What do we know about interstellar space? We know a few things now. As of 2024, NASA’s Voyager 1 spacecraft has long since passed the edge of the solar system and is now exploring interstellar space. Here’s an updated overview of Voyager 1’s current status and recent developments:
Current Location and Mission
Voyager 1 is now more than 15 billion miles (24 billion kilometers) from Earth, making it the most distant human-made object in space. It has been operating in interstellar space since 2012, well beyond the heliopause – the boundary where the solar wind’s influence ends and interstellar space begins. Voyager 1 is currently traveling at a speed of approximately 38,026.77 mph (61,204 km/h) relative to the Sun. Voyager 1 has the highest velocity of any human spacecraft[19].
Voyager 2, as of August 15, 2024, is approximately 12.7 billion miles (20.4 billion kilometers) from Earth, 136 AU (Astronomical Units) from Earth, 13 billion miles (20.4 billion kilometers) from the Sun, or 137 AU from the Sun[14]. Voyager 2 is traveling at an estimated speed of 34,390.98 mph (15.3471 kps) with respect to the Sun[14]. The one-way light time (time for a signal to reach Earth from the spacecraft) is about 18 hours, 53 minutes[14]. Several Voyager 2 instruments are still operational, including the Cosmic Ray Subsystem (CRS), Low-Energy Charged Particles (LECP), Magnetometer (MAG), Plasma Wave Subsystem (PWS), and Plasma Science (PLS)[14].
Recent Challenges and Resolutions
In November 2023, Voyager 1 experienced a significant technical issue that prevented it from sending readable science and engineering data back to Earth. This was described as the “most serious challenge” the probe had faced in recent years.
Recovery and Current Status
NASA engineers worked diligently to resolve the issue:
- In April 2024, they partially fixed the problem, enabling Voyager 1 to resume sending engineering data.
- On May 19, 2024, the team successfully commanded the spacecraft to begin returning science data.
- By June 2024, all four of Voyager 1’s science instruments were once again returning usable data.
Ongoing Science
Voyager 1 continues to provide valuable insights into interstellar space:
- Its instruments study plasma waves, magnetic fields, and particles in the interstellar medium.
- This data helps scientists understand how particles and magnetic fields evolve far from Earth’s influence.
- Voyager 1 and its twin, Voyager 2, are the only spacecraft directly sampling interstellar space.
Galactic Cosmic Ray (GCR) Measurements
Voyager 1’s Ultraviolet Spectrograph (UVS) has been measuring GCR flux since at least 1992. The UVS detector is sensitive to GCRs with energies around 300 MeV.
Key Observations:
- After May and August 2012, the GCR count rate measured by UVS stabilized, showing only minor fluctuations around a constant value.
- These small variations were determined to be responses to changing anisotropy rather than changes in the overall flux.
- Long Mean Free Paths: Models suggest very long parallel mean free paths in the VLISM, ranging from 104 to 105 AU at 100 MeV. This indicates that GCR particles can travel great distances along field lines before being scattered.
- This anisotropic behavior in the VLISM contrasts with the more isotropic diffusion observed within the heliosphere.[2]
Anisotropy is exhibiting properties with different values when measured in different directions. The observations indicate that GCRs diffuse much more easily along magnetic field lines than perpendicular to them.
Heliopause Crossing:
- In August 2012, at approximately 122 AU from the Sun, Voyager 1 experienced significant changes in GCR measurements, indicating it may have crossed the heliopause.
- Two rapid increases in GCR fluxes for particles >70 MeV were observed by Voyager 1:
- 110 days before the heliopause crossing
- At the heliopause itself
Post-Heliopause Observations:
- Since crossing the heliopause, neither Voyager 1 nor Voyager 2 has observed measurable long-term changes in GCR fluxes.
- The spectral slope of GCRs around the 300 MeV range has remained unchanged since August 2012, suggesting the end of heliospheric modulation at these energies.
Interstellar Medium:
In the very local interstellar medium (VLISM), GCR diffusion is believed to be highly anisotropic. The ratio of perpendicular to parallel diffusion coefficients is estimated to be very low (η = κ⊥ < 10-5κ∥) due to the very low intensity of magnetic fluctuations.
In the VLISM, magnetic field lines are nearly straight. The nearly straight magnetic field lines in the VLISM are a result of the region’s unique properties, including low turbulence and distance from solar influences, which allow the large-scale interstellar magnetic field to dominate the local structure.
It is important to note that there are no actual magnetic field lines. “There are no field lines. We draw field lines so that their density is proportional to the strength of the field around them.”[3] Magnetic field lines do not originate from specific points but rather form continuous loops or extend to infinity, serving as a visual representation of the magnetic field’s strength and direction throughout space. They do not have a beginning or end point. Even inside permanent magnets, the field lines join from south to north poles.
Galactic Cosmic Ray (GCR) Flux Values
- Pre-Heliopause:
- Before crossing the heliopause, Voyager 1 observed GCR fluxes in the range of approximately 1 to 2 particles per cm² per second for particles with energies greater than 70 MeV.
- At the Heliopause:
- At the time of crossing the heliopause, the flux increased significantly, reaching levels of approximately 4 to 5 particles per cm² per second or higher for similar energy thresholds.
- Post-Heliopause:
- After crossing into interstellar space, the GCR flux continued to increase, stabilizing at around 5 to 10 particles per cm² per second for higher energy cosmic rays.
Good to know what we have to deal with on those long trips to other planets. The numbers above are supposedly from sources listed below.[9][10][11][12][13] Of course, we now know that most galactic cosmic rays (GCRs) have energies above 1 GeV. The Cosmic Ray Subsystem (CRS) on Voyagers can detect electrons from 3-110 MeV and cosmic ray nuclei from 1-500 MeV/n, but the bulk of GCRs have energies above 1 GeV. The GCR spectrum peaks around 100 MeV to 1 GeV for protons and extends to much higher energies, with some particles reaching energies of 1020 eV or more. While the CRS doesn’t cover the full energy range of GCRs, it is still highly sensitive to the lower energy portion of the GCR spectrum. This makes it valuable for detecting changes in the GCR flux, especially as Voyager transitions from the heliosphere to interstellar space.
No Wait, It’s Probably Much Higher
I think the flux values given above are only a fraction of the real total GCR flux, only what the Voyager instruments can measure. Models indicate that the GCR flux can increase dramatically at energies above 1 GeV, potentially reaching values in the range of 20 to 50 particles per cm² per second or more, depending on the specific conditions and models used. While the observed GCR flux of 5 to 10 particles per cm² per second gives a baseline for the lower energy cosmic rays detected by Voyager 1, the actual total flux, especially when considering particles above 1 GeV and up to 1 TeV, would be expected to be significantly higher. Accurate estimates would require integrating data from multiple sources and models to account for the full spectrum of cosmic rays present in the interstellar medium.
What generates magnetic fields present in the VLISM ?
Based on our current understanding of interstellar magnetic fields, the magnetic fields present in the Very Local Interstellar Medium (VLISM) are generated and influenced by several factors:
1. Galactic magnetic field:
The VLISM magnetic field is primarily an extension of the large-scale galactic magnetic field. This field is thought to be generated by dynamo processes operating on a galactic scale, involving the rotation of the galaxy and the motion of ionized gas.
2. Stellar sources:
Nearby stars, especially those that have gone supernova, can contribute to the local magnetic field structure. Stellar winds and explosions can compress and shape the interstellar magnetic field.
3. Cosmic rays:
High-energy cosmic rays, while not directly generating magnetic fields, can influence the structure and strength of existing fields through their interactions with the interstellar medium.
4. Interstellar turbulence:
Although the VLISM has relatively low turbulence compared to the heliosphere, small-scale turbulent motions in the interstellar plasma can affect the local magnetic field structure.
5. Heliospheric influence:
While the VLISM is beyond the direct influence of the solar wind, the interaction between the heliosphere and the interstellar medium at the heliopause can affect the magnetic field structure in the nearby VLISM.
6. Large-scale interstellar structures:
The local interstellar magnetic field is part of larger structures, such as the Local Bubble, which can shape the field on scales larger than the VLISM.
7. Historical events:
Past events, such as nearby supernova explosions or the passage of dense interstellar clouds, may have left imprints on the magnetic field structure observed in the VLISM today.
The magnetic field in the VLISM is relatively stable and ordered compared to the more turbulent fields within the heliosphere. Voyager 1 has observed nearly straight magnetic field lines in the VLISM, with some variations and structures such as pressure fronts and magnetic humps that are still being studied and explained.
Size of the VLISM
The exact size of the VLISM is not precisely defined, but it extends beyond the heliopause, which is approximately 120-130 astronomical units (AU) from the Sun. The VLISM is considered to be the region of interstellar space immediately surrounding our solar system that is influenced or modified by heliospheric processes.
Future Outlook
While Voyager 1 has resumed normal science operations, some minor work remains:
- Engineers need to resynchronize timekeeping software in the spacecraft’s onboard computers.
- Maintenance on the digital tape recorder is planned.
As Voyager 1 continues its journey through interstellar space, it remains a testament to human ingenuity and our quest to explore the unknown reaches of our cosmic neighborhood.
Read More
[1] https://events.icecube.wisc.edu/event/84/contributions/5553/attachments/4713/5163/cra2017_florinski.pdf
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9287243/
[3] https://physics.stackexchange.com/questions/687272/do-magnetic-field-lines-originate-from-the-same-point-on-a-magnet
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9085707/
[5] https://www.texasgateway.org/resource/201-magnetic-fields-field-lines-and-force
[6] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/interstellar-magnetic-field
[7] https://iopscience.iop.org/article/10.3847/1538-4357/aa7685
[8] https://iopscience.iop.org/article/10.3847/1538-4357/acd6eb
[9] https://www.aanda.org/articles/aa/full_html/2014/03/aa22705-13/aa22705-13.html
[10] https://www.researchgate.net/publication/331515134_Galactic_Cosmic-Ray_Anisotropies_Voyager_1_in_the_Local_Interstellar_Medium
[11] https://ui.adsabs.harvard.edu/abs/2013AGUFMSH11B1976L/abstract
[12] https://www.aanda.org/articles/aa/abs/2014/03/aa22705-13/aa22705-13.html
[13] https://www.redalyc.org/journal/568/56875427004/html/
[14] https://science.nasa.gov/mission/voyager/where-are-they-now/
[15] https://www.npr.org/2023/04/30/1172921603/nasa-voyager-2-2026-backup-power-space
[16] https://news.satnews.com/2024/04/24/nasas-voyager-1-resumes-sending-engineering-updates-to-earth/
[17] https://www.jpl.nasa.gov/news/nasas-voyager-1-resumes-sending-engineering-updates-to-earth
[18] https://theskylive.com/voyager2-info
[19] https://voyager.jpl.nasa.gov/
