Hubble Space Telescope Starts Search for Kuiper Belt Objects as New Horizons Races Toward Pluto

Artist's impression of New Horizons at a KBO
An artist’s rendering of the New Horizons spacecraft, encountering a Kuiper Belt Object during its extended mission following its Pluto flyby. The Hubble space telescope has started searching for a suitable KBO so that the spacecraft’s extended mission can take place. Image Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

With the clock ticking toward NASA’s New Horizons’ upcoming Pluto flyby in July of next year, time is running out for the discovery of a suitable Kuiper Belt Object that the spacecraft can study, following its close encounter with the dwarf planet. Having systematically searched the skies for years with some of the biggest telescopes on the ground to no avail, the mission’s science team has just been given the green light to use the Hubble Space Telescope, whose superior observing capabilities represent the last best chance for assuring a successful extended mission phase for the New Horizons spacecraft.

The study of at least one member of the hundreds of thousands of icy planetesimals that are believed to reside in the vast region beyond the orbit of Neptune, known as the Kuiper Belt, had been one of the main science goals for all the robotic mission concepts to Pluto that were under consideration by NASA during the 1990s, like the Pluto Fast Flyby and Pluto Kuiper Express. Based on these previous mission proposals, which were ultimately cancelled by the space agency due to budget constrains, New Horizons retained the exploration of the Kuiper Belt as a main science objective, provided that a suitable KBO located close enough to the spacecraft’s flight path through the Pluto system could be discovered. To that end, following the spacecraft’s launch in 2006, the mission’s science team implemented an extensive observing campaign utilising some of the largest ground-based telescopes, like the twin 6.5-m Magellan Telescopes in Chile and the 8.2-m Subaru and 3.6-m Canada–France–Hawaii Telescopes on Mauna Key in Hawaii. Yet, even though they were able to discover a total of 52 candidate KBOs in the same area of the sky as Pluto, scientists found none that laid close enough for New Horizons to visit.

The Subaru Telescope dome (left) is silhouetted by the Milky Way, as the telescope searches for KBOs. The search area is among the star clouds in the upper left of the image. Image Credit/Caption: John Spencer/JHU/APL
The Subaru Telescope dome (left) is silhouetted by the Milky Way, as the telescope searches for KBOs. The search area is among the star clouds in the upper left of the image. Image Credit/Caption: John Spencer/JHU/APL

Part of the reason for the failure of the ground-based searches to locate a suitable KBO has been the area of the sky that scientists have had to observe. Currently positioned in the Sagittarius constellation where the center of the Milky Way lies, Pluto and any neighboring KBOs are easily lost in the intense glare coming from the densely packed stars of the galactic core. In addition, ground-based telescopes have to contend with the blurring effects of the Earth’s atmosphere, which further hamper the search for objects as small as 50 km across and as dark as charcoal, observed from a distance of more than three billion miles away. The New Horizons science team has also proved to be overtly optimistic in its previous estimates of the number of very small objects that could lie in the Kuiper Belt, which turned out to be fewer than expected. “Despite intensive searches using the largest telescopes and most talented astronomers in the world, we haven’t yet been able to identify any suitable KBO that is close enough to be within the spacecraft’s fuel supply,” said Dr. Alan Stern, Principal Investigator for the New Horizons mission, during a recent interview with AmericaSpace.

Now, all hope for locating these long sought for ice planetesimals that are properly aligned with the spacecraft’s flight path, lies on the superior resolving power of NASA’s Space Hubble Telescope, for which the mission’s team had requested to be allocated a small amount of observing time on last October. After weighting in the scientific merits of their application, the Space Telescope Science Institute’s Hubble Space Telescope Time Allocation Committee finally announced its much-anticipated decision to the mission team late last week to give the go-ahead for the New Horizons Hubble observations. “I am pleased that our science peer-review process arrived at a consensus as to how to effectively use Hubble’s unique capabilities to support the science goals of the New Horizons mission,” says Dr. Matt Mountain, Director of the Space Telescope Science Institute (STScI) in Baltimore, Md.

The New Horizons team will use Hubble’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in order to scan the part of the sky in the Sagittarius constellation where Pluto currently resides, during a two-fold observing campaign. During the first part of observations, which have been ongoing since last week, the space telescope will use 40 of its orbits (the equivalent of 2.5 days worth of observations) to turn in such a way as to match the predicted rate with which KBOs are moving across the sky. This way, the telescope’s cameras will depict the stars in a total of 20 different star fields as streaks in the resulting images, whereas any KBOs present would appear as pinpoint sources of light.

These preliminary observations aim to showcase that Hubble has a statistically strong chance to find a pair of KBOs that New Horizons could visit, by mapping the number of KBOs in the region near Pluto. If this pilot search proves successful, the Space Telescope Science Institute will allocate the mission team another 156 to 160 Hubble orbits for the duration of the summer, in order to conduct a more thorough set of observations that would hopefully succeed in identifying New Horizons’ future targets following the Pluto encounter. The reason for this two-part approach to Hubble’s KBO search is the extremely limited amount of available observing time on the space telescope, compared with the observing time requested by the plethora of applications from the astronomy community each year. “Astronomers around the world apply for observing time on the Hubble Space Telescope,” states NASA. “Competition for time on the telescope is extremely intense and the requested observing time significantly exceeds the observing time available in a given year.”

The first raw image from the New Horizons KBO search, taken with the Hubble space telescope. Additional observations are needed, for scientists to be able to differentiate between KBOs and stars in the images. Image Credit: Alex Parker/HST
The first raw image from the New Horizons KBO search, taken with the Hubble space telescope. Additional observations are needed for scientists to be able to differentiate between KBOs and stars in the images. Image Credit: Alex Parker/HST

With the Pluto flyby fast approaching, the New Horizons team must find a suitable KBO preferably before the end of summer, if it hopes to guide the spacecraft accordingly next year through the Pluto system. Since New Horizons is speeding through space at a record speed of 52,000 km/h, and its onboard fuel reserves are quite limited, any trajectories toward a KBO must be planned well in advance, many months before the spacecraft commences its Pluto encounter operations early next year. Any delays in discovering an appropriate KBO further reduce the chances for an extended mission beyond Pluto. Yet, despite being in a race against time, the New Horizons team is confident that the Hubble KBO search will prove to be successful. “Computer model estimates indicate a 95 percent chance we will be able to locate [with Hubble] a suitable KBO object for the spacecraft to study,” says Stern.

The optimism showcased by the New Horizons team regarding the outcome of the ongoing KBO search isn’t unwarranted. These new observations by Hubble are the latest in a long series of past Solar System observations by the space telescope that have led to important, groundbreaking discoveries, like Pluto’s four smaller moonsNix, Hydra, Kerberos, and Styxwhich were discovered during the last decade and had all gone undetected by previous searches with ground-based observatories. Contrary to the saying that past performance is not necessarily indicative of future results, which is so prevalent in the financial sector, Hubble’s past accomplishments are indicative of its unsurpassed abilities as an astronomical observatory to probe deep into the uncharted territory that is the vast expanse beyond Neptune. “The planned search for a suitable target for New Horizons further demonstrates how Hubble is effectively being used to support humankind’s initial reconnaissance of the Solar System,” says Mountain. “Likewise, it is also a preview of how the powerful capabilities of the upcoming James Webb Space Telescope will further bolster planetary science. We are excited by the potential of both observatories for ongoing Solar System exploration and discovery.”

With all the effort and hard work that the whole New Horizons team has put into the Hubble KBO observing campaign, we may take our first glimpses in a few weeks of the small primordial icy worlds that await us in the furthest reaches of our Solar System. With any luck, in the next few years these small worlds will turn from mere pixels in a blurry image into full-fledged planetary vistas.


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  1. Since the first impact of a man-made object on the Moon in 1959, countless spacecraft have been flying from our planet, leaving an eternal legacy of an intelligent species to perhaps being discovered one day. What a perspective!

    The Kuiper Belt exploration will add a significant wealth of information to our knowlege of the solar system.

    Maybe, just maybe, our presence will be detected.

    Another gem, Leonidas!

    • Thank you Tom!

      Indeed, New Horizons represents our first foray into the Solar System’s third realm of planetary objects – the Kuiper Belt. Akin to the great discoveries of the previous decades of planetary exploration, it will be such a wondrous mission!

      As for the future, who knows? The sure thing is that New Horizons will be humanity’s fifth interstellar robotic emissary – a wondrous achievement in itself.

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