The Hubble Space Telescope’s celebration of thirty years in space this April is a time to not just look back to the tremendous accomplishments that have happened over that span, but also to look forward. Originally conceived as a Great Observatory, Hubble’s flexibility enables a wide range of science. Its ability and willingness to adapt to the changing scientific landscape have contributed in large part to Hubble’s continuance as a prominent astrophysics mission. Much of the science being done now was unanticipated at the time of Hubble’s launch. The most widely touted example is exoplanets, an area that had not had its nascence at the time of launch, but which now makes up roughly 20% of science observing. The versatility of the science instruments and active involvement of the user community in setting the science program are fundamental to ensuring that this innovation will continue.
The future for Hubble is bright, even with the much-anticipated launch of JWST and development of the WFIRST mission concept for the middle of the decade. Hubble has unique aspects that are not currently replicated in NASA’s astrophysics missions on the horizon – its ability to provide high-resolution imaging in the ultraviolet, as well as spectroscopic capabilities in this wavelength range at a variety of resolving powers, are in high demand on the observatory. These ensure complementarity with science questions to be pursued by JWST.
The 30th anniversary comes 11 years after the most recent servicing mission, when astronauts repaired and replaced instruments and parts on the observatory to keep its capabilities current. The instrument performance in the ensuing decade-plus have been outstanding; there are realities to dealing with the harsh environment of space and slow degradation of components. The observatory, however, is remarkably reliable, especially because of continued vigilance with calibration and innovative observing and analysis techniques. The prospect of continued operations for at least the next five years is sound.
The science areas being investigated with Hubble now and in the future are the topics being investigated by NASA’s Astrophysics Science Division: namely, how the universe works, how the universe began and evolved, and the search for Earth-like planets.
On the Hubble constant with the Hubble Space Telescope
The Hubble Space Telescope has played a central role in fundamental physics. One of the original key projects with Hubble was to determine the value of the local expansion rate of the universe, characterized by the term Hubble constant. At the time the controversy existed because the age of the universe implied by the value of the Hubble constant, or H0, was younger than the ages of the oldest stars derived using an independent method. Refinements to H0 using observations with Hubble brought the two measurements into accord. Further measurements of distant supernovae with Hubble were influential in confirming and extending the evidence for the acceleration of the expansion of the universe and thus the presence of dark energy, a feat for which three astronomers received the Nobel Prize in 2011.
Interest in refining the uncertainty in this constant has continued, especially with observations of the early universe by other ground- and space-based telescopes in the recent past. Currently, astronomers’ best measurements of the Hubble constant have an astounding 2% uncertainty (the goal for early Hubble observations was only 10%), and some methods show significant differences with the value of the constant derived from early universe observations.
Three different observing programs are currently underway with the Hubble Space Telescope to attack the problem of the value of the Hubble constant, and especially how it compares with the value derived from the early universe. This is important, because confirming the discrepancy potentially opens a window on new physics that we would otherwise not be able to access. Is this a clue to a property of dark energy? Or is it perhaps some unknown aspect of dark matter that causes the difference between the nearby universe and the early universe, or an aspect of cosmology revealed beyond the standard model often assumed?
The first observing program continues the legacy which began with Henrietta Swan Leavitt and Edwin Hubble’s original observations. For Cepheid variable stars, there is a fundamental relationship between the star’s intrinsic brightness and the timescale for the star’s variability – the so-called Leavitt law. Cepheid variable stars exist in our galaxy, as well as others. By refining this relationship for Cepheid stars in our galaxy of known distance (thanks to the Gaia satellite), the relationship can be extended to Cepheid variable stars in other galaxies which host supernovae used for distance measurements. These observations make use of an observing mode developed since the last servicing mission, which “scans” across the detector and enables very high precision measurements of bright objects. In this way, the rungs of the “distance ladder” are related to each other, and effects that would creep in to this link and nudge the Hubble constant value systematically higher or lower can be reduced, and simultaneously extended to much more distant galaxies.
Cepheid variable stars in the Large Magellanic Cloud used to refine measurements of the Hubble constant.
A complementary method currently in use by a different set of observers uses a different set of stars to the Cepheid variable stars to provide a constraint on the distances to nearby galaxies. This method utilizes particular evolved stars, which have reached the peak of their brightness in the course of their evolutionary changes; they are known as tip of the red giant branch stars. These stars can be found in a wider range of galaxies, and are typically located in regions of a galaxy not subjected to current star formation; they are thus less affected by stellar crowding. Recent results demonstrate a value for the Hubble constant intermediate between those of the Cepheid method and what has been inferred for the early universe.
A third independent technique utilizes quasars, and gravitational lensing, to measure Hubble’s constant and determine the expansion rate of the universe. Quasars are the powerful black holes at the centers of some galaxies. There are special lines of sight in our universe where the light from a distant quasar will travel through an intermediate galaxy. The mass of this galaxy will “bend” the light of the quasar, resulting in multiple images of the background quasar. Each image has taken a slightly different path on its way to detectors here at Earth. Quasars are notoriously variable, as the light emitted from the central black hole changes, and the slightly different path lengths of each image of the quasar means that the variability will be detected at different times for each image. By investigating this delay for each image, many quantities which depend on distance “cancel out” and enable a constraint on H0. This is independent of the local distance ladder (necessary for the previous two methods), and with a large enough sample of quadruple-imaged quasars, an uncertainty rivaling that of the Cepheids can be achieved. These types of measurements require high-resolution imaging, with high-quality constraints on how the light in an image is distributed.
Exoplanets
Hubble has played a ground-breaking role in the search for, and characterization of, worlds beyond our solar system. The versatility of Hubble’s instrumentation shines through, as detectors manufactured years before the first exoplanet detections are now routinely used to characterize those systems. Hubble’s spectrographs and ability to return high-precision measurements of an exoplanet’s atmosphere make it the workhorse for studying the make-up of exoplanets that transit their host star. Hubble provided the first direct measurement of an exoplanet’s atmosphere, the first detection of organic molecules on an exoplanet, and was the first to detect changes in an exoplanet’s atmosphere. Recently, with the discovery of Earth-sized planets around nearby red dwarfs, Hubble has performed the first atmospheric study of such alien worlds. Access to near-infrared wavelengths is key to tracing water on these worlds.
Some exoplanets appear to have a hazy atmosphere, while others show the existence of patchy clouds, and yet others have clearer atmospheres. What causes this diversity? Building up a census of the diversity of exoplanets is important for understanding the processes at work in exoplanet formation and evolution.
The treasure trove of exoplanet discoveries with the recent Kepler/K2 mission, and now TESS, means that Hubble’s instruments will be busy characterizing the best candidates for the next several years. The access to ultraviolet and visible light with which to study exoplanets, along with near infrared measurements, does a far better job in constraining the properties of exoplanets than with only the near infrared wavelengths. Hubble’s capabilities will thus still be sorely needed when JWST launches, to fill in these missing wavelength ranges and enhance interpretation of JWST data.
Exoplanet studies extend beyond detection and characterization of the exoplanets themselves, and include understanding the environment in which the planet exists. Recent results have shown the impact of the host star on influencing the atmospheric chemistry of nearby exoplanets. Random variations in brightness of the star’s ultraviolet light in the form of flares can change the chemistry of the exoplanet and may be harmful to life on the surface. Similarly, the overall radiation of the host star can actually remove part of the exoplanet’s atmosphere. Hubble can study both of these effects: its ultraviolet spectral recordings of host stars details the rate and type of ultraviolet flares, and atmospheric characterization of exoplanets with similar properties, but different distances from their host stars, reveals the influence of the star on the planet’s atmosphere.
Hubble’s Dedication to Solar System Astronomy
Even as planetary science missions can visit constituents in our solar system and obtain up-close recordings of their properties, Hubble provides unique information about solar system objects. The long temporal baseline of observations enables monitoring of changing conditions on the surfaces of planets, such as changes in Jupiter’s spots, and appearance of dark vortices in Neptune’s atmosphere. Hubble observations can provide a global perspective to complement in situ measurements; ultraviolet studies of aurorae, clouds and storms on Jupiter with Hubble nicely fill in global details that the finer-grained measurements of NASA’s Juno mission provide.
Hubble’s unique access to ultraviolet wavelengths, coupled with high spatial resolution, were key to the discovery of water geysers on the icy moon Europa, a moon of Jupiter in our solar system. This initial discovery spurred additional observing programs for further characterization of the plumes, with initial follow-up results indicating possible venting activity. This is important for the study of an additional source of water in our solar system. The existence of these plumes means that future planetary missions targeting Europa can sample Europa’s subsurface ocean without needing to drill through its large ice layer.
Hubble has a long history of providing measurements crucial to the success of NASA’s planetary science missions. The high-precision photometric monitoring enables discovery. Hubble has discovered four of the five moons of Pluto; the partnership with the New Horizons mission for its initial encounter with Pluto ensured the safe passage of the spacecraft on its way to its original destination, then was key in finding the next Kuiper Belt Object for New Horizons to target in its extended mission. Such observations help advance the objectives to study the origins and constituents of the solar system, and put our solar system in perspective of what else is out there in the universe. In the near term, Hubble is partnering with Lucy, a new planetary science mission to be launched in 2021, to investigate the Trojan asteroids. Recent results with Hubble have shown that Eurybates, one of the targets of the Lucy mission scheduled for a flyby in 2027, has a small satellite. This is important to determine that Lucy’s mission to study the Trojan asteroid, a building block left over from the formation of the giant planets, can be achieved safely.
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