What is Dark Energy?

There is a dark side to the universe; in fact, most of what makes up our cosmos is dark. In our post, entitled “What Is Dark Matter?” we introduced this pie chart that shows the relative composition of everything in the universe.

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Composition of matter in the universe. These numbers have been revised by results from the Planck mission. More info here. Credit: NASA/ESA

This deceptively simple diagram shows the percentages of everything the universe is made of. Embedded in this uncomplicated, straightforward pie chart is a story full of surprises and anxiety.

Measuring the Universe

With the exception of Einstein’s “biggest blunder,” few prior to the 1990’s had any expectation that a cosmological force, such as dark energy, even existed. It was thought that the universe was solely comprised of normal matter and dark matter. There was much debate on the nature of dark matter.  How much is there? How much is made of exotic undiscovered particles versus the more mundane but visibly dark stuff like planets, small stars, etc.?  Much has been learned, but dark matter is still largely a mystery today.  Theories and experiments abound to find all constituents of the missing dark matter, particularly the exotic variety that does not contain normal matter, i.e., those particles that do not interact with normal matter other than via gravitational force.

Dark matter and normal matter both have one thing in common: gravity. Thus, the expectation for astronomers was that they would observe some decrease in the expansion rate of the universe over time due to the pull of gravity from all of the matter in the universe. In the 1990’s, two groups of astronomers attempted to measure the deceleration rate of the universe independently by looking at a whole bunch of Type 1a supernovae.  Type 1a supernovae are the explosions of a certain type of star, where the explosions themselves all have the same intrinsic brightness. You can determine how far away the star is by how bright it appears to us; the dimmer a Type 1a supernova appears, the farther away it must be.  Just like the equivalent of a standard 60–watt lightbulb, finding these “standard candles” allows astronomers to accurately measure the distances, and thus the time in the past, where these explosions took place.

Click here for more information on how Type 1a supernovae were used to measure distances.

What the astronomers actually discovered was far more surprising, and it was important that two different groups did this because, if only one had done it, no one would have believed what actually happened. These teams of astronomers noticed that distant supernovae, whose light from the early epochs of the universe was just now reaching our telescopes, were fainter and thus farther away than expected. In 1998, these two groups both declared that the universe wasn’t decelerating at all – it was accelerating!

This was a completely unexpected result — no one saw it coming. I mean, the universe is full of normal matter and dark matter, all gravitationally pulling on each other as the universe expands. Shouldn’t that mean the universe is slowing down its expansion? One could hear hyperventilating cosmologists from across the globe.

After everyone started to calm down, astronomers began to ask, “OK, so what does it take to have an accelerating universe?”

The answer is, you need something else besides matter. Whatever that is, we call it dark energy.

But What Does Dark Energy Mean?

After the initial surprise of finding an accelerating universe wore off and people started thinking about it, astronomers did something they rarely do — they accepted the idea rather quickly. Usually, an unexpected result like this generates huge debates among scientists, and this did too. The thing is, the notion of a cosmological force like dark energy now solved a lot more problems than it created. In an uncharacteristically short period of time, people started warming to the idea of dark energy.

As a function of time, galaxies are moving away from us at a faster and faster rate, and that is what is meant by an accelerating universe. The discovery of dark energy has brought the ultimate fate of our universe back into question. Will dark energy continue to increase its dominance over gravity and cause our universe to rip apart — a potential fate known as the Big Rip? Or will the repulsive force of dark energy and the attractive force of gravity balance out so that the universe expands forever at a constant, non-accelerating rate? With the current understanding of dark energy, it seems improbable that gravity will reverse the expansion and collapse the universe back in on itself. However, the nature of dark energy is not well understood yet.

 

What’s Next for Dark Energy?

Right now, astronomers are making observations designed to constrain some of the many dark energy models that are out there.  The nature of this research is often done from the ground so that wide areas of the sky can be observed for a very long time. This kind of campaign is not well-suited to a high-demand telescope like the Hubble Space Telescope. The idea is to “constrain,” or better understand, the expansion rate of the universe, and measure the growth of large–scale structure (like galaxy clusters).

Past surveys like the Sloan Digital Sky Survey have made some progress, and current projects like the Dark Energy Survey (DES) has started its observing runs. DES will observe 5,000 square degrees of the night sky over 525 nights, making measurements that should help us whittle down some of the many dark–energy models presently being considered. Currently being built is the Large Synoptic Survey Telescope, an 8.4–meter ground-based telescope in Chile, which will image the entire sky every few nights at several wavelengths, and will no doubt play a large role in helping us understand dark energy.

Space-based telescopes do have an essential role to play in characterizing dark energy. For example, Hubble has played a key role in getting data on distant supernovae — hence the discovery of dark energy. It is the combination of ground-based large surveys with space-based pointed deep follow-ups that give us our breakthroughs. Future missions are being envisioned to build on the best of both ground-based surveys and space-based observations. The Wide-Field Infrared Survey Telescope (WFIRST) will use a Hubble-class, space-based telescope to survey a large portion of the sky in an effort to better constrain the nature of dark energy through the history of the universe.

Frontier Fields and Dark Energy

While the Frontier Fields were not designed to capture the large numbers of supernovae needed to explore dark energy through cosmic time, the observations of strong galaxy cluster lensing will be used in combination with cosmological measurements from other missions to help constrain the nature of dark energy.  Stay tuned for more!

 

James Edwin Webb: Turning Imagination into Reality

by Holly Ryer and Ann Jenkins

The Frontier Fields program peers into the universe’s distant past, yet it also offers a glimpse of the future work that the powerful James Webb Space Telescope will conduct. Webb, known as Hubble’s successor, will use infrared vision to detect galaxies beyond even Hubble’s reach.

But the man for whom the Webb telescope is named is not commonly linked to space science. James Edwin Webb (1906–1992) wasn’t a scientist or engineer; he was a businessman, attorney, and manager. Still, many believe that this second administrator of NASA, who ran the fledgling agency from 1961 to 1968, did more to advance science and space exploration than perhaps any other government official. He laid the foundations at NASA for one of the most successful periods of astronomical discovery, one that continues today.

James Edwin Webb, the second administrator of NASA, was a staunch champion of space exploration. Photo credit: NASA.

James Webb was born in Granville County, N.C. He completed his college education at the University of North Carolina at Chapel Hill, where he received a degree in education. Webb then became a second lieutenant in the United States Marine Corps and served as a Marine Corps pilot. Afterward, he studied law at the George Washington University Law School in Washington, D.C. and was admitted to the Bar of the District of Columbia in 1936.

Webb’s long career in public service included serving as director of the Bureau of Budget and Under Secretary of State under President Harry Truman. In 1961, when he was selected by President John Kennedy to serve as the NASA administrator, Webb was reluctant to take the job. He assumed that it might be better handled by someone with a firmer grasp of science or technology. However, Kennedy wanted a leader with keen political insight and management skills for the position.

Webb oversaw great progress in the Space Program while serving as NASA’s administrator. During his tenure, NASA developed robotic spacecraft, which explored the lunar environment so that astronauts could do so later. On his watch, NASA also sent scientific probes to Mars and Venus. By the time Webb retired, NASA had launched more than 75 space science missions to study the stars and galaxies, our own Sun and the as-yet-unknown environment of space above the Earth’s atmosphere.

Webb also weathered the turmoil of the 1967 Apollo 1 tragedy, in which three astronauts—“Gus” Grissom, Edward White, and Roger Chaffee—died in a flash fire during a simulation test on the launch pad at Kennedy Space Center in Florida. Firmly committed to getting NASA back on its feet after this terrible setback, Webb strove to maintain support for the program. His success helped to pave the way to future NASA triumphs, such as the historic Apollo moon landing, which took place shortly after his retirement from NASA in 1968.

Webb remained in Washington, D.C., where he served on several advisory boards and as a regent of the Smithsonian Institution. In 1981, he was awarded the Sylvanus Thayer Award by the United States Military Academy at West Point for his dedication to his country. Former NASA Administrator Sean O’Keefe said of Webb: “He took our nation on its first voyages of exploration, turning our imagination into reality.”

Edwin Hubble Expands Our View of the Universe

by Donna Weaver and Ann Jenkins

American astronomer Edwin Powell Hubble (1889–1953) never lived to see the development or launch of his namesake, the Hubble Space Telescope. But like the telescope that bears his name, Dr. Hubble played a crucial role in advancing the field of astronomy and changing the way we view the universe. As Hubble’s namesake is breaking new ground in the exploration of the distant universe via the Frontier Fields, let us take a step back and learn more about Hubble, the man.

This is an illustration of Dr. Edwin Powell Hubble.

Edwin Hubble is regarded as one of the most important observational cosmologists of the 20th century. Illustration credit: Kathy Cordes of STScI.

As a young boy, Edwin Hubble read tales of traveling to undersea cities, journeying to the center of the Earth, and trekking to the remote mountains of South Africa. These stories by adventure novelists Jules Verne and H. Rider Haggard stoked young Hubble’s imagination of faraway places. He fulfilled those childhood dreams as an astronomer, exploring distant galaxies with a telescope and developing celestial theories that revolutionized astronomy.

But Hubble didn’t settle immediately on the astronomy profession. He studied law as a Rhodes Scholar at Queens College in Oxford, England. A year after passing the bar exam, Hubble realized that his love of exploring the stars was greater than his attraction to law, so he abandoned law for astronomy. “I chucked the law for astronomy and I knew that, even if I were second rate or third rate, it was astronomy that mattered,” Hubble said. (1)

Our Galaxy Is Not Alone

He studied astronomy at the University of Chicago and completed his doctoral thesis in 1917. After serving in World War I, he began working at the Mount Wilson Observatory near Pasadena, Calif., studying the faint patches of luminous “fog” or nebulae — the Latin word for clouds — in the night sky. Hubble and other astronomers were puzzled by these gas clouds and wanted to know what they were.

Using the 100-inch reflecting Hooker Telescope — the largest telescope of its day — Hubble peered beyond our Milky Way Galaxy to study an object known then as the Andromeda Nebula. He discovered special, “variable stars” on the outskirts of the nebula that changed in brightness over time. These stars brightened and dimmed in a predictable way that allowed Hubble to determine their distances from Earth. Hubble showed that the distance to the nebula was so great that it had to be outside the Milky Way Galaxy. Hubble realized that the Andromeda Nebula was a separate galaxy much like our own. The discovery of the Andromeda Galaxy helped change our understanding of the universe by proving the existence of other galaxies.

Hubble also devised the classification system for galaxies, grouping them by sizes and shapes, that astronomers still use today. He also obtained extensive evidence that the laws of physics outside our galaxy are the same as on Earth, verifying the principle of the uniformity of nature.

The Expanding Universe

As Hubble continued his study, he made another startling discovery: The universe is expanding. In 1929 he determined that the more distant the galaxy is from Earth, the faster it appears to move away. Known as Hubble’s Law, this discovery is the foundation of the Big Bang theory. The theory says that the universe began after a huge cosmic explosion and has been expanding ever since. Hubble’s discovery is considered one of the greatest triumphs of 20th-century astronomy.

Albert Einstein could have foretold Hubble’s discovery in 1917 when he applied his newly developed General Theory of Relativity to the universe. His theory — that space is curved by gravity — predicted that the universe could not be static but had to expand or contract. Einstein found this prediction so unbelievable that he modified his original theory to avoid the problem. Upon learning of Hubble’s discovery, Einstein immediately regretted revising his theory.

For his many contributions to astronomy, Hubble is regarded as one of the most important observational cosmologists of the 20th century.

(1) As quoted by Nicholas U. Mayall (1970). Biographical memoir. Volume 41, Memoirs of the National Academy of Sciences, National Academy of Sciences (U.S.). National Academy of Sciences. p. 179.

Frontier Fields Public Lecture

Want to hear about the Frontier Fields project straight from the scientist? On August 5, 2014, principal investigator Dr. Jennifer Lotz gave a public lecture entitled “The Frontier Fields: a Sneak Peek at the First Billion Years of the Universe” and the recorded webcast is available at the link below.

Dr. Jennifer Lotz
The Frontier Fields: a Sneak Peek at the First Billion Years of the Universe

August 5, 2014

How we far can we go? What are the faintest objects the Hubble Space Telescope can possibly see? Can we get a sneak peek at the early universe before the James Webb Space Telescope is launched? These are the key questions we hope to answer with the Frontier Fields campaign. Over this three year program, astronomers at the Space Telescope Science Institute will attempt to push the Hubble Space Telescope’s capabilities to its limits. This ambitious effort will combine the power of Hubble with the natural gravitational telescopes of massive clusters of galaxies that magnify more distant galaxies. Hubble will obtain the deepest ever optical and infrared images of six massive clusters, in parallel with the deep images of six neighboring “blank” fields. These observations will reveal galaxies about 10-20 times fainter than any previously seen, allowing astronomers to study the birth of galaxies like our own Milky Way.

https://webcast.stsci.edu/webcast/detail.xhtml?talkid=4287

This lecture is part of the monthly public lecture series at the Space Telescope Science Institute in Baltimore, Maryland. Each month addresses a different cosmic topic, usually related to Hubble, but always venturing to some fascinating part of the universe. For more information, check out the web page on HubbleSite:  http://hubblesite.org/about_us/public_talks/

 

Seeing Double (or More!) in Frontier Fields Images

The immense gravity in this foreground galaxy cluster, Abell 2744, warps space to brighten and magnify images of far-more-distant background galaxies as they looked over 12 billion years ago, not long after the big bang.  This is the first of the Frontier Fields to be imaged.

Galaxy cluster Abell 2744, the first of the Frontier Fields to be imaged.

Take a long look at this image. You’re seeing a lot of distant galaxies magnified by the natural “gravitational lens” of galaxy cluster Abell 2744. But you aren’t seeing as many as you think.

Gravitational lenses, natural magnifiers created in space when light is bent by the enormous mass of galaxy clusters, distort and enlarge the images of distant galaxies behind the cluster. But they do more than that: sometimes they replicate them, like multiple images in a funhouse mirror.

abell multiple

Galaxy cluster Abell 2744, with multiple images of individual galaxies marked. These multiple images are produced by the cluster’s gravitational lens.

In the above image, we’ve marked the galaxies that are actually images of the same galaxy by overlaying them with numbered triangles. Each galaxy has a number. The multiple images are identified by letters. The galaxies labeled 1a, 1b, and 1 c, for instance, are one galaxy, its image repeated three times. (Only numbers and letters are significant. The colors don’t represent anything, but are used to make it easier to distinguish the various numbered galaxies.)

In previous posts, we explained that mass distorts space. Light from a distant galaxy follows space’s curve like a ball rolling along a putting green. (Think of space as a miniature golf course with fewer animatronic dinosaurs.)

Sometimes, the level of distortion sends the light to multiple places. If you’ve ever seen a single candle reflected multiple times in the bottom of a wineglass, you’ve seen this distorting effect occur in lenses. In fact, gravitational lensing is so similar to glass lensing that you could replicate the distortions of a gravitational lens by grinding a glass lens to the same proportions and bumps.

And cosmic lenses are quite lumpy. The galaxies of the cluster, embedded in halos of dark matter, create bumps of mass. Light can take multiple paths around the galaxy cluster as it encounters the distortions in space-time created by the cluster’s mass. The closer the light of more-distant galaxies passes to the lens, the stronger the deflection. If the light passes close enough to the lens, these multiple images are likely to appear. The individual galaxies in the cluster also add small deflections, and occasionally help produce multiple paths for the light to reach us.

When astronomers look at a lensed image, they’re looking at a giant puzzle. They need to figure out where all the mass is in the image – most of it, being dark matter, is invisible. Pinpointing the multiple images of identical galaxies helps accomplish this because they’re a good indicator of how dramatically the light is being deflected.

Abell2744-multilens-1+markers Abell2744-multilens-3+markers

Some of the multiple images are obvious. Galaxy images 1a, 1b and 1c (left image) are good examples – they’re blue galaxies with red centers, and they look very like one another. The green-hued galaxy identified by 3a, 3b and 3c (right image) is another good example. Astronomers seek out those obvious candidates to start with, then try to build a model of how the mass in the cluster is distributed. Based on that model, they start identifying the multiple images that aren’t so obvious: Does this reddish galaxy to the side have a counterpart where the model says it should be? Analysis of attributes like color, and especially distance, also play an important role in determining which galaxies are multiples — a technique that comes in handy in many situations.

Thanks to reader Judy Schmidt for the idea for this post.

 

How Hubble Observations Are Scheduled

This is the third in a three-part series.

After observing time is awarded, the Institute creates a long-range plan. This plan ensures that the diverse collection of observations are scheduled as efficiently as possible. This task is complicated because the telescope cannot be pointed too close to bright objects like the Sun, the Moon, and the sunlit side of Earth. Adding to the difficulty, most astronomical targets can only be seen during certain months of the year; some instruments cannot operate in the high space-radiation areas of Hubble ’s orbit; and the instruments regularly need to be calibrated. These diverse constraints on observations make telescope scheduling a complex optimization problem that Institute staff are continually solving, revising, and improving.”

Preparing for an observation also involves selecting guide stars to stabilize.the telescope’s pointing and center the target in the instrument’s field of view. The selection is done automatically by the Institute’s computers, which choose two stars per pointing from a catalog of almost a billion stars. These guide stars will be precisely positioned within the telescope’s fine guidance sensors, ensuring that the target region and orientation of the sky is observed by the desired instrument.”]

A weekly, short-term schedule is created from the long-range plan. This schedule is translated into detailed instructions for both the telescope and its instruments to perform the observations and calibrations for the week. From this information, daily command loads are then sent from the Institute to NASA’s Goddard Space Flight Center to be uplinked to Hubble.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md.  In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

Hubble’s Flight Operations Team resides in the Space Telescope Operations Control Center at NASA’s Goddard Space Flight Center in Greenbelt, Md. In addition to monitoring the health and safety of the telescope, they also send command loads to the spacecraft, monitor their execution, and arrange for transmission of science and engineering data to the ground.

The journey from proposal through selection and scheduling culminates in the email informing astronomers that their data is ready to be accessed. Usually, the process takes more than a year from idea to data—sometimes even longer. Of course, that’s when the real work begins—the analysis of the data and the hard work of uncovering another breakthrough Hubble discovery!

How Hubble Observations Are Planned

This is the second in a three-part series.

Researchers awarded telescope time based on the scientific merit of their Phase I proposal must submit a Phase II proposal that specifies the many details necessary for implementing and scheduling of the observations. These details include such items as precise target locations and the wavelengths of any filters required.

Once an observation has occurred, the data becomes part of the Hubble archive, where astronomers can access it over the Internet. Most data is marked as proprietary within the Institute computer systems for 12 months. This protocol allows observers time to analyze the data and publish their results. At the end of this proprietary-data-rights period, the data is made available to the rest of the astronomical community. (Most of the very large programs, such as Frontier Fields, have given up proprietary time as part of their proposal.)

This is a view of the many computers that are part of the Barbara A. Mikulski Archive for Space Telescopes (MAST), located at the Space Telescope Science Institute (STScI) in Baltimore, Md. The archive is named in honor of the United States Senator from Maryland for her career-long achievements and becoming the longest-serving woman in U.S. Congressional history. MAST is NASA’s repository for all of its optical and ultraviolet-light observations, some of which date to the early 1970s. The archive holds data from 16 NASA telescopes, including current missions such as the Hubble Space Telescope and Kepler. Senator Mikulski is in the center, STScI Director Matt Mountain at her right, and STScI Deputy Director Kathryn Flanagan at her left. The plaque to image right is a photo of Supernova Milkuski, an exploding star that the Hubble Space Telescope spotted on Jan. 25, 2012. It was named in honor of the Senator by Nobel Laureate Adam Riess and the supernova search team with which he is currently working. The supernova, which lies 7.4 billion light-years away, is the titanic detonation of a star more than eight times as massive as our Sun.

This is a view of the many computers that are part of the Barbara A. Mikulski Archive for Space Telescopes (MAST), located at the Space Telescope Science Institute (STScI) in Baltimore, Md. The archive is named in honor of the United States Senator from Maryland for her career-long achievements and becoming the longest-serving woman in U.S. Congressional history. MAST is NASA’s repository for all of its optical and ultraviolet-light observations, some of which date to the early 1970s. The archive holds data from 16 NASA telescopes, including current missions such as the Hubble Space Telescope and Kepler. Senator Mikulski is in the center, STScI Director Matt Mountain at her right, and STScI Deputy Director Kathryn Flanagan at her left. The plaque to image right is a photo of Supernova Milkuski, an exploding star that the Hubble Space Telescope spotted on Jan. 25, 2012. It was named in honor of the Senator by Nobel Laureate Adam Riess and the supernova search team with which he is currently working. The supernova, which lies 7.4 billion light-years away, is the titanic detonation of a star more than eight times as massive as our Sun.

Along with their Phase II proposal, observers can also apply for a financial grant to help them process and analyze the observations. These grant requests are reviewed by an independent financial review committee, which then makes recommendations to the Institute director for funding. Grant funds are also available for researchers who submit Phase I proposals to analyze non-proprietary Hubble data already archived. The financial committee evaluates these requests as well.

Up to 10 percent of Hubble ’s time is reserved as director’s discretionary time and is allocated by the Institute director. Astronomers can apply to use these orbits any time during the course of the year. Discretionary time is typically awarded for the study of unpredictable phenomena such as new supernovae or the appearance of a new comet. Historically, directors have allocated large percentages of this time to special programs that are too big to be approved for any one astronomy team. For example, the observations of the Frontier Fields use director’s discretionary time.

Gravitational Lensing in Action

In my previous blog post, Visual “Proof” of General Relativity, I discussed how gravitational lensing demonstrates the effects of Einstein’s theory of general relativity in a direct, visual manner. Images created by gravitational lenses show features that are not possible in Newton’s version of gravity.

Although seeing general relativity with your own eyes is kinda awesome, there’s one unsatisfying aspect: you only see the result, not the process. Since you don’t know exactly what those galaxies looked like before the gravitational lensing, it is hard to fully appreciate the magnitude of the distortions. We have no on/off switch for the mass of the galaxy cluster to be able to examine the un-lensed image and compare against the lensed one.

lensing_sim_trio-1600x550

A simulation of gravitational lensing by a cluster of galaxies (click on image for larger version). The galaxies of cluster Abell 2744 (left) are inserted into the Hubble Ultra Deep Field (right) to produce the combined image with gravitational lensing (center).

But we can demonstrate the process of gravitational lensing through scientific visualization. The images above show a simulation of gravitational lensing by a galaxy cluster. On the left is an image of only the galaxies that belong to galaxy cluster Abell 2744; all of the foreground and background objects have been removed. On the right is a deep field image of galaxies. In the center is a simulation of how the galaxies of Abell 2744 would distort the galaxy images in the deep field.

By carefully comparing galaxy images between the right and center panels, one can see how the un-lensed galaxies transform to their distorted lensed versions.  The elongated streaks and arcs in the center image generally come from compact, ellipse-shaped galaxies in the right image. But not all galaxies are changed, a fact easily seen by examining the larger, yellow galaxy in the lower right.

The explanation comes from the details of the simulated lensing. The deep field used above is a portion of the Hubble Ultra Deep Field (HUDF), and includes only galaxies for which we have a good measure of their distance. Using those distances and the distance to Abell 2744, we were able to place the galaxies of Abell 2744 at their correct positions within the deep field. HUDF galaxies which are closer than the galaxy cluster would not be lensed, and appear the same in the right and center images. Only those galaxies behind the cluster were transformed by the simulated lensing. Thus, the central image provides a proper simulation of what would be seen if Abell 2744 suddenly wandered across the sky and ended up in the middle of the HUDF.

I note that all of the background galaxies were combined into a single image at a set distance behind the cluster for simplicity. The full, and rather tedious, 3D calculation could have been performed, but was deemed unlikely to provide a significant visual difference for a public-level illustration. I further note that it is an occupational hazard of being a scientist that one feels compelled to provide such full-disclosure details.

The really difficult challenge is to do the reverse of this simulation. Start with an image of gravitational lensing and then work out the mass distribution of the galaxy cluster from the distribution of streaks and arcs. But, hey, no one said being an astrophysicist was easy.

In the final part of this series of blog posts, I’ll provide a more down-to-earth example of gravitational lensing.

 

How Hubble Observations Are Proposed

This is the first in a three-part series.

Time on the Hubble Space Telescope is a precious commodity. As a space telescope, Hubble can observe 24 hours a day, but its advantageous perch also attracts a large number of astronomers who want to use it. The current oversubscription rate—the amount of time requested versus time awarded—is six to one.

The process of observing with Hubble begins with the annual Call for Proposals issued by the Space Telescope Science Institute to the astronomical community. Astronomers worldwide are given approximately two months to submit a Phase I proposal that makes a scientific case for using the telescope. Scientists typically request the amount of telescope time they desire in orbits. It takes 96 minutes for the telescope to make one trip around the Earth, but because the Earth usually blocks the target for part of the orbit, typical observing time is only about 55 minutes per orbit.

Longer observations require a more compelling justification since only a limited number of orbits are available. Winning proposals must be well reasoned and address a significant astronomical question or issue. Potential users must also show that they can only accomplish their observations with Hubble ’s unique capabilities and cannot achieve similar results with a ground-based observatory.

The Institute assembles a time allocation committee (TAC), comprising experts from the astronomical community, to determine which proposals will receive observing time. The committee is subdivided into panels that review the proposals submitted within a particular astronomical category. Sample categories include stellar populations, solar system objects, and cosmology. The committee organizers take care to safeguard the process from conflicts of interest, as many of the panel members are likely to have submitted, or to be a co-investigator, on their own proposals.

The time allocation committee (TAC) discusses which proposals will receive observing time on Hubble.

The time allocation committee (TAC) discusses which proposals will receive observing time on Hubble.

Proposals are further identified as general observer (GO), which range in size from a single orbit to several hundred, or snapshot, which require only 45 minutes or less of telescope time. Snapshots are used to fill in gaps within Hubble ’s observing schedule that cannot be filled by general observer programs. Once the committee has reviewed the proposals and voted on them, it provides a recommended list to the Institute director for final approval.

In my next post, I will discuss how observations are planned.