Our SEAES undergraduates undertaking a 3 and 4 year degree programme in Earth and Planetary Science complete a 3rd year ‘Planetary Articles’ module, where they are asked to communicate a planetary topic of their choice in a popular science style to a wide audience.
Andrew O’Mara, an MSci. student, wrote about the Voyager missions and his article has been abridged here for your interest. Thanks to Andrew!
On the Final Frontier: The Voyager Interstellar Mission
What is the solar system? A fairly straightforward question you would expect to have a fairly straightforward answer. But is that really the case? There is more to the solar system than you would think. For instance, how big is the solar system, really? Where does the solar system end? And what is beyond?
Well thanks to Voyager the environment at the edge of the solar system has been investigated by the two spacecraft. And more is being discovered about this region of space each day. This is because the two spacecraft, Voyager 1 and Voyager 2, are the two furthest objects sent from Earth.
Both of the spacecraft have passed through the Kuiper belt which is the region of space where icy dwarf planets can be found and short term comets (<200 year orbits) are thought to originate. Such Kuiper belt objects are thought to be leftovers from the formation of the solar system. The Voyager spacecraft both managed to traverse this 3750 million km wide region of the solar system and move on to the termination shock (see here for more details).
The termination shock is a boundary in space towards the edge of the solar system that is controlled by the interaction of the solar winds with interstellar winds (see photo below). Solar winds are streams of charged particles that form the heliosphere which surrounds the solar system. These solar winds originate from the Sun’s corona which heat them up and give them energy enough to reach speeds of around a million mph throughout the solar system. The termination shock boundary is a feature that is distinguished by the drop in solar wind speeds from supersonic to subsonic and the change in plasma flow direction and magnetic field orientation as a result of the interaction with interstellar winds.
These spacecraft are now almost leaving the solar system which is the ultimate goal of the Voyager Interstellar Mission (VIM). This will involve the spacecraft passing through the heliopause that denotes the end of the solar system. This is where the solar winds meet the interstellar winds creating a boundary (see above). This is the limit of the Sun’s influence such that the solar winds cease to have any effect, interstellar particles become more dominant and the direction of the magnetic field at the edge of the solar system changes.
The timeline so far…
Originally, the two Voyager spacecraft were only sent out to explore the more local parts of solar system where the outer planets are found. They were first launched back in 1977 on the 20th August (Voyager 2) and 5th September (Voyager 1).
As is shown on the NASA Voyager Timeline webpage, the interstellar mission was set up as an extension after the initial Voyager mission of exploring the giant planets was complete in 1989. The spacecraft were to be sent out towards the edges of the solar system to the limit of the Sun’s influence, and hopefully beyond. The VIM began on the 1st January 1990 at which point Voyager 1 was 40 AU (astronomical unit) from Earth and Voyager 2 was 31 AU from Earth. The last images from Voyager came on the 14th February of the same year and these were photos of the whole solar system from the spacecrafts point of view.
During this time the Voyager spacecraft were still in the first phase of the interstellar mission called the termination shock phase. This was the earliest part of the VIM in which the two spacecraft passed through an environment of the solar system made up of supersonic solar winds. This region is controlled by the Sun’s magnetic field where the plasma particles are dominated by these solar winds. The boundary of this region is what’s referred to as the termination shock which signifies the border with the heliosheath. This first phase was completed by both spacecraft once they passed through the termination shock with Voyager 1 and 2 crossing on the 15th December 2004 and 5th September 2007 respectively.
At the fringes of the solar system
The crossing through the termination shock led to the second phase of the mission – the heliosheath exploration phase. The heliosheath is the next region of space that stretches out to the very edge of the solar system before reaching interstellar space. The environment is still dominated by the Sun’s magnetic field and solar winds. This current phase of the mission is exploring the environment of the outer solar system and searching for the heliopause. This is the limit of the Sun’s influence (magnetic field and solar wind) and would end the heliosheath exploration phase. As this region of the solar system has never been explored before, it is unknown how wide the heliosheath is. It is estimated to be tens of AU across which would take years for both Voyager spacecraft to travel. The predicted extent of the heliosheath can be seen in the photo below.
So how do the Voyager spacecraft work?
As both of the spacecraft travel through the heliosheath, they are collecting and assessing data on the environment found at the edge of the heliosheath. The characteristics of the environment (see mission science objectives) include the strength and orientation of the Sun’s magnetic field; the composition, direction and energy spectra of the solar wind particles and interstellar cosmic rays; the strength of radio emissions which are thought to come from the heliopause, the border with interstellar space; and the distribution of hydrogen within the outer heliosphere. The data gathered is used by five different investigation teams that work on the VIM. In addition, data is being collected from the UV Spectrometer Subsystem (UVS) and the captured data is made available to interested scientists.
Both of the Voyager spacecraft are identical and are three-axis stabilized systems that use a celestial or gyro referenced attitude control so that the high-gain antennas are kept pointed at the Earth. The scientific equipment consisted of 10 instruments for 11 investigations including radio science. There are now only five investigator teams that are supported, although data are collected for two other instruments. Apart from the plasma investigation instrument on Voyager 1, all of the instruments are still working well and able to continue operating in the upcoming environment expected. Data are also collected by the Planetary Radio Astronomy instrument and the UVS on Voyager 1. The Flight Data Subsystem and an 8-track digital tape recorder provide the data handling functions. The FDS configures the instruments and takes care of their operations. It also collects engineering and science data and formats the data for transmission. The DTR is used to record the high-rate plasma wave investigation data. All data are played backed every six months.
The command computer subsystem is used to sort out the sequence and the controls of the instruments onboard the spacecraft. The CCS also has fixed routines such as decoding commands sent to the spacecraft, detecting any faults with Voyager, corrective routines, information to point the antenna the right way and spacecraft sequencing information.
The Attitude and Articulation Control Subsystem controls which way the spacecraft is orientated and manoeuvres and positions the scan platform.
Uplink communications is via S-band (16-bits/sec command rate) while an X-band transmitter provides downlink telemetry at 160 bits/sec and 1.4 kbps for playback of high-rate plasma wave data. All of the data are sent and received via the 3.7m high-gain antenna.
Over time as the electrical power decreases, some of the instruments on the spacecraft must be turned off so that the demand onboard does not exceed the supply. On Voyager 1 all platform instruments, but the UVS have been powered down. The Voyager 1 scan platform would have been turned off in late 2000, but will be left on to investigate an unexpected excess in UV in the solar wind from upwind of the spacecraft.
Well what has been found?
Two of the three major signs that Voyager 1 is nearing the heliopause have been seen from the data collected. These are a decrease in solar wind charged particles and increase in interstellar wind charged particles. It was thought that Voyager 1 had left the solar system; however, the Voyager team was quick to respond confirming that this was not so: it has not encountered the changes in the orientation of the magnetic field that are to be expected when reaching the heliopause. It was only thought to have left the solar system because of Voyager 1’s position in a region of space dubbed by scientists as the “magnetic highway”. This is where changes in the charged particles at the boundary of the solar system are seen due to the connection between the Sun’s magnetic field and interstellar space magnetic field. This connection allows charged particles from within our heliosphere to get out and interstellar particles to get in to the solar system. This indicates that Voyager 1 is on the precipice of interstellar space.
So what does the future hold?
For now all that is left for the Voyager spacecraft to do is reach the heliopause and pass through into interstellar space. The only thing that will determine that now is the third and final indicator of interstellar space which is the change in the orientation of the magnetic field. This will mean that both of the Voyager spacecraft will leave the solar system for good and to go boldly where no-one has gone before.
For further reading: this article is based on the information made available by NASA on the Voyager Interstellar Mission website. See below for the specific webpages.