The Hubble Space Telescope As Cosmic-Ray Detector

The Hubble Space Telescope requires little or no introduction. For more than three decades, the telescope has stood at the forefront of astronomical research, while its visually stunning images have served as a paragon of awe and inspiration.

But all this time, Hubble has also been secretly acting as a particle detector. And a good one at that, as a team of scientists demonstrated in a pilot study to appear in The Astrophysical Journal (preprint available here).

Energetic charged particles, or cosmic rays, populate every corner of the solar system. A near-constant flow of galactic particles originates from supernova remnants (and the stellar cinders at their cores). The Sun also sheds energetic particles, mostly protons, during solar flares and coronal mass ejections.

These cosmic rays wiggle and push their way through the solar system under the influence of the magnetized solar wind. Many end up at Earth, where they threaten astronauts, disrupt satellites, and affect atmospheric chemistry.

Cosmic rays are every astronomer’s nightmare. Neither ground nor space telescopes are immune to the barrage of the energetic particles, and when a particle travels through a camera, it leaves a sharp and bright trace on the resulting image. Needless to say, astronomers try to remove this pollution from their data.

But now scientists have demonstrated that one person’s garbage is another’s treasure.

Hubble’s Treasure Trove

“The idea for this project started five or six years ago at a conference about space weather,” says Susana Deustua (Ars Metrologia, formerly Space Telescope Science Institute), who led the study. An international group of astronomers, particle physicists, and planetary scientists put their heads together and came up with a plan. “Cosmic rays interact with the geomagnetic field,” Deustua adds. “We know that Hubble collects charged particles on its detector, therefore we should be able to glean information about the field from Hubble.”

The team dug into Hubble’s rich data archive. They looked for calibration images that were particularly well-suited for their plan and ended up with almost 100,000 images collected over the past 25 years.

Algorithms for finding and removing cosmic-ray traces from astronomical images have been around for decades. But rather than simply getting rid of the traces, the team wanted to learn as much as possible about the cosmic rays that caused them. “For example, we wanted to know how many pixels on the camera each cosmic ray affected and how much energy the particle lost in the process,” explains graduate student Nathan Miles (University of California), who is first author on the study.

Miles developed software to extract such information, using cloud computing services to carry out the time-demanding computations. His algorithm harvested more than 1 billion cosmic rays from the images.

A Cosmic-ray Image of Earth’s Magnetic Field

The results provide an important proof of concept with encouraging results. The cosmic ray properties from Hubble data match those detected by the PAMELA experiment, a defunct particle detector in low-Earth orbit. The team also saw in their data the South Atlantic Anomaly, the famous dip in the Earth's geomagnetic field. And they observed the expected response of cosmic rays to the solar cycle.

Claudio Corti (University of Hawai‘i at Manoa), who was not involved in the study, was pleasantly surprised by the work. “There is always interest in a better understanding of the geomagnetic field and the effect it has on the particle radiation for astronauts and electronics on the satellites,” he says. The data may prove valuable to understand cosmic ray populations in the solar system.

The analysis so far has only scratched the surface. The team is looking forward to unleashing the full power of the data to better understand the relation between galactic cosmic rays, the Sun, and Earth’s environment. “One of our interests is to look if we can find subtle secular changes in the geomagnetic field,” Deustua says. “We also want to make comparisons with geophysical observatories.”

Hubble’s uninterrupted monitoring of cosmic rays over a quarter of a century nicely complements other cosmic ray detectors. “From one single-point measurement, it is hard to get information on the global space environment,” Corti says. “The more points you have, the better it is.”