Subaru Telescope: An IceHunters datasource

Quite a few people have been asking why we’re using telescopes back on Earth to search for KBOs, when we’ve got cameras on the New Horizons spacecraft already more than halfway out there- why don’t we use the onboard cameras for the search?  It’s a good question, and it has at least two good answers.

1) Sensitivity. Our biggest “telescope” on New Horizons is the LORRI camera, which has a mirror 20.8 centimeters (about 8 inches) in diameter, the size of a modest backyard telescope.  Compare that to the giant Subaru telescope in Hawaii that we’ve been using for the KBO search, which has an 8.2-meter (323 inch) diameter mirror.  The area of the Subaru telescope’s mirror is 1500 times that of LORRI’s mirror (mathematically speaking, area varies with the square of the diameter), so Subaru can gather 1500 times as much light in the same amount of time.  The KBOs we’re looking for are very faint, so LORRI’s camera just doesn’t gather enough light to see them- we need the light-gathering power of those giant telescopes on Earth.  Sure, New Horizons is closer to the Kuiper Belt, but we’re still just over halfway to Pluto and most of the Kuiper Belt objects are quite a bit further away than Pluto, so the KBOs aren’t a whole lot brighter seen from New Horizons than from Earth.  In fact even after we’ve discovered a target KBO, we don’t expect it to be bright enough to detect with New Horizons’ cameras till a couple of months before we fly past it (when it’s about 40 million miles away, compared to New Horizon’s current distance from the Kuiper belt of about 2 billion miles).  Till then, we have to do all our steering towards the KBO based on information from telescopes on Earth (or near Earth, like the Hubble Space Telescope).

Artist's rendering of New Horizons

You might ask why, if we can detect KBOs with New Horizons from 40 million miles away, we don’t just wait till we’re in the Kuiper Belt, use New Horizons to find a “nearby” KBO closer than 40 million miles, and set a course towards it?  Part of the answer is that we’d have to look over the entire sky, and we can’t afford to take and downlink enough photos from New Horizons to cover the whole sky to look for them- see point 2) below.  The other part of the answer is that we’d be unlikely to find one that close which was aligned along the direction we’re traveling, so we’d probably have to make a pretty sharp turn to reach it, and we don’t have enough fuel onboard New Horizons for sharp turns.  We need to find our KBO early enough, years in advance, so that a gentle nudge will send us in its direction.  If you’re flying from Los Angeles to New York and you’re re-routed to JFK instead of La Guardia, you don’t have to make much of a course correction if you get the news while flying over Denver.  But if you don’t learn of your new destination till you’re over Manhattan, you’ll need to hang a sharp right.

2) Data volume. On our 1-night observing run on the Subaru telescope in early July [add a link to that blog post], we collected 57 Gigabytes of data, and that was just one of about 25 nights of searching we’re doing in 2011.  For comparison, the total amount of data New Horizons will collect during its 2015 Pluto flyby will be about 5 Gigabytes, and that will take months of expensive time on the Deep Space Network to send back to Earth.  We just don’t have the time (or the money) to downlink enough data from the spacecraft for a viable KBO search.

A related question is why we’re not taking pictures of all the unknown objects we’re passing by on our way to Pluto.  There’s a similar answer- while there are undoubtedly unknown worlds out there, our onboard cameras are only sensitive enough to detect objects that are relatively close, and space is big and empty enough that there just aren’t many objects that are close enough.  Also, they could be anywhere in the sky, and again we can’t afford to search the whole sky from New Horizons.

We did give some thought to using ground-based telescopes to search for objects between Jupiter and Pluto that we could either alter our course to fly past for a close-up view on our way to Pluto, or failing that, photograph from a distance from our current course. But while there are some small bodies (called Centaurs), orbiting the sun in that region, they are not nearly so numerous as the objects beyond Pluto in the Kuiper Belt.  When we ran the numbers we found that the chances of  finding a conveniently-placed Centaur were too small to justify the huge amount of telescope time we’d need to look for it.

I hope this helps to clear up some of your questions- if not, let us know and we’ll try to answer any follow up questions in a later post.