Interstellar studies toy with our expectations. Those of us who think about sending probes to other stars share the frustration of the long time-scales involved, not just in transit times but also in arriving at the technologies to make such missions happen. But the other half of interstellar studies, the observation and characterization of targets, is happening at a remarkable rate, with new instruments coming online and an entire class of extremely large telescopes in the pipeline. Exoplanet studies thrive.
In between, upcoming events are encouraging. Having identified two interstellar objects – 1I/ʻOumuamua and comet 2I/Borisov – in our own Solar System, we will shortly be able to expand the number of such confirmed interlopers enormously. That puts us in position to build intercept missions to study and sample material from another stellar system in relatively short order. The Legacy Survey of Space and Time (LSST), being planned for the now under construction Vera C. Rubin Observatory in Chile, should be able to detect interstellar materials passing through our system in abundance.
Image: An artist’s impression of a small, rocky interstellar object hurtling from the upper right toward the inner Solar System. The orbits of the four inner planets (Mercury, Venus, Earth, Mars) are fully visible, drawn as teal concentric circles around the bright ball of the Sun at the center. We see the orbits from a slightly elevated angle, so that the circular paths appear oval. The black background is sprinkled with points of starlight. The interstellar object looks like an elongated potato above the Sun, streaming toward the Sun from the upper right, with a short tail of gas and dust trailing behind. Credit: Rubin Observatory/NOIRLab/NSF/AURA/J. daSilva.
The LSST has crept into almost every discussion we’ve had in these pages about our two known interstellar visitors, along with the lament that had we found these objects sooner, we would have had been able to collect much more data from them. A 10-year survey of the southern sky (from the El Peñon peak of Cerro Pachón in northern Chile), the survey will use a large-aperture wide-field instrument called the Simonyi Survey Telescope (SST) to study half the sky every three nights in six optical bands. It will deploy the largest digital camera ever constructed, with a 9.6 square degree of view.
Using three refractive lenses, the LSST Camera will take a pair of 15-second exposures of each field, operating throughout the night. Astronomers plan over 5.2 million exposures in ten years, creating views that will be sensitive to redshifts up to z=3. Recall the terminology: z=3 means that the observed wavelength of light from a distant object is three times longer than the rest wavelength (when the light was emitted.
Because the z parameter represents the stretching of wavelength owing to the expansion of the universe, higher values of z represent more distant (and hence older) objects, receding from us at a significant percentage of the speed of light. I’ve seen a redshift of z=0.0043 for the galaxy M87, which is roughly 55 million light years from Earth. A redshift of z=3 implies an object whose light has been traveling 11 billion years to reach us. That would make the actual distance to the object today over 18 billion light years because of the continuing expansion of the universe as the light travels. Las Cumbres Observatory offers an excellent backgrounder on all this.
Forgive the digression – this is how I learn stuff. But the point is that what the LSST will create is what its planners call a ‘movie,’ one summing that decade of observation and exposures and sensitive to extraordinarily distant objects. To get a sense of this, consider that the LSST project will take 15 terabytes of data every night, yielding an uncompressed data set of 200 petabytes. And with this kind of sensitivity, interstellar objects moving into our own Solar System should appear with some regularity.
Michele Bannister (University of Canterbury, NZ), a member of the Rubin Observatory/LSST Solar System Science Collaboration, comments:
“Planetary systems are a place of change and growth, of sculpting and reshaping. And planets are like active correspondents in that they can move trillions of little tiny planetesimals out into galactic space. A rock from another solar system is a direct probe of how planetesimal formation took place at another star, so to actually have them come to us is pretty neat. We calculate that there are a whole lot of these little worlds in our Solar System right now. We just can’t find them yet because we aren’t seeing faint enough.”
Image: This image captures not only Vera C. Rubin Observatory, a Program of NSF’s NOIRLab, but one of the celestial specimens Rubin Observatory will observe when it comes online: the Milky Way. The bright halo of gas and stars on the left side of the image highlights the very center of the Milky Way galaxy. The dark path that cuts through this center is known as the Great Rift, because it gives the appearance that the Milky Way has been split in half, right through its center and along its radial arms. In fact, the Great Rift is caused by a shroud of dust, which blocks and scatters visible light. This dust makes the Great Rift a difficult space to observe. Fortunately, Rubin is being built to conduct the Legacy Survey of Space and Time (LSST). This survey will observe the entire visible southern sky every few nights over the course of a decade, capturing about 1000 images of the sky every night and giving us a new view of our evolving Universe. Rubin Observatory is a joint initiative of the National Science Foundation and the Department of Energy (DOE). Once completed, Rubin will be operated jointly by NSF’s NOIRLab and DOE’s SLAC National Accelerator Laboratory to carry out the Legacy Survey of Space and Time. Credit: RubinObs/NOIRLab/NSF/AURA/B. Quint.
The LSST has uses far beyond interstellar interlopers, of course, with implications for the study of dark energy and dark matter as well as the formation of the Milky Way and the trajectories of potentially hazardous asteroids. But its emergence, beginning with first operations in late 2024, puts us on the cusp of studying planet formation using materials from other stellar systems. That brings intercept missions into the discussion, a topic we’ve considered in these pages before through the work of my friend Andreas Hein (University of Luxembourg). On a broader level, consider that expansion into the Solar System already has interstellar aspects, as I’ll discuss soon with a look at what we are learning about interstellar dust, and how missions beyond the heliosphere can inform our views of the local bubble in which we move.
I think you are conflating 2 ideas here. z implies distance when the object is expected to be very distant, like a galaxy. A local object would have a very low z value due to the universe’s expansion, but an observable object traveling along our line of sight at a high velocity, say 0.2c, would have a higher absolute value of z. So while ‘Oumuamua was traveling at relatively low velocity, a real alien ship approaching or traveling through our solar system might have a negative z and a positive one as it leaves our system.
If we are talking about high velocity and relativistic velocity star ships, perhaps even of Rama/worldship size, then z values for local relative velocity would be an obvious technosignature. If the ship was using something as primitive as a fusion drive, then the bright exhaust exhibiting a Doppler ship due to the ship’s velocity would be such a technosignature.
So z is both useful for detecting natural distant objects like galaxies, and as a technosignature for local, artificial objects, traveling at high velocity.
[If a ship was at rest in our system and started up its photon drive, the exhaust would not show a Doppler shift. However, if the drive had a relativistic exhaust of plasma, would the emitted light from the plasma have a Doppler shift? – I assume it must.]
Alex, I’m not talking about z in terms of interloper detection, but rather just running a digression on what z means, because LSST will be able to detect objects as faint as incredibly distant galaxies. I didn’t mean to imply anything about using z for local objects. I just meant that LSST is so incredibly sensitive that finding interstellar objects approaching our system should be straightforward. Your thoughts on using z as a technosignature are intriguing; hadn’t thought of that.
I should have clearly used Doppler shift” and “z” where appropriate. I was trying to get at something more subtle regarding identification. The relation between redshift and distance was made using various “standard candles” although the early work described by Fred Hoyle in his book “Frontiers of Astronomy” (pub. 1955) was interesting as various means for estimating object distance were used. As a result, redshift alone can be used to estimate distance under stable assumptions. [In 1955, the age of the universe was thought to be about 1/2 the current estimate, and Hoyle’s extrapolations suggested galaxies and stars older than the universe!] As the default is to assume everything is natural, any faint object with a redshift is considered a distant object.
But consider when we detect a faint, high redshift object. What if it isn’t natural, but artificial? The best hope is that its movement has some rapid movement against the stars suggesting it is close, rather than distant. Even better, if there is a blueshift, this would indicate this object is probably not distant due to the universe’s expansion. The object would have to be emitting [a lot of] light, which suggests perhaps a rocket exhaust of hydrogen or hydrogen/helium.
Given that artificial, relativistic starships are probably vanishingly rare, they are not something we would look for. However, I envisage that future all-sky scans will be analyzed by computers whose algorithms may include the identification of all sorts of objects and might flag anomalies that may be artificial. Anomalous objects might be detected using z values as one variable to flag them.
There are a couple of statements that may lead to confusion.
“sensitive to redshifts up to z=3”
That doesn’t mean anything because no instrument is “sensitive” to redshift. The instrument has a sensitivity function across that a range of wavelengths. The instrument is designed to be suitably sensitive for its science mission(s).
There are wavelengths at which molecules and atoms will radiate (or absorb) photons, and redshift means those will be observed at longer wavelengths. Correlating the observation with the molecule or atom allows the calculation of the redshift.
“z=3 means that the observed wavelength of light from a distant object is three times longer than the rest wavelength (when the light was emitted)”
The correct term for this usage is 1 + z, not z.
The wavelength shift Alex is talking about is usually described as Doppler shift, leaving use of z for cosmological observations. Same thing really, just a matter of convention.
“The correct term for this usage is 1 + z, not z.”
Can you amplify on this, Ron, in relation to the z=3 discussion? I’m unclear on it.
Have a look the table in the referenced section, Paul, and you’ll see the difference between z and 1+z. The latter is the ratio to be used for wavelength emitted / wavelength observed. I used to mix them up all the time!
https://en.wikipedia.org/wiki/Redshift#Measurement,_characterization,_and_interpretation
Clearly stated and very helpful. Thanks for clarifying this, Ron.
The equations are shown on the Wikipedia RedShift page.
More about Z
An interloper approaching us would be blueshifted, and one departing our system, redshifted. But the shifted light would be reflected sunlight; that is, solar spectral lines would be displaced on the spectrum by the motion of the reflecting body. Of course, if the body were shining from its own light (a blazing fusion drive for example) that would look very different!
I get the impression this “Z detector” is used for cosmological research. Would it be sensitive enough to detect Doppler shifts on rocks (or spacecraft) traveling only a few tens or hundreds of km/sec relative to earth?
Detecting Doppler shifts on objects moving at these speeds would be an interesting exercise, I would think. But I seem to have led everyone astray with the discussion of z. I brought it up simply to show how sensitive the LSST will be to extremely faint objects, and got off on the digression about z simply as background (and out of personal interest). The real point is that if LSST can detect objects as faint as its specs indicate, then we’re going to start picking up interstellar objects in much higher numbers than we have so far.
Cosmologically interesting objects are distant galaxies, faint, but fixed on the celestial sphere, so very long time exposures would allow their light to be integrated on a detector. An interloper of the same brightness would have some motion across the line of sight, either its own, or a motion imposed on it by the earth’s orbital motion around the sun. This might interfere with the ability to determine Doppler shifts on interlopers, but it might also give an opportunity to use these line-of-sight displacements to help pick them out of the background of stars.
The problem of directly detecting Doppler is whether you know the spectrum well enough to know that what you’re measuring is different and by how much. That is why emission and absorption lines are so important. You won’t have much luck measuring Doppler shift of a black body spectrum, whether it is by direct emission or reflection.
My guess ( I didn’t bother looking it up) is that LSST data will be used to determine object trajectory by ordinary astrometric methods. That is, the more apparent position/time data is collected the greater the space of possible trajectories (direction & velocity) is constrained. The absolute minimum is 3 points, and more is better. Many more is likely necessary for objects so far away that they are weakly affected by solar gravity.
The difference here, as Paul alludes to, is the instrument sensitivity to small, distant objects. Measurements can be difficult since these objects are tumbling with varying brightness and will may not be detected in every observation.
Fun fact: 1+z and z converge for high values of z.
Vector & scalar velocity & speed.
The deliberate use of Doppler shift for detecting starships is probably not appropriate. Doppler shift should be sensitive enough as it is used for radial velocity measurement to detect exoplanets that are less than 1 m/s.
Once the ship is close enough, laser ranging for distance, direction, and velocity would be a better method.
Hi Paul
Another interesting post, These new telescopes and there findings are going to be really interesting to follow.
Cheers Edwin
If we do find an object of interest will we do anything about it? What would the cost be if we push hard from the first few days of discovery onward? I’m curious what people think. Is there any plan by any space faring group or nation to attempt closeup analysis and even sampling?
ESA’s Comet Interceptor mission is interesting on this score. Planned for launch in 2029, the craft will orbit the Sun-Earth L2 Lagrangian until it finds a target comet. And yes, interception and perhaps sampling seem planned.
https://www.cosmos.esa.int/web/comet-interceptor
Time appeared to move 5 times more slowly in 1st billion years after Big Bang, quasar ‘clocks’ reveal.
https://www.space.com/quasar-clocks-universe-time-dilation
Another point of view is that new matter in the universe runs five times slower then our local clock…
Another words quasars are new born and ejected from the core of galaxies…
“Halton Arp”
For reasons of my own, have to review “z” from time to time.
And put it into a grand design. Hope that this is of some help:
Z = Change of wavelength lambda ( lambda- lambda0) divided by lambda0
Lambda zero being a reference zero recession value for a specific absorption or emission line.
This can be equivalenced to square root of [ (1 + v/c)/(1-v/c)] – 1.
Hence, you can get some pretty high values of Z when v approaches the
speed of light.
Now when it comes to “cosmological” issues, and ignoring cases where galaxies in a cluster are moving randomly relative to ours in a foreground ( e.g., the Andromeda), the velocity of recession is calibrated against various candles with characteristic luminosities: catalogued bright stars, especially variables and
supernovas that blow up the same way with the same intensity as far as we know. The farther away the dimmer. Hence a 70 or 80 km/sec recession velocity is estimated for each additional mega-parsec of distance.
Now if we have a comet either entering the solar system or exiting it, the radial velocity toward us would give a negative z value by the above definition. The recession velocity would operate much the same way as the cosmological, but it would not be connected with the distance scale. However, since it is a longer wavelength of a given line, and if we know nothing else about the object, it could be mistaken for an object at cosmological distance related to that recessional velocity.
At this point one could well wonder how the Vera Rubin Observatory takes that problem on. I think the answer to that would be in observed mean motion or else or parallax. If the observatory in the space of a year can detect a shift against deep space reference frame, well there you go. It’s an interloping object.
The LSST, I suspect, has some recent instrument innovations that make it sensitive to faint Oort Cloud objects. Probably good resolution for wavelength shifts at IR or other low wavelengths characteristic of cold objects.
A New Way to Measure the Expansion Rate of the Universe: Redshift Drift
Writing an entry is like trying to take a wobble out of a chair.
Would like to make at least one correction.
The ratios v/c should be squared – but remain in the square root expressions.
Also, “low wavelengths” . Not so much what it should be, but what would be more clear: It could be low energy wavelengths, say, in the infra red or millimeter range representative of cold matter in the Oort Cloud or interstellar space.
Now I will just hang this on the walll, and see if I notice anything else askew…
Hey Wes, we all do this now and then! I thought it best to simply run your corrections rather than trying to insert them.
It’s a good point, Wes; so many of our discussions assume Z is purely due to spacial expansion, ignoring relative motion to the observer, which, in this case, might be particularly important. Thanks for reminding us and posting the equation.
This is misleading. Adding to what I said previously, z is a metric used, by convention, to describe spacetime curvature between points far apart. It can be used to specify Doppler shift due to relative motion for, typically, less than cosmological distances, but it is not the best choice.
In the majority of cosmology cases, there is indeed relative motion between, say, two stars in a remote galaxy when, in cases, we can measure it. For example, quasar jets. However, when expressed as a velocity/Doppler shift, it is so much less than that due to the spacetime curvature that it is generally negligible in comparison and not especially meaningful.
This discussion of Doppler versus z is a good example for either of these cliches: “going down a rat hole” and “making a mountain out of a molehill.”