How Gaia Is Already Shaping Our Interstellar Adventures

The space telescope has refined the stellar flybys of the Voyager and Pioneer probes—how might it help us chart our way to the stars in the future?

The Gaia space telescope [ESA]

When looking up on a starry night, it can be difficult to comprehend that those stars are not fixed in the sky. Sure, on timescales of a human lifetime, or even the entirety of human history, the stars don’t appear to move too much. But look over longer timescales—tens of thousands, to millions of years—and it becomes clear that the stars in the sky are in motion. This means the constellations we see today will be misshapen (or even non-existent!) in a few hundred thousand years’ time.

This poses an interesting question: If humanity were to send a spacecraft on an interstellar mission—an endeavor that could take thousands of years, depending on how ambitious the target—aiming it directly at a distant star would be a mistake. Depending on how far away that star is, by the time the spacecraft reaches its target, the star could have moved a few light-years away. This is why precision astrometry—the astronomical measurement of a star’s position, speed and magnitude—will be needed to predict where a target star will be, and not where it currently is, when our future interstellar mission gets there.

To test this, we don’t need to wait until humanity has the means to build a starship, however. We have a bunch of interstellar probes that have already started their epic sojourns into the galaxy.

Interstellar Interlopers

Earlier this year, NASA’s Voyager 2 spacecraft departed the Sun’s sphere of influence and became humanity’s second interstellar mission, six years after its twin, Voyager 1, made history to become the first human-made object to drift into the space between the stars. Both Voyagers are still transmitting telemetry to this day, over 40 years since their launch. Another two spacecraft, the older Pioneer 10 and 11 missions, are also on their way to interstellar space, but they stopped transmitting decades ago. A newcomer, NASA’s New Horizons mission, will also become an interstellar mission in the future, but it has yet to finish its Kuiper belt explorations and still has fuel to make course corrections, so predictions of its stellar encounters will remain unknown for some time.

Both Voyager 1 and 2 have left the Sun’s heliosphere to become humanity’s first interstellar missions [NASA]

Having explored the outer planets in the 1970’s and 80’s, the Voyagers and Pioneers barreled on, revealing stunning science from the outer solar system. In the case of Voyager 1 and 2, when each breached the heliopause (the invisible boundary that demarks the limit of the Sun’s magnetic bubble, between the heliosphere and interstellar medium), they gave us a profound opportunity to experience this distant alien environment, using their dwindling number of instruments to measure particle counts and magnetic orientation.

But where are our intrepid interstellar interlopers going now? With the help of precision astrometry of local stars observed by the European Space Agency’s Gaia space telescope, two researchers have taken a peek into the future, seeing which star systems the spacecraft will drift past in the next few hundred thousand to millions of years.

Previously, astronomers have been able to combine the spacecrafts’ trajectory with stellar data to see which stars they will fly past, but in the wake of the Gaia Data Release 2 (GDR2) last year, an unprecedented trove of information has been made available for millions of stars in the local galaxy, providing the most precise “road map” yet of those stars the Voyagers and Pioneers will encounter.

“[Gaia has measured] the positions and space velocities of nearby stars more precisely than before and so has more precisely characterized the encounters with stars we already knew about,” says astronomer Coryn Bailer-Jones, of the Max Planck Institute for Astronomy in Heidelberg, Germany.

Artist’s impression of Voyager 1’s Jupiter encounter [NASA]

Close Encounters of the Voyager Kind

Bailer-Jones and colleague Davide Farnocchia of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., published their study in Research Notes of the American Astronomical Society, adding another layer of understanding about where our spacecraft, and the stars they’ll encounter, are going. Although their work confirms previous estimates of some stellar close encounters, Bailer-Jones tells Astroengine.com that there have been some surprises in their calculations—including encounters that have not been identified before.

For example, the star Gliese 445 (in the constellation of Camelopardalis, close to Polaris) is often quoted as being the closest encounter for Voyager 1, in approximately 40,000 years. But with the help of Gaia, which is giving an extra layer of precision for stars further afield, the researchers found that the spacecraft will come much closer to another star, called TYC 3135-52-1, in 302,700 years.

“Voyager 1 will pass just 0.30 parsecs [nearly one light-year] from that star and thus may penetrate its Oort cloud, if it has one,” he says.

This is interesting. Keep in mind that the Voyager and Pioneer spacecraft include the famous Golden Records and plaques (respectively), revealing the location, form, and culture of a civilization living on a planet called “Earth.” For an alien intelligence to stumble across one of our long-dead spacecraft in the distant future, the closer the stellar encounter the better (after all, the likelihood of stumbling across a tiny spacecraft in the vast interstellar expanse would be infinitesimally small). Passing within one light-year of TYC 3135-52-1 is still quite distant (for instance, we currently have no way of detecting something as dinky as a Voyager-size probe zooming through the solar system’s Oort Cloud), but who knows what the hypothetical aliens in TYC 3135-52-1 are capable of detecting from their home world?

The Pioneer plaque is attached to the spacecrafts’ antenna support struts, behind Pioneer 10 and 11’s dish antennae, shielding the plaques from erosion by interstellar dust [NASA]

Another interesting thought is that these Gaia observations can help astronomers find stars that are currently very far away, but now we know their speed and direction of travel, some of those stars will be in our cosmic backyard in the distant future.

“What our study also found, for the first time, is some stars that are currently quite distant from the Sun will nonetheless come very close to one of the spacecraft within the next few million years,” says Bailer-Jones. “For example, the star Gaia DR2 2091429484365218432 is currently 159.5 parsecs [520 light-years] from the Sun (and thus from Voyager 1), but Voyager 1 will pass within 0.39 parsecs [1.3 light-years] of it in 3.4 million years from now.”

In some cases, given unlimited time, you may not have to go to a star, the star will come to you!

Our Interstellar Future?

While pinpointing the various stellar encounters for our first interstellar probes is interesting, the observations being made by Gaia will be important for when humanity develops the technology to make a dedicated effort to travel to the stars.

“It will be essential to have extremely precise astrometry of any target star,” explains Bailer-Jones. “We must also measure its velocity and its acceleration precisely, because these affect where the star will be when the spacecraft arrives.”

Although this scenario may seem a long way off, any precision astrometry we do now will build our knowledge of the local stellar population and boost the “legacy value” of Gaia’s observations, he adds.

“Once a target star has been selected, we would want to make a dedicated campaign to measure its position and velocity even more precisely, but to determine the accelerations we need data measured at many time points over long periods (at least tens of years), so Gaia data will continue to be invaluable in the future,” Bailer-Jones concludes.

“Even now, astrometry from the previous Hipparcos mission—or even from surveys from decades ago or photometric plates 100 years ago!—are important for this.”

An artist’s impression of the Icarus Interstellar probe, a concept for a fusion-powered, un-crewed starship that may be used to travel to the stars [Icarus Interstellar/Adrian Mann]

For more on how Gaia observations are being used, see my previous interview with Coryn on how these data were used to find the possible origins of ‘Oumuamua, the interstellar comet.

Is Pluto Affected by the Pioneer Anomaly?

From Pluto, looking at its icy moons in the Kuiper belt (NASA)

The Pioneer Effect is a mysterious observation of a number of man-made probes that venture through and beyond the Solar System. Originally noticed in the slight drift of the Pioneer 10 and Pioneer 11 spacecraft (launched in 1972 and 1973) from their calculated trajectories, scientists have been at a loss to explain the tiny, yet constant, extra-sunward acceleration.

Some theories suggest that invisible clouds of dark matter are slowing these probes down, causing them to be influenced by the Sun’s gravity more than expected. Other suggestions include ideas that Einstein’s theory of General Relativity needs to be tweaked when considering interplanetary distances.

However, there are other, more mundane ideas. Perhaps there is a tiny fuel leak in the probes’ mechanics, or the distribution of heat through the spacecraft is causing a preferential release of infrared photons from one side, nudging them off course.

Finding an answer to the Pioneer effect probably won’t surface any time soon, but it is an enduring mystery that could have a comparatively simple explanation, within the realms of known science, but there’s also the possibility that we could also be looking at some entirely new physics.

In an attempt to single out whether the Pioneer anomaly is an artefact with the spaceships themselves, or unknown in the physics of the Universe, astronomers decided to analyse the orbits of the planets in the outer Solar System. The rationale being that if this is a large-scale influence, some observable periodic effects should be evident in the orbit of Pluto.

So far, no effect, periodic or otherwise, has been observed in the orbit of Pluto. If the effect isn’t big enough to influence Pluto, does this mean we can narrow the search down to spaceship-specific artefacts?

Not so fast.

Gary Page and John Wallin from George Mason University in Virginia and David Dixon from Jornada Observatory in New Mexico, have published a paper pointing out that the suggestion that the Pioneer effect doesn’t influence Pluto is flawed. Pluto’s orbit is far less understood than the orbits of the inner Solar System planets, as, let’s face it, Pluto is far away.

We simply don’t possess the data required to cancel out the Pioneer effect on planetary bodies in the outer-Solar System to reach the conclusion the anomaly doesn’t influence Pluto.

Of course, this does not mean that the Pioneer effect exists. It does mean that we cannot deny the existence of the Pioneer effect on the basis of motions of the Pluto as currently known.” — Page et al., 2009

So, back to the drawing board. This is a fascinating study into a true Solar System mystery; bets are on as to the real reason why our interplanetary probes are being knocked off course…

Source: The Physics arXiv Blog