BY DR EMILY BALDWIN
Posted: 07 April, 2009
LISA, NASA and ESA’s Laser interferometer Space Antenna, which will attempt to detect gravitational waves, will also turn its “noise” into useful information about near-Earth asteroids.
Gravity waves are associated with the warping of the space-time continuum, believed to be caused by supernovae events or colliding black holes sending ripples through the Universe. These ripples are what LISA is hoping to detect. The mission will comprise three satellites connected by laser beams, and if a gravitational wave passes them by, their separation should change by a distance less than the width of an atom.
Planetary scientists also realised that they too could exploit LISA, since asteroids would also make the spacecraft wobble, leaving a distinct signature in the data being collected. Pasquale Tricarico of the Planetary Science Institute in Tucson, Arizona, developed this idea to predict the number of asteroid encounters LISA can expect and how those encounters can be used to determine the mass of passing asteroids.
The three LISA spacecraft will be placed in orbits that form a triangular formation separated by 5 million km. Ripples in the fabric of space and time will cause LISA to wobble. Passing asteroids will also cause the spacecraft to wobble, providing useful insight into the properties of NEAs. Image: NASA.
Mass may seem like an obvious vital statistic to know about a planetary body, but Tricarico reveals that only the mass of asteroids that have been visited by spacecraft or the mass of a few binary asteroids observed from Earth are known. “We always wonder about the porosity, the density, and this will give us measurements from additional asteroids,” he says. Because LISA will look at known Near-Earth Asteroids (NEAs), data on their trajectories will already be well-defined. “So from the signal, we can indirectly measure the asteroid’s mass because that’s the only uncertainty in the equation,” he adds. Tricarico also points out that if a known asteroid passes one of the satellites and doesn’t leave a signature then an upper limit can be placed on the mass of that asteroid.
The mission could also provide information on the distribution of sizes of the NEA population. “We don’t have good constraints on the size distribution for small asteroids because they have to come very close to Earth for us to observe them using ground-based telescopes,” says Tricarico, who predicts that LISA will ‘feel’ one or two known near-Earth asteroids a year, and a total of around ten during the expected mission lifetime. But if LISA starts detecting five asteroids a year instead of two or three, this could modify theories concerning the distribution of sizes in the NEA population.
There will be a while to wait before any results are seen, however, for LISA will be launched no earlier than 2018. Tricarico’s paper describing the way in which LISA can be exploited for studying asteroids will be published in the Classical and Quantum Gravity journal. His research was supported by NASA’s Applied Information Research Program.