The Voyager 1 and 2 spacecraft have passed beyond the bubble created by the Solar Wind, entering interstellar space. Both have sent back interesting data from out there. However, neither were designed to provide the answers astronomers seek about the space between the stars. Consequently, NASA and partners are considering a probe carrying instruments built for this specific purpose. Some details of those plans have been revealed at the European Geosciences Union General Assembly 2021.
"The Interstellar Probe will go to the unknown local interstellar space, where humanity has never reached before," Dr Elena Provornikova of the Johns Hopkins Applied Physics Lab said in a statement. "For the first time, we will take a picture of our vast heliosphere from the outside to see what our solar system home looks like."
Provornikova was presenting the work of a four-year “pragmatic concept study”, now approaching completion. The study is not tasked to produce detailed plans, but instead to outline possible mission objectives, the sort of instruments required and possible trajectories. "Our approach is to lay out the menu of what can be done in such a space mission," Provornikova said. A previous version proposed in 1999 never came to fruition.
Even if the mission ever gets off the ground it may be considerably diminished once the costs of such a project have been considered. To even get to that point the Probe will have to compete for priority with missions to Jupiter's Trojans, and many other options, but advocates hope it will launch in the next 10-12 years, taking another 15 to reach the boundary of the heliosphere – less than half the Voyagers' journey time.
Simply in terms of distance, the mission would be far more ambitious than anything humanity has done previously. Voyager 1 is currently 152 astronomical units (the distance of the Earth to the Sun) away. The Interstellar Probe is proposed to collect data 1,000 AUs from the Sun, making Mars missions look puny. The Voyagers, Pioneers and New Horizons, will reach such distances eventually of course, but by the time they do it's likely, we will have lost contact.
The Interstellar Probe's most obvious purpose is to sample the Interstellar medium, measuring aspects such as composition, ionization and magnetic field.
The mission may also learn about the Solar System in ways only possible from outside it, including confirming its currently speculative shape. It will use energetic neutral atoms instead of photons to form images of the heliosphere, and investigate the interactions between the Sun and the wider galaxy. Even more ambitious options include trying to “Observe extragalactic background light from the early times of our galaxy formation,” Provornikova said.
The probe could also investigate the greater abundance of cosmic rays beyond the Solar Wind's protection.
Interstellar space is not uniform. The Sun is currently traversing a region called the Local Interstellar Cloud, defined by its higher density compared to the wider Local Bubble. It is thought to have entered the cloud 40,000-150,000 years ago, and there are suspicions we are in the process of leaving. The Interstellar Probe may reveal what to expect when that occurs.
Ambitious as this mission is, even its 50 year timeline will not see it pass outside the more generous definition of the Solar System, which takes in all the space where the Sun's gravity dominates over that of other stars. Such a voyage would take us a good portion of the way to the nearest stars.
An abstract of Provornikova's presentation is available.