LISA Frequently asked questions
LISA Frequently asked questions (FAQs)
This is a list of the most frequently asked questions that are asked
about LISA. If you feel that your specific question (or indeed
answer) is missing, do not hestitate to get
in contact with us.
If you are interested in some questions specific to NASA, please
visit their website or 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 or Virgo make a direct detection?
- We have guaranteed verification binaries known from conventional
electromagnetic astronomy, and will have ten thousand or more
additional white dwarf binaries. The rates from black hole mergers
and extreme mass ratio inspirals are somewhat uncertain, but even
the most pessimistic rate estimates allow LISA a few exciting
sources during the mission lifetime.
- LISA can see merging black holes to a redhift of 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
1.4 Since LISA is sensitive to all of the sources at the same time, won't there be source confusion?
- LISA and the ground-based detectors are
complementary. Observations from ground-based detectors 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 0.01 mHz to 1 Hz, and the
ground-based detetors are sensitive from
about a few hertz to 1.5 kHz. They look at entirely different sources, and entirely
- LISA has guaranteed sources
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 signal-to-noise ratios of 100 or
- Observations by ground-basewd detectors will provide useful information on mergers
of stellar-mass binaries, from neutron star binaries to black hole
binaries of roughly 10 solar mass.
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 centres, and about the interaction
of such massive black holes with the galaxy formation process out to
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.
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 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
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
For some sources such as massive black hole binaries, the internal
dynamics of the source allows a significant improvement in
localization (10 arc-minutes).
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)
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). The science objectives of LISA can be found here.
- 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 main technical challenges 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
- 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 digitises 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
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 50 000 km.
- Since the LISA spacecraft are in independent orbits, neither
station keeping nor any major constellation maintenance is
required. The only propellant required is for the micronewton
thrusters to maintain inertial flight.
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
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
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 μrad half angle. The full width half maximum
(FWHM) of the beam is ~ 2.6 μrad, 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 two 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
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.
- The three spacecraft are stacked vertically in a single launch
vehicle, each attached to a propulsion module. Once launched, the
spacecraft separate and follow three independent trajectories to
their final orbital injection points, taking 10 — 13 months to arrive
on station. Once on station, the sciencecraft separates from the
propulsion module and then goes through an acquisition
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?
- As of today, there are two partner agencies, ESA and NASA. Both
agencies have extensive experience with international
collaborations, and LISA is not the first project of this kind. The
approach of equal partnership during the early 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 and responsiblities.
- Unlike NASA, ESA's science programme does not provide direct
funding to research groups. Intereste researchers should apply
directly to their respective national funding agencies.
- The current planning foresees a substantial support by the
national funding agencies for the LISA data analysis.
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.
31 Jan 2011