[meteorite-list] Pluto-bound Probe Faces Crisis (New Horizons)

Ron Baalke baalke at zagami.jpl.nasa.gov
Thu May 22 14:01:46 EDT 2014



http://www.nature.com/news/pluto-bound-probe-faces-crisis-1.15261

Pluto-bound probe faces crisis

NASA scientists scramble to find an object in the outer Solar System's 
Kuiper belt in time for a close-up visit.

Alexandra Witze
Nature News
20 May 2014

Nearly 4.3 billion kilometres from Earth, and most of the way to Pluto, 
NASA's New Horizons spacecraft is in danger of missing out on half of 
its mission. Project managers face a looming deadline to identify an icy 
object in the outer Solar System for the probe to fly by after it passes 
Pluto.

A visit to a Kuiper belt object, or KBO, was always meant to be a key 
part of New Horizons' US$700-million journey, which began in 2006. But 
there is only a slim chance that astronomers will find a suitable KBO 
with their current strategy of using ground-based telescopes - and securing 
time on the orbiting Hubble Space Telescope is far from guaranteed.

New Horizons will fly past Pluto in July 2015. Soon afterwards, it must 
fire its engines and set itself on course to fly past a selected KBO. 
Project scientists must identify a KBO in the next several months if they 
are to determine the necessary trajectory well enough for New Horizons 
to aim accurately and meet its target.

"They're running out of time," says Mark Sykes, director of the Planetary 
Science Institute in Tucson, Arizona, who is not involved in the mission. 
"We're not just talking about science being lost - we're talking about 
getting return on our investment."

New Horizons scientists have asked for 160 orbits' worth of observing 
time on the hugely oversubscribed Hubble. It is a rare request for a NASA 
mission already in operation. The committee that allocates Hubble time 
will make a decision by 13 June.

The two targets of New Horizons - Pluto and a KBO - each promise different 
rewards. "Two different scientific communities are getting excited about 
it," says Will Grundy, mission co-investigator at the Lowell Observatory 
in Flagstaff, Arizona. The Pluto fly-by will be the first close-up glimpse 
of this geologically active world and its moons. The KBO visit, 2-3 years 
later (see "Far horizons"), would be the best look yet at one of these 
primordial icy bodies - time capsules from the early days of the Solar 
System.

In theory, project scientists should have identified a suitable KBO long 
ago. But they postponed their main search until 2011, waiting for all 
the possible KBO targets to begin converging on a narrow cone of space 
that New Horizons should be able to reach after its Pluto encounter. Starting 
to look for them before 2011 would have been impossible, says Grundy, 
because they would have been spread over too much of the sky.

Now that the hunt for KBOs is on, the New Horizons researchers have mainly 
been using the 8.2-metre Subaru Telescope in Hawaii and the 6.5-metre 
Magellan Telescopes in Chile. They have found about 50 new KBOs; none 
is close enough for New Horizons to reach.

Part of the problem is that the search area is in the Galactic Plane, 
where most of the Milky Way's stars are. The bright stars tend to outshine 
the light coming from the faint KBOs, and the crowded field of view means 
that the discovery rate depends strongly on observing conditions. Bad 
weather and poor-quality observation on many nights has ruined much of 
the search.

Early on, the team also overestimated the number of KBOs that it would 
find. In the past couple of years, astronomers have discovered that there 
are fewer dim KBOs than was extrapolated on the basis of the number of 
bright ones. "There just aren't as many faint ones," says Grundy - and 
faint, faraway KBOs are what New Horizons must detect.

For a mission that has been carefully planned for many years, the failure 
to find a KBO target is striking. But Sykes is not alarmed. "That should 
have been in the calculus, but it really wouldn't have changed their ground-based 
campaign," he says.

The team has another eight nights with Subaru in late June and July, and 
hopes to win some more time in August. It has also applied for extra observing 
time with Magellan in October. Even if conditions are near perfect, the 
chances of snaring a KBO from the ground this year are less than 40%. 
With the requested Hubble time, those chances soar to greater than 90%.

Without Hubble, it might still be possible to identify a KBO target from 
the ground early next year. But that would complicate the mission, because 
New Horizons would have to wait for longer after visiting Pluto to fire 
its engines. In that case, the spacecraft would need to execute a sharper 
turn than team scientists would like to reach the correct trajectory.

In the worst-case scenario, New Horizons would be forced to observe a 
KBO from far away. Several possible long-distance targets have already 
been spotted (S. D. Benecchi et al. Icarus http://doi.org/ssn; 2014). 
Even with its puny 21-centimetre telescope, New Horizons would get a better 
glimpse of a distant KBO than the 2.4-metre Hubble can manage from an 
Earth orbit.

Not that the team has yet given up hope of a close KBO target. "If we 
can find one," says Grundy, "we will happily take it."

Nature
509, 407–408 (22 May 2014) doi:10.1038/509407a



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