PicSat
PicSat is a French nano-satellite, a CubeSat made of 3 units, designed to measure the transit of the planet Beta Pictoris b in front of its star Beta Pictoris. PicSat was designed and built by a small team of scientists and engineers led by Dr. Sylvestre Lacour, astrophysicist and instrumentalist at the High Angular Resolution in Astrophysics group of the LESIA laboratory at the Paris Observatory / PSL Research University / CNRS. The satellite was launched on January 12, 2018. It operated for more than 10 weeks, then fell silent on March 20, 2018.
The Beta-Pictoris Star System
With an age of about 23 million years, Beta Pictoris is a very young star, astronomically speaking. Compared to the Sun, which is halfway through its adult life at 4.5 billion years, Beta Pictoris is about twice the mass and twice the size. Beta Pictoris is relatively close to the Sun, just 63.4 light years away, which means it is bright and easy to observe. This makes Beta Pictoris interesting for study as it allows astronomers to learn more about the very early stages of planet formation.In the early 1980s a large disk of debris of asteroids, dust and gas was found surround Beta Pictoris, leftover from the formation of the star. In 2009 a giant gas planet was discovered by a team of French astronomers led by Anne-Marie Lagrange from Grenoble, France. The planet, named Beta Pictoris b, is about seven times as massive as Jupiter, the largest planet in the Solar System. It orbits the star at some ten Astronomical Units, which is ten times the distance between the Earth and the Sun and the same distance as planet Saturn orbits the Sun.
A possible transit of Beta-Pictoris b or its Hill Sphere
In 2016 there were predictions published that the Beta Pictoris b Hill Sphere, or perhaps the planet itself, would be passing in front of its star as seen from the Earth. The detailed observation of such a phenomenon would allow learning more about the young planet. For example, if the planet itself were to transit and we would be able to observe this transit, then its exact size and the extension and composition of its atmosphere could be derived. Knowledge of the size of the planet, in combination with its mass, leads to its density. The density is directly related to the chemical composition: a lower density means more gaseous material, a higher density more rocky material. Because the planet Beta Pictoris b is so young, knowing these parameters gives more insight into the formation of giant planets and planetary systems in general.However, the moment of transit can only be estimated roughly, because the orbit of Beta Pictoris b is not that well known. The transit has been predicted to occur between the summer of 2017 and the summer of 2018. A transit of the planet would only last for up to a few hours. A transit of the Hill Sphere can last anywhere from days to months. In order to capture the phenomenon, continuous accurate monitoring of the star system from Space is the only possible way. It cannot be done from Earth, because on the one hand professional observatories are in high demand and cannot be used for long term continuous monitoring, and on the other hand even smaller observatories and high level amateurs all suffer from the same problem: the Earth's varying atmospheric conditions and day-night cycle. Even the most tenuous cloud passing in front of a telescope could be misinterpreted as a transit phenomenon.
However, observations are being done non-continuously using Earth-based observatories. At the time of writing, no signs of any transit have yet been detected from these observations.
During a transit the amount of the light that reaches us from the star is slightly diminished while the planet is blocking out a small part of it. This little dip in the star's brightness is the signature of the transit and can be captured by a sensitive instrument, a photometer, that accurately measures the light from the star collected by a telescope. The main goal of PicSat is exactly that: To observe the brightness of the star Beta Pictoris continuously, so as to capture the small dip in that light as the planet Beta Pictoris b or its Hill Sphere passes in front of it.
The PicSat Project
PicSat, a contraction of "Beta Pictoris" and "Satellite", is a so-called CubeSat. PicSat is composed of three standard cubic units, called a "3U", each 10x10x10cm in size.PicSat is the very first CubeSat to be operated by the CNRS. It is different from most CubeSat projects in that it has been developed by professionals, not by students. The project started in 2014 when Sylvestre Lacour, astrophysicist and instrumentalist at the French CNRS at the LESIA laboratory / Paris Observatory, got the idea to use a CubeSat to observe the predicted transit of planet Beta-Pictoris b. He gathered a small local team and together they designed and built PicSat.
PicSat is one of the few CubeSats worldwide with an astrophysical science goal, and the first CubeSat in the challenging field of exoplanetary science. The PicSat science case has been defined in collaboration with Dr. Alain Lecavelier des Etangs, from the Institut d’Astrophysique de Paris, who has been working on the Beta Pictoris system for many years. The PicSat project also results from the fruitful collaboration with CCERES, the space "Center & Campus" of PSL Research University, and with French Space Agency CNES experts.
PicSat was launched into Low Earth Orbit on 12 January 2018 by the Indian ISRO PSLV C40 mission.
PicSat Techspecs
PicSat consists of three cubic units. The top and middle cubic units hold the satellite's payload, the bottom unit contains the onboard computer.The top cubic unit of PicSat contains a small telescope, with a five centimeter diameter mirror. Due to the fact that Beta Pictoris is such a bright star, this small mirror size is sufficient to collect enough light from the star.
There are two important innovative technical aspects for PicSat. The first is the innovative way the satellite will fine track the star, the second is the use of an optical fibre in space to direct the light from the star into the photodiode.
In the middle cubic unit a tiny optical fibre, three micrometer in diameter, about a fifth the size of a thin human hair, collects the light from the star. Used frequently at Earth based observatories, it will be the first time an optical fibre is flown in space for astronomical observations. Thus the telescope system in the top unit sends the light of the star onto its focal plane at the bottom of the unit and into the fibre. The tiny fibre guides the light into a sensitive photodiode in the middle unit that accurately measures the arrival time of each photon individually. The idea of using an optical fibre is that due to its small size, it eliminates all disturbing light sources from entering the photodiode, and thus allows for a very accurate measurement of the star's brightness. These are for example stray light from the sky and scattered light within the optical system.
However, the standard pointing accuracy of a CubeSat is not sufficient to allow the telescope to send the light from the star exactly into the small opening of the fibre all the time. The telescope will wiggle and wobble too much as it orbits the Earth. The PicSat team devised an innovative solution for this problem by connecting the optical fibre to a small plate, a so-called piezoelectric actuator, that can move very fast. By moving the fibre very fast around the star, it can track where the star is drifting off to, and then immediate follow it, so as to remain on target.
The bottom cubic unit of PicSat contains the onboard computer for the operation of the satellite, communication with Earth, batteries, raw pointing of the telescope and other important monitoring tasks.
The whole satellite is clothed in arrays of solar panels that deploy once in Space. They provide with energy for the operation of all the systems. The total weight of PicSat is about 3.5 kg and the power consumption is about 5 Watt.
PicSat Orbit and Communication
PicSat was launched into Low Earth Orbit on 12 January 2018 by Indian ISRO on the PSLV PSLV-C40 mission.The orbit of PicSat is a polar orbit. This means that PicSat will orbit over the poles as the Earth rotates below it, which will allow it to observe Beta Pictoris continuously. Each orbit will take 94 minutes to complete.
The satellite is operated from the PicSat Ground Station at the Paris Observatory in Meudon, France. However, the Ground Station is only able to see the satellite about 30 minutes per day. PicSat communicates at radio amateur frequencies. This has been made possible thanks to the involvement of the Réseau des Émetteurs Français. Anyone with radio receiving capabilities can tune in to the satellite's transmissions when it is passing overhead, receive information from the satellite, and upload it to a data base via the PicSat website. A large network of radio amateurs is called upon to collaborate with the tracking the satellite, receive its data and transmit it to the Ground Station. Interested person are referred to the PicSat website to register, follow the updates and become part of the radio network. Radio amateurs that are licensed to transmit can use PicSat as a transponder, when it is not doing science observation or other communication, to talk to other amateurs.
The website will display this information. It will also show the up-to-date light curve of the star as measured by PicSat. The light curve is the brightness of the star as a function of time.
PicSat was predicted to operate for one year. It operated for approximately 10 weeks before contact was lost on March 20, 2018. Attempts to re-establish contact were made, and on March 30, it was believed that contact was re-established by a team at Morehead State University but this turned out to be a signal from a different satellite. The mission was officially closed April 5.