that there will be interference (confusion) between some sources that appear at the same frequency, such as galactic binaries at low frequencies, but LISA will be able to detect and characterize thousands of sources without confusion.
1.6 How does LISA get positions for its sources?
- LISA results will be of strong value to astronomers and cosmologists interested in the initial growth of structure in the universe, as well as to many other astrophysicists. Ground-based gravitational wave detectors are likely to provide strong new tests of the predictions of general relativity under strong field conditions, and LISA will provide even stronger tests. However, such tests are only one of the four main scientific objectives of LISA.
1.7 We don't know gravitational waves exist, so isn't LISA premature?
- LISA's directional sensitivity derives to a large part from the orbital motion of the detector and to a smaller part from the intrinsic directionality of the detectors response. The orbital motion in combination with the proper motion of the detector result in a modulation of the signal in both amplitude and phase as the antenna pattern sweeps across the position of the source, and this modulation helps to determine the position, typically to about one degree (see for example http://adsabs.harvard.edu/abs/1996CQGra..13A.279P or http://prola.aps.org/abstract/PRD/v57/i12/p7089_1). For some sources such as massive black hole binaries, the internal dynamics of the source allows a significant improvement in localization (10 arc-minutes, see for example http://arxiv.org/abs/0710.3795).
1.8 If gravitational waves exist, why bother to look for them?
- While it is true that gravitational waves have never been detected directly, there is overwhelming evidence that they exist. The rate of orbit shrinkage of binary pulsars (e.g. 1916+13 that led to the Hulse-Taylor Nobel Prize see for example http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm) is exactly as predicted by GR if gravitational waves are carrying off the orbital energy of the pulsars. So we are very confident that GR correctly describes gravitational waves in weak field gravity even though we haven't detected them directly. LISA will use those waves to do astrophysics that cannot be done any other way, employing them to probe strong-field gravity for which we still have no direct tests other than via gravitational waves.
1.9 What can LISA tell us about General Relativity?
- LISA's goal is not limited to discovering low frequency gravitational waves, but to use them as a new window into astrophysical and physical phenomena that cannot be studied any other way. Gravitational waves carry information from objects that have no electromagnetic signature (such as the capture of neutron stars by massive black holes), whose electromagnetic signature is obscured by dust (GW are not absorbed) or is too weak (LISA detects amplitude, not power and can observe GW events out to redshifts of z~20)
- We know General Relativity (GR) is an extremely good approximation to Nature in the weak field. In the strong field we have only rather circumstantial evidence for the validity of GR, and no clear demonstration that massive compact objects are really described by the vacuum uncharged Kerr metric. Nonlinear GR is so complicated that we can't be sure that there are not other stable solutions that ordinary matter might produce. Furthermore even if Einstein was right and Gμν=8πTμν, GR does not tell us what the stress-energy tensor is for highly relativistic compact objects that generate strong (and therefore non-linear) gravitational fields Tμν. Unexpected nearby matter, charge configurations or exotic new fields (soliton stars, boson stars) might exist near or in some or all massive compact objects which we call "black holes".
2.1 What are the technical tall poles for LISA?
2.2 Doesn't LISA require formation flying with nanometer precision, which has never been demonstrated?
- Three important technologies for LISA are
- Gravitational Reference Sensor (GRS) - The GRS is the test mass assembly with supporting subsystems. Those subsystems include the test mass itself, the reference housing with sensing and forcing electrodes, the front-end electronics for sensing and forcing, the charge sensing and control subsystem, the caging subsystem and the vacuum system. The LISA Pathfinder (LPF) qualification model, which is designed to meet the LISA requirements, has been built and tested to verify that it meets all requirements for LPF. The flight model is built, and is currently undergoing integration. The LISA GRS performance will be further demonstrated on-orbit by the LISA Pathfinder mission.
- The micronewton thrusters - These quiet, finely adjustable thrusters are the actuators used in the drag-free control system to position the spacecraft. The LISA Pathfinder mission will demonstrate the performance of two different microthruster technologies on-orbit, except for the lifetime requirement. ESA has completed development and testing of an engineering model of the Field Emission Electric Propulsion thruster for LISA Pathfinder based on cesium and a slit geometer. NASA has delivered two flight units of the colloidal micronewton thrusters to ESA, and they have been integrated onto the Pathfinder spacecraft. The LISA lifetime requirements are being demonstrated in accelerated ground testing.
- The phasemeter - The phasemeter digitizes the fringe signal from the interferometer and determines its frequency relative to a local oscillator. The phasemeter has demonstrated all requirements in a laboratory environment and is on track to demonstrate engineering model flight readiness by the end of 2011.
2.3 Micronewton thrusters are new technology. What is LISA doing to develop this technology?
- No. LISA is not formation flying. Each spacecraft freely follows its own Keplerian path, and the arm lengths vary by ~ 1% or about 50,000km. (See for example: http://arxiv.org/abs/gr-qc/0410093.)
- Since the LISA spacecraft are in independent orbits, no station keeping is required. The only propellant required is for the micronewton thrusters that are keeping the GRS drag free.
2.4 How can you test thruster 5-year lifetime in time for launch this decade?
- Two different microthruster technologies will be demonstrated on ST7 and LISA Pathfinder, the Colloid Micro-Newton Thruster (CMNT) developed in the US, and the Field Emission Electric Propulsion (FEEP) thruster developed in Europe. The CMNT flight hardware has been already been tested and qualified for the expected vibrational and thermal environments, and been delivered to ESA and integrated onto the LPF spacecraft.. Direct measurements of thrust on a microthrust stand have shown that the CMNT thrusters meet the performance requirements with margin including thrust noise (<0.1 μN/√Hz), precision (<0.1 μN), and range (5-30 μN). These performance requirements are the same for Pathfinder and LISA. A 3400-hour long duration test has shown that the CMNT thrusters can meet the 90-day Pathfinder mission lifetime with extra margin. The FEEP thruster system has also been put through many performance and long duration tests, passing each one. The FEEPs are currently at TRL 5. Flight hardware fabrication of the FEEPs is ongoing.
2.5 How can you get the telescopes of the three spacecraft correctly pointed at each other?
- We have already accumulated 50,000 hours (~ 7 years) of testing experience on many different thruster systems in many different long duration tests. This has helped us uncover and solve many of the failure modes. In the near future, we plan to use accelerated life testing techniques and verify predictive models (similar to what is used in the semiconductor industry to predict pump laser lifetimes) to validate our thruster designs. By mid-2010 we'll begin a long duration test of a flight-like LISA microthruster system. The thruster will run autonomously, accumulating approximately 8000 hours of operation per year, reaching the mission lifetime requirement of 40,000 hours in five years, and accumulating life margin before the microthruster subsystem critical design review. Using both the validated predictive models and the test results, we will be able to verify that the thrusters will meet the LISA lifetime requirements.
2.6 What if one spacecraft fails? Is LISA then a total loss?
- The process of aligning the telescopes to each other is called "Spatial Acquisition" (to distinguish it from the process of acquiring the correct laser frequencies, called frequency acquisition). Once the spacecraft are in the correct orbits, the positions and velocities of each are estimated from the ground. The spacecraft determine their angular orientation using on-board star-trackers. In addition, the proof masses are used as a gyro (accelerometer mode) to steady attitude knowledge drifts while waiting for the acquisition process to complete. One spacecraft is chosen to activate the laser on one arm. It points to the estimated direction of the other spacecraft. The spacecraft at the other end of the arm is commanded to point toward the estimated position of the first spacecraft with its laser off. The spacecraft with the active laser scans to cover the region of uncertainty in the knowledge of where the other spacecraft lies. The size of this region is mainly dominated by the accuracy of the star tracker, as well as those of the orbits, and is estimated to be ~ 9 urad half angle. The full width half maximum (FWHM) of the beam is ~ 2.6 urad, hence a scan of the uncertainty region is required. The other spacecraft uses its telescope and a CCD detector to look for a signal. When it sees a signal, it orients itself to center the signal on the CCD and turns on its laser. When the scan completes the spacecraft with the active laser turns off the laser and receives the signal from the other spacecraft, since it should be correctly pointed. The first spacecraft corrects its angular orientation using an array of sensors, from a CCD detector all the way to heterodyned wavefront sensing, thus acquiring one link. The process in repeated for the opposing link to acquire one LISA arm.
- Once one arm is acquired, the process moves to the next arm and is repeated until all three arms of LISA are acquired. Note that each link requires 2 rotational degrees of freedom per spacecraft, and each spacecraft has four: the three rigid-body rotations and the telescope articulation. Therefore is it possible to maintain each link as the next one is acquired. Furthermore, the spacecraft motion becomes smaller as each link is established, so typically it is hardest to acquire the first link and becomes easier as each link is acquired. Estimated worst-case time for acquisition of a link is about one hour per link.
- A more complete description of the process may be found in http://www.iop.org/EJ/article/0264-9381/22/10/038/cqg5_10_038.pdf
2.7 Why isn't there a fourth spacecraft for redundancy?
- Yes, if one spacecraft fails completely, then the mission fails. However, it is extremely unlikely that an entire spacecraft would fail. The spacecraft are designed with enough redundancy to avoid credible single-point failures, and it is possible to accomplish the scientific goals of the mission with only four of the possible six one-way links between spacecraft working.
2.8 How do the three spacecraft get into the proper orbits?
- Cost efficiency. For the same probability for mission success, it is more cost-effective to add redundancy in each of the three spacecraft than adding an additional spacecraft, either in orbit or on the ground to be launched as a replacement.
3.1 Isn't LISA too complicated to manage because there are many agencies involved?
3.2 Does LISA provide support for postdoctoral researchers and students?
- NASA has extensive experience with international collaborations, and LISA is not the first project of this kind. The approach of equal partnership during the Formulation Phase of the project is deliberate and allows in-depth understanding of the technical issues to be fostered on both sides. During the Implementation Phase there will be a clear division of roles with one agency in charge.
- LISA has been operating successfully under a formal agreement between Agency headquarters since 2004 that spells out the roles and responsibilities for the different agencies.
- NASA's SARA program funds all university grants, and it responds to proposal pressure and mission popularity. Send in proposals.
4. LISA Pathfinder (LPF)
4.1 Could the results of Pathfinder result in a major delay of the mission?
4.2 How long will it take to analyze Pathfinder data?
- Possible but unlikely. The development and ground testing work for Pathfinder will shorten the development time for LISA. It is unlikely that flight testing will uncover a major problem not already uncovered during ground testing, but if so, of course the LISA design may have to be changed. After all, this is the purpose and advantage of a technology precursor mission: to identify problems before launch. Results from Pathfinder will be known at least 5 years before LISA CDR, way before any design is finalized or flight hardware built. Therefore major delays to the LISA mission are not expected due to results from Pathfinder.
4.3 What if Pathfinder blows up on the launch pad?
- The core Pathfinder mission lasts 6 months, after a 2-month cruise phase and one month of commissioning. Data analysis will be almost immediate, so the primary results should be available 6-9 months after launch.
- Of course this would be a loss, but there is a very large value in the development work that was done to reach the launch pad in the first place. New techniques for ground-testing developed during Pathfinder development have retired many risks we previously thought would require in-flight testing, so we now think it pretty unlikely that flight will uncover a major problem. Therefore, if Pathfinder is not able to successfully reach operational status for some reason beyond our control, all of the LISA technologies will still been developed and tested to flight readiness.