Centauri Dreams

It seems to be a week for endings. Following the retirement of the wildly successful Kepler spacecraft, we now say goodbye to Dawn following an extraordinary eleven years that took us not only to orbital operations around Vesta but then on to detailed exploration of Ceres. The spacecraft ran out of hydrazine, with the signal being lost by the Deep Space Network during a tracking pass on Wednesday. No hydrazine means no spacecraft pointing, vital in keeping Dawn’s antenna properly trained on a distant Earth.

I immediately checked to see if mission director and chief engineer Marc Rayman had gotten off a post on his Dawn Journal site, but he really hasn’t had time to yet. It will be interesting to see what Dr. Rayman says, and it’s appropriate here to thank him for the continuing updates and insights he provided throughout the Dawn mission. Keeping space exploration in front of the public is essential for continuing funding of deep space robotic missions, as both the Dawn and New Horizons team clearly understand (and there is much ahead for New Horizons!)

Image: This photo of Ceres and one of its key landmarks, Ahuna Mons, was one of the last views Dawn transmitted before it completed its mission. This view, which faces south, was captured on Sept. 1 at an altitude of 3570 kilometers as the spacecraft was ascending in its elliptical orbit. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The ability to orbit one target, move on to another and subsequently establish an orbit there is a demonstration of what ion engines can do, and a reminder of how significant the Deep Space 1 mission was in terms of testing these technologies. If DS1 established ion propulsion as a viable tool, Dawn took it to the next level, highlighting the kinds of exploration that may lie ahead as we contemplate missions to multiple Kuiper Belt objects (see Game-changer: A Pluto Orbiter and Beyond). Ambitious missions grow out of the gentle, efficient thrust of ion engines.

Ponder this: The orbiters we’ve sent to Mars had to burn their engines for orbital insertion. Consider something on the order of 300 kilograms of propellant for conventional rocketry to meet this requirement. Dawn could make the same change in speed (about 1000 meters/second) using fewer than 30 kilograms of xenon. But of course we wouldn’t use ion engines for this purpose. Ion engines demand patience: It would take Dawn three months to achieve this result while conventional engines in our orbiters complete this maneuver in 25 minutes. But missions with multiple targets or complex operations in deep space are suited to engines like Dawn’s, which accumulated 2,141 days (5.9 years) of thrust time in the course of its travels.

Describing Dawn’s changing trajectories and the intricate gravitational dance they involved, Rayman wrote this paean to ion propulsion in his Dawn Journal:

Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the exotic world in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

And now the mission is over, though the spacecraft will remain in Ceres orbit for at least twenty years, and mission engineers believe with 99 percent confidence that its orbit will last at least 50 years, keeping planetary protection protocols in place for the near future. As we look back, consider Dawn’s accomplishments. It was the first spacecraft to orbit a body between Mars and Jupiter, the first to visit a dwarf planet and the first to orbit two destinations beyond Earth.

Image: Dawn’s interplanetary trajectory (in blue). The dates in white show Dawn’s location every Sept. 27, starting on Earth in 2007. Note that Earth returns to the same location, taking one year to complete each revolution around the Sun. When Dawn is farther from the Sun, it orbits more slowly, so the distance from one Sept. 27 to the next is shorter. Credit: NASA/JPL.

“In many ways, Dawn’s legacy is just beginning,” said principal investigator Carol Raymond at JPL. “Dawn’s data sets will be deeply mined by scientists working on how planets grow and differentiate, and when and where life could have formed in our solar system. Ceres and Vesta are important to the study of distant planetary systems, too, as they provide a glimpse of the conditions that may exist around young stars.”

Dawn’s discoveries at Vesta and Ceres have been described before in these pages, but we’ll track new conclusions from Dawn’s datasets well into the future. For now, I’m simply thinking about Dawn’s beginnings and all that has happened since. Congratulations to the entire team.

Image: Dawn climbs to space on Sept. 27, 2007, from Cape Canaveral Air Force Station. Dawn launched at dawn (7:34 am EDT). Credit: KSC/NASA.

The interstellar object called ‘Oumuamua continues to inspire analysis and speculation. And no wonder. We had limited time to observe it and were unable to obtain a resolved image to find out exactly what it looks like. This morning I want to go through a new paper from Shmuel Bialy and Abraham Loeb (Harvard University) considering the role radiation pressure from the Sun could play on this deep sky wanderer. Let’s also review what we do know about it, which I’ll do with reference to this paper’s introduction, where recent work is discussed. For it seems that each time we look at ‘Oumuamua anew, we find something else to talk about.

Discovered in October of 2017 by the Pan-STARRS survey (Panoramic Survey Telescope and Rapid Response System) in Hawaii, ‘Oumuamua stood out because of its hyperbolic trajectory, flagging it as an interstellar object, the first ever discovered passing through the Solar System. The object’s lightcurve indicated both that it was tumbling and had an aspect ratio of at least 5:1 and perhaps higher, an unusual shape for known asteroids and comets. I won’t provide all the references here, as they’re easily found both in the paper cited below and in other related work.

Image: The track of `Oumuamua as it passed through the inner solar system in late 2017. Credit: Brooks Bays / SOEST Publication Services / Univ. of Hawaii.

What stood out in 2018 was detection of non-gravitational acceleration in ‘Oumuamua’s motion, which could be consistent with cometary activity, although as we’ve seen in other posts on this site, no such activity has been noted by way of a cometary tail or gas emission and absorption lines. This despite a relatively close approach to the Sun of 0.25 AU. The Micheli et al. paper noting the acceleration was addressed in another 2018 paper from Rafikov et al., which pointed out that torque from any cometary outgassing should have had effects on the object’s spin, but this does not appear in our admittedly limited observations.

What Bialy and Loeb consider in today’s paper is the possibility that solar radiation pressure — imparted by the momentum of photons from the Sun — is responsible for the acceleration, expressed as an excess radial acceleration ?a ? r^?2, where r is the distance of ’Oumuamua from the Sun. If so, ‘Oumuamua would of necessity be a thin object with a small mass-to-area ratio — this is required in order to make the radiation pressure effective.

We can work out constraints on the object’s area through its observed magnitude. The paper proceeds to show that a thin sheet roughly 0.3 mm thick and some 20 meters in radius will allow the non-gravitational acceleration computed in the Micheli paper.

I was intrigued enough at this point to ask Dr. Loeb about those dimensions, which vary with albedo (the incident light reflected by a surface). He told me that the 20-meter figure would be the radius if the object is a perfect reflector, though the size would be larger if the value for the albedo is smaller. We do see variations in reflected light as ‘Oumuamua rotates over an eight-hour spin period. Thus, considering the object as a thin surface, we could imagine a conical or hollow cylindrical shape. “You can easily envision that by rotating a curved piece of paper and looking at its net surface area from different viewing angles,” Loeb told me.

So let’s back up a moment. We are asking what properties ‘Oumuamua would have to have if its non-gravitational acceleration is the result of solar radiation pressure. We do not know that solar radiation is the culprit, but if it is, the object would need to be a thin sheet with a width in the range of 0.3 mm. This scenario explains the acceleration but forces the question of what kind of object could have these characteristics. A major problem is that, as mentioned above, there are too many degrees of freedom in our observations to nail down what ‘Oumuamua looks like. We did not have observations sensitive enough to produce a resolved image.

Image: Oumuamua as it appeared using the William Herschel Telescope on the night of October 29. Credit: Queen’s University Belfast/William Herschel Telescope.

We do know that if ‘Oumuamua is accelerating because of solar photons, it must represent what the paper calls ‘a new class of thin interstellar material.’ The researchers note the possibility that such material is naturally produced in the interstellar medium, but go on to consider an artificial origin. Could ‘Oumuamua be debris from a technological civilization, a discarded lightsail?

A fascinating speculation indeed. From the paper:

Considering an artificial origin, one possibility is a lightsail floating in interstellar space as debris from an advanced technological equipment (Loeb 2018). Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative. The lightsail technology might be abundantly used for transportation of cargos between planets (Guillochon & Loeb 2015) or between stars (Lingam & Loeb 2017). In the former case, dynamical ejection from a planetary system could result in space debris of equipment that is not operational any more (Loeb 2018) and is floating at the characteristic speed of stars relative to each other in the Solar neighborhood.

We’re past the stage where we can image ‘Oumuamua with our telescopes and it’s too late to get a mission off to chase it with chemical rockets, which means that the only way we have of pressing the investigation forward is to look for other such objects in the future. But we can look at the properties of thin films to determine whether an object like a lightsail could survive interstellar travel, given encounters with dust and gas between the stars. Returning to the paper:

Collisions with dust grains at high velocities will induce crater formation by melting and evaporation of the target material. Since the typical time between dust collisions is long compared to the solidification time, any molten material will solidify before the next collision occurs, and thus will only cause a deformation of the object’s surface material, not reduction in mass. On the other hand, atoms vaporized through collisions can escape and thus cause a mass ablation.

Bialy and Loeb find that for the mass-to-area ratio they have calculated for a thin-film ‘Oumuamua, the object could travel through much of the galaxy before losing a significant fraction of its mass. Collisions with gas particles in the interstellar medium as well as the stresses of centrifugal and tidal forces are also considered. None of these present problems for the object’s survival until we reach a maximal travel distance in the range of 10 kpc (well over 32,000 light years). Earth is approximately 25,000 light years from the center of the galaxy.

Image: This is Fig. 1 from the paper. Caption: The maximum allowed travel distance through the interstellar medium (ISM), as a function of (m/A). The blue and red lines are limitations obtained by slow-down due to gas accumulation, and vaporization by dust-collisions, respectively. The plotted results are for a mean ISM proton density of (n) ? 1 cm^?3. All lines scale as 1/(n). The dashed magenta line is our constraint on ’Oumuamua based on its excess acceleration. The Solar Galactrocentric distance is also indicated. Credit: Bialy & Loeb.

Avi Loeb has recently written about how we might find evidence for extraterrestrial civilizations long gone. From that perspective, he considered the possibility that ‘Oumuamua is just such an artifact, and told me this in an email:

With the perspective of my recent essay in Scientific American, `Oumuamua could be defunct sails floating under the influence of gravity and stellar radiation. Similar to debris from ship wrecks floating in the ocean. The alternative is to imagine that `Oumuamua was on a reconnaissance mission. The reason I contemplate the reconnaissance possibility is that the assumption that `Oumumua followed a random orbit requires the production of ~10<sup>15</sup> such objects per star in our galaxy. This abundance is up to a hundred million times more than expected from the Solar System, based on a calculation that we did back in 2009. A surprisingly high overabundance, unless `Oumuamua is a targeted probe on a reconnaissance mission and not a member of a random population of objects.

Loeb also made a comment in his email that ties in to what Breakthrough Starshot is attempting to quantify, a series of missions to the same target — hundreds if not thousands of probes — sent swarm-like to ensure that at least one or a few come close to the world under observation. If something like this were happening with ‘Oumuamua, and given that PAN-STARRS barely detected the object at closest approach, we would not know about any of its fellow probes.

Addendum: Just as I was publishing this I learned of a paper by Eric Mamajek (University of Rochester), who notes that ‘Oumuamua appears to have originated at the Local Standard of Rest (LSR), which is the galactic frame of reference. Quoting Mamajek: “Compared to the LSR, ‘Oumuamua has negligible radial and vertical Galactic motion…” According to Dr. Loeb in a subsequent email, less than one star in 500 is at that frame of reference to the same precision. (The Mamajek paper, “Kinematics of the Interstellar Vagabond 1I/’Oumuamua (A/2017 U1),” can be found here in preprint form).

It’s an interesting point. If ‘Oumuamua turned out to be artificial, would there be an advantage in such a position? Perhaps so, for as Loeb goes on to say in his email:

I view a sail (like `Oumuamua) floating in interstellar space with stars (like the Sun) running into it as if it were a buoy floating on the ocean surface with boats colliding with it. An artificial origin would naturally place floating sails at the LSR, perhaps as relay stations.

Another item of interest:

Trilling et al., “Spitzer observations of `Oumuamua and `Oumuamua’s density and shape,” as presented at the American Astronomical Society DPS meeting #50, finds no detection of thermal emission from `Oumumua, implying that it must be very reflective or small, which is consistent with the Bialy and Loeb paper. The abstract of the Trilling paper is here.

And a final point: While catching up with ‘Oumuamua would be immensely challenging, Andreas Hein and colleagues have suggested how it might be done. For more, see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua (formerly A/2017 U1), the Interstellar Asteroid.

The Bialy & Loeb paper is “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” (preprint). The Micheli et al. paper on ‘Oumuamua’s acceleration is “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua),” Nature 559 (27 June 2018), 223-226 (abstract).