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Proposal ID = 12237 Principle Investigator = William M. Grundy - Lowell Observatory Title = "Orbits, Masses, Densities, and Colors of Two Transneptunian Binaries" Time = Mar 18, 2012 10:57:30 - 11:54:16 Target = 2005EF298 Instrument = WFC3/UVIS
The mass of an astronomical object can be determined by observing the velocity of something in orbit around it. This works for everything from measuring the mass of the earth based on the orbit of the moon, to measuring the mass of the Milky Way galaxy based on the velocity of the sun as it rotates around the center of the galaxy, to measuring the mass of a black hole, even though it is not possible to see the black hole itself. This is also why binary stars have played a critical role in astronomy for many years, providing the most direct measurements of stellar masses. Similarly, transneptunian (i.e., outside the orbit of Neptune) binaries are valuable because they allow us to measure the masses of objects in the outer regions of the solar system. Once the mass of an object is determined, the next question is what the density is. Is it nearly a point source like a black hole, or is it a large object with a low density like Jupiter. This proposal will use measurements from both Hubble and the Herschel spacecraft, which observes in the infrared part of the spectrum where most of the light from these very cold objects is emitted.
Paraphrasing from the abstract:
Binaries are the key to learning many crucial bulk properties of transneptunian objects (TNOs) including their masses. Perhaps the most interesting mass-dependent property of a TNO is its bulk density, which provides unique information about its bulk composition and interior structure. Densities have so far only been measured for a handful of binary TNO systems. This proposal seeks to determine orbits and thus masses of two more binary TNOs, both of which are also to be observed at thermal infrared wavelengths by the Herschel spacecraft. Combining the masses from Hubble with the sizes from Herschel will enable us to compute their densities. We will also obtain multi-wavelength photometric colors of the individual components of each binary system. It is imperative to link colors to the physical properties measurable in binary systems in order to use the remnant planetesimals in today's Kuiper belt to learn more about the early history of our own solar system, and more generally about how planetesimals form in nebular disks and subsequently evolve. The job of Hubble is to obtain relative astrometry, from which we determine the mutual orbits of the binary bodies, giving us their masses. Meanwhile, Herschel deterines their surface areas. When we put these pieces of information together, we get the bulk densities.
You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "12237" in the Prop. ID box.