Keith Bechtol at UW Space Place

We are so very fortunate here in southern Wisconsin to have evening public lectures the 2nd Tuesday every month of the year at the University of Wisconsin Space Place, expertly organized by Jim Lattis. On Tuesday, November 12th, Clif Cavanaugh (retired physics and astronomy professor at the UW in Richland Center) and I made the trek (as we often do) from Spring Green-Dodgeville to the Space Place in Madison. This month, we were treated to an excellent presentation by Keith Bechtol, an Observational Cosmologist in the Physics Department at UW-Madison. His topic was The Big Picture: Science with Astronomical Surveys. Keith is an early career scientist with a bright future. His presentation was outstanding.

I’d like to share with you some of the highlights.

Before the talk, which is mostly about the Large Synoptic Survey Telescope (LSST), currently under construction in Chile and expected to see first light in 2020, I asked Keith about whether LSST would be renamed the Vera Rubin Telescope as was announced at AAS 234 in St. Louis this past summer. As it turns out, Keith has been a vocal advocate for naming LSST after Vera Rubin, though no final decision has yet been made.

Before I get into notes from the talk, I wanted to share with you the definition of the word synoptic in case you are not familiar with that word. The Oxford English Dictionary defines the word synoptic as “furnishing a general view of some subject; spec. depicting or dealing with weather conditions over a large area at the same point in time.” But rather than the traditional meteorological definition, here we are referring to a wide-field survey of the entire night sky visible from Cerro Pachón in Chile, latitude 30˚ S.

Keith first talked about how astronomical imaging is currently advancing along two fronts. The first is high-resolution imaging, as recently illustrated with first image of the event horizon of a black hole from the Event Horizon Telescope, where an amazing resolution of around 25 microarcseconds was achieved.

In general, the larger the telescope aperture, the smaller the field of view.

The Hubble Space Telescope’s Ultra Deep Field is only 3.1 arcminutes square

A survey telescope, on the other hand, must be designed to cover a much larger area of the sky for each image.

Not only can a survey telescope detect “anything that changes” in the night sky, but it also allows us to probe the large-scale structure of our universe. Three still-mysterious entities that are known to affect this large-scale structure are dark energy, dark matter, and neutrinos. Keith indicates that “these names are placeholders for physics we don’t yet fully understand.”

Dark energy, which is responsible for driving galaxies apart at an accelerating rate, is unusual in that it maintains a constant density as the universe expands. And its density is very low.

Supernovae are a very useful tool to probe the dark-energy-induced accelerating expansion of the universe, but in any particular galaxy they are exceedingly rare, so by monitoring large areas of the sky (ideally, the entire sky), we can discover supernovae frequently.

The mass distribution of our universe subtly affects the alignment and shapes of distant galaxies through a phenomenon known as weak gravitational lensing. Understanding these distortions and correlations requires a statistical approach looking at many galaxies across large swaths of sky.

Closer to home, small galaxies that have come too close our Milky Way galaxy are pulled apart into stellar streams that require a “big picture” approach to discover and map. The dark matter distribution in our Milky Way galaxy plays an important role in shaping these stellar streams—our galaxy contains about ten times as much dark matter as normal matter.

With wide-field surveys, not only do we need to cover large areas of sky, but we also want to be able to see the faintest and most distant objects. That latter property is referred to as “going deeper”.

The LSST will provide a dramatic increase in light gathering power over previous survey instruments. The total number of photons collected by a survey instrument per unit time is known as the étendue, a French word, and it is the field of view (in square degrees) × the effective aperture (in m2) × the quantum efficiency (unitless fraction). The units of étendue are thus m2deg2. Note that the vertical axis in the graph below is logarithmic, so the LSST will have a significantly higher étendue than previous survey instruments.

The largest monolithic mirrors in the world are fabricated at the Steward Observatory Mirror Lab at the University of Arizona in Tucson. The largest mirrors that can be produced there are 8.4 meters, and LSST has one of them.

Remember the Yerkes Observatory 40-inch refractor, completed in 1897? It has held the record as the largest lens ever used in an astronomical telescope. Until now. A 61.8-inch lens (L-1) and a 47.2-inch (L-2) have been fabricated for use in the LSST camera.

L-1, the largest lens ever produced, is the front lens of the LSST camera

LSST will utilize a camera that is about the size of a car. It is the largest camera ever built for astronomy.

The LSST camera will produce 3.2 gigapixel images. You would need to cover about half a basketball court with 4K TV screens to display the image at full resolution.

An image will be produced every 15 seconds throughout the night, every clear night, and each patch of sky will be reimaged every three nights. That is a HUGE amount of data! ~10 Tb of data each night. Fiber optical cable will transport the data from Cerro Pachón to the National Center for Supercomputing Applications in Urbana, Illinois, where it will be prepared for immediate use and made publicly available to any interested researcher. The amount of data is so large that no one will be downloading raw data to their local computer. They will instead be logging in to the supercomputer and all processing of the data will be done there, using open source software packages.

There are many data processing challenges with LSST data needing to be solved. Airplane, satellite, and meteor trails will need to be carefully removed. Many images will be so densely packed with overlapping objects that special care will be needed separating the various objects.

One LSST slide that Keith presented showed “Solar System Objects: ~ 6 million” and that piqued my interest, given my ongoing research program of observing stellar occultations by asteroids and trans-Neptunian objects for IOTA. Currently, if you endeavor to observe the highest probability occultation events from a fixed observatory location each night, you will be lucky to record one positive event for every ten negative events (no occultation). The reason for this is that our knowledge of the orbital elements of the small bodies of the solar system is not yet precise enough to accurately predict where stellar occultation events will occur. Gaia DR3, scheduled for the latter half of 2021, should significantly improve the precision of small body orbits, and even though LSST does not have nearly the astrometric precision of Gaia, it will provide many valuable astrometric data points over time that can be used to refine orbital elements. Moreover, it is expected that LSST will discover—with its much larger aperture than Gaia—at least 10 times the number of asteroids and trans-Neptunian objects that are currently known.

During the question and answer period after the lecture, I asked Keith what effect the gigantic increase in the number of satellites in Earth orbit will have on LSST operations (global broadband internet services provided by organizations like SpaceX with its Starlink constellation). He stated that this definitely presents a data processing challenge that they are still working on.

An earlier version of Keith’s presentation can be found here. All images in this article except the first (OED) come from Keith’s presentation and have not been altered in any way.

References

Bechtol, Keith, “The Big Picture: Science with Astronomical Surveys” (lecture, University of Wisconsin Space Place, Madison, November 12, 2019).

Bechtol, Ellen & Keith, “The Big Picture: Science and Public Outreach with Astronomical Surveys” (lecture, Wednesday Night at the Lab, University of Wisconsin, Madison, April 17, 2019; University Place, Corporation for Public Broadcasting, PBS Wisconsin).

Jones, R. L., Jurić, M., & Ivezić, Ž. 2016, in IAU Symposium, Vol. 318, Asteroids: New Observations, New Models, ed. S. R. Chesley, A. Morbidelli, R. Jedicke, & D. Farnocchia, 282–292. https://arxiv.org/abs/1511.03199 .

Oxford English Dictionary Online, accessed November 17, 2019, https://www.oed.com/ .

Interstellar Object 1I/2017U1 ‘Oumuamua

After years of searching and hypothesizing, we have finally discovered a macroscopic object passing through our solar system that came from interstellar space!  An elongated rocky object with approximate dimensions 755 × 115 × 115 ft. entered the solar system from the direction of the constellation Lyra at a velocity (v) of 26 km/s (16 mi/s or 58,000 mph), and will exit the solar system at essentially the same speed in the direction of the constellation Pegasus, within the Great Square.

This interstellar object (ISO) is called 1I/2017U1 ‘Oumuamua.  What’s in a name?  A lot!  Let’s separate the three different parts of this designation, discussing each in turn.

1I – “I” stands for “interstellar” and “1” indicates that it is the first interstellar solar system visitor discovered.

2017U1 – indicates that it was the first object discovered during the half-month October 16-31 in the year 2017.

‘Oumuamua [pronunciation] is a Hawaiian word for “scout”, reflecting how this object is like a scout or messenger reaching out to us from the distant past.

‘Oumuamua Enters the Solar System

Here’s a brief timeline of the encounter.

September 9, 2017 – Closest approach to the Sun (0.26 AU)

October 14, 2017 – Closest approach to the Earth (0.16 AU)

October 19, 2017 – Discovered by Robert Weryk with Pan-STARRS

It is very difficult for us to discover objects coming towards us from the inner solar system and the glare of the Sun, so it is not surprising that ‘Oumuamua was discovered after it had passed by the Earth on its way out of the solar system.

‘Oumuamua in the Inner Solar System

NASA Animation Showing ‘Oumuamua’s Journey Through the Inner Solar System

‘Oumuamua Exits the Solar System

Rob Weryk, a post-doc at the University of Hawaii Institute for Astronomy, discovered ‘Oumuamua in images taken by the Pan-STARRS1 1.8-meter Ritchey–Chrétien telescope at the summit of the dormant volcano Haleakalā on the island of Maui.  Pan-STARRS is an acronym for “Panoramic Survey Telescope and Rapid Response System” and is primarily used to search for Near Earth Objects (NEOs).  It has been estimated that Pan-STARRS should be able to detect an interstellar object like ‘Oumuamua passing through our solar system about once every 5 years.

But the 8.4-meter Large Synoptic Survey Telescope (LSST) in Chile, which will see first light in 2020, is expected to be able to detect at least one interstellar object passing through our solar system each year.

While we don’t know ‘Oumuamua’s place of origin, we do know that it originated outside our solar system, and that is exciting.  Was it ejected from a binary system?  Or through a chance encounter with a giant planet in its outer solar system?  Is it an “extinct” interstellar comet?  Perhaps it is a former asteroid of a dying star.  Even our own Sun, which is expected to reach a peak luminosity of 5200 L as a red giant star in a few billion years, will lose mass and transition to a white dwarf, causing a dynamical reshuffling that will eject a large number of asteroids, trans-Neptunian objects, and comets from our solar system (Seligman & Laughlin 2018).  Perhaps ‘Oumuamua long ago suffered a similar fate.

A detailed astrometric study (ground-based and HST) of ‘Oumuamua’s trajectory through the inner solar system has revealed a small non-gravitational acceleration component directed radially away from the Sun (Micheli et al. 2018).  After ruling out other known gravitational and non-gravitational accelerators, the authors conclude that the most probable explanation is cometlike outgassing, though ‘Oumuamua displayed no detectable coma during its all-too-brief apparition.  Furthermore, no change in the rotational state of ‘Oumuamua occurred during the month-long interval over which it was observed.  If the anomalous acceleration away from the Sun was caused by cometary activity, a measurable effect on ‘Oumuamua’s rotation should have been seen (Rafikov 2018).

‘Oumuamua wasn’t discovered until 40 days after perihelion, and Zdenek Sekanina, JPL, argues that it is a dwarf interstellar comet that disintegrated before perihelion, so that during the period of observation it was an extremely low density debris plume whose orbital motion was affected by solar radiation pressure and not outgassing (Sekanina 2019).   As such, he notes the difficulty in trying to reconstruct its original shape and place of origin.

Could there be some other cause of the anomalous acceleration?  It is worth considering that ‘Oumuamua might be of artificial origin (Bialy & Loeb 2018) .  It could be a lightsail that long ago was ejected from its solar system of origin, and this interstellar debris just happened to encounter our solar system.  Or, perhaps, it is (or was) an operational space probe purposefully directed towards Earth’s vicinity by an alien civilization.  Incidentally, no radio emissions were detected from ‘Oumuamua (yes, we looked).

There may yet be some other explanation for the acceleration ‘Oumuamua experienced during its journey through our solar system.  Our experience with the Pioneer anomaly (now explained), or the still unexplained flyby anomaly, might lead us towards new insights.  The possibility that ‘Oumuamua is a highly elongated or flattened object only adds to the mystery.

References
Bialy, S., Loeb, A. 2018, ApJL, in press (arXiv: 1810.11490)
McNeill, A., Trilling, D. E., Mommert, M. 2018, ApJL, 857, L1 (arXiv:1803.09864)
Micheli, M., Farnocchia, D., Meech, K.J., et al. 2018, Nature,
https://www.nature.com/articles/s41586-018-0254-4
Rafikov, R. R. 2018, arXiv preprint 1809.06389
Sekanina, Z. 2019, arXiv: 1901.08704)
Seligman, D. & Laughlin, G. 2018, AJ, in press (arXiv:1803.07022)