The Value of Long-Term Observations of Space

Space has an infinite number of stories, and they get told many different ways. It’s been a while since I’ve attended a press conference where Voyager observation results have been discussed. So, it was with great interest I went to a talk this morning describing how these venerable spacecraft are helping astronomers probe the makeup of interstellar clouds beyond our solar system.

The presentation was doubly interesting because it also talked about a result that began astronomers on the path of discovery in the clouds of the local instellar medium. I first heard about in 1996 when I was at CU as a graduate student. The first study was made by dissecting starlight streaming through the so-called Local Interstellar Cloud. The work was done by Dr. Jeffrey Linsky and colleagues, using the Goddard High Resolution Spectrograph on Hubble Space Telescope. Those observations allowed Linsky to “illuminate the cloud” and use the starlight streaming through to figure out its rough extent. He’s continued to work on the problem, and in recent years, people have turned to starlight to understand more about the chemical makeup and structure in the cloud.

So, fast forward to today, where a student researcher described using data from Voyagers 1 and 2, plus the Hubble Space Telescope, to further probe the cloud.

The Voyager Contribution

While today’s story included HST data, the tale really began in 1977. That’s when the Voyager 1 and 2 spacecraft left Earth on a jaunt past several outer solar system planets on their way out of the solar neighborhood. They are now on a one-way road trip to the stars, and are sampling the interstellar medium as they go. The ISM, as it’s called for short, is filled with gases and elements ejected from long-gone stars, as well as clouds of hydrogen gas. As they go along, their instruments are studying the ISM and sending data back to Earth.

HST Gets into the Act

The Hubble Space Telescope, which came along well after the Voyagers had finished their planetary missions, has been measuring the ISM along a line of sight that includes the paths that both Voyagers are following. This is the work that Linsky has been doing. HST has been finding evidence for multiple clouds of hydrogen along the path, clouds that also contain other elements from stars. All this data from all the probes is giving us a new and more nuanced look at the ISM, and the solar system’s

A student who was likely a small child when Linsky was doing his first observations told us today that her team’s work will synthesize the data from the Voyagers — which are our “on the spot” reporters. Along with the Hubble data (which is giving us the long view of the starlight passing through the clouds), the observations give us a much better “view” of the interstellar medium both from Earth orbit and “in situ“.

The good news is that the Voyagers are still working and should be able to give us maybe 10 years of data as they probe the clouds of gas and dust beyond the solar system. And, as long as HST is orbiting and working, we’ll probably hear a lot more about our neighborhood in the galaxy.

For example, Hubble found that Voyager 2 will move out of the interstellar cloud that surrounds the solar system in perhaps 2,000 years. After that, astronomers think that the spacecraft will spend 90,000 years in a second cloud before passing into a third interstellar cloud.

The Sun’s Progress

Based on the HST data, it also seems that the Sun is passing through clumpier material in nearby space. This which may affect the heliosphere, which is the large bubble containing our solar system that is produced by solar wind. At its boundary, called the heliopause, the solar wind pushes outward against the interstellar medium. Hubble and Voyager 1 made measurements of the interstellar environment beyond this boundary, where the wind comes from stars other than our sun.

The heliosphere gets compressed when the Sun moves through dense material, but it expands back out when the star passes through low-density matter. So, HST gives good insight into that, while the Voyagers are still our outermost scouts probing the space beyond the heliosphere.

I love stories that demonstrate the value of long-term studies. It’s one thing to look at the sky quickly and see an object and its events. But, if you really want to know what’s happening over time, the work of generations of astronomers and their instruments is the way to go!

(Note: this story came from a press conference and press release by STScI and the AAS.)