The Voyager Interstellar Mission sounds like something out of Star Trek, but it is in fact the extended mission of the doughty spacecraft that taught us so much about the outer Solar System. An extended mission can be just as valuable, and sometimes more so, than the original — think about the continuing adventures of our Mars rovers, working well beyond their projected timelines. In Voyager’s case, we’re learning much about how the Solar System behaves as it moves through the interstellar medium, and about the heliopause, where the Sun’s solar winds effectively lose their dominance over the winds from other stars.
Now the Deep Impact spacecraft, which provided such spectacular scenes of Comet Tempel 1, will acquire an extended mission of its own, and in two parts. The one that catches my eye is called Extrasolar Planet Observation and Characterization (EPOCh), which will turn the spacecraft loose on the study of several nearby bright stars already known to have gas giant planets around them. The collected data should tell us more about the planets and their atmospheres as they transit between Deep Impact and the stars they orbit. We may also learn whether some of these planets have moons or rings, with the possibility of finding smaller, hitherto undetected planets in orbit around the same stars.
At the end of 2006, Emily Lakdawalla discussed EPOCh on the Planetary Society Weblog with principal investigator Drake Deming (NASA GSFC), an e-mail conversation that yielded this description of the mission:
“EPOCh will do photometry of giant planets transiting several nearby bright stars (it’s like Kepler in that respect, but Kepler is locked-in to a particular field in Cygnus, and won’t look at nearby bright transiting systems). We exploit the fact that the Deep Impact telescope is out of focus (allows us to get better photometry). We are sensitive to terrestrial planets via their perturbations on the transit times of the giant planets (which we measure very precisely).”
Out of focus? It turns out that Deep Impact’s high-resolution camera does indeed have a flaw, but it’s one that may prove advantageous to the mission, as Deming goes on to explain:
“We are doing very precise photometry, measuring the brightnesses of the stars. As the giant planets pass in front of them (“transit” the stars) we will see the dip in the star’s light. This dip lasts for several hours, but we want to time its occurrence very precisely (to about 1 second accuracy). It’s those changes in the time of the giant planet transits that will indicate the presence of terrestrial planets. That means we have to measure the stellar brightness very accurately, so that the curve of brightness versus time is very “smooth”, i.e. has high signal-to-noise ratio, and we can find the center time accurately. But to get high signal to noise, we have to collect lots of photons from the star. That’s where the defocus helps. Each pixel of the CCD has a limited capacity to collect photons before it saturates. With a defocused image, we have about 75 pixels collecting light for us, so we can collect lots of photons in each exposure without saturating, and that gives us the high signal-to-noise ratio that we need.”
This interesting search will occur as Deep Impact proceeds toward the second goal of its extended mission, a flyby of the comet Boethin planned for December 5, 2008. Both investigations are intriguing ways to stretch existing resources, and they’re complemented by an extended mission for the Stardust spacecraft, which will revisit Comet Tempel 1. Deep Impact’s encounter with Tempel 1 was on July 4, 2005; Stardust, now flying as the New Exploration of Tempel 1 (NExT) mission, is to arrive on February 14, 2011. Encounters with Tempel 1 seem irresistibly drawn to holidays.
When you’ve got a budget as tight as NASA’s, it’s important to find ways to stretch your dollars, and according to the agency’s Alan Stern (who knows something about dollar-stretching through his work as principal investigator for the New Horizons mission), these extended missions will accomplish their work for about 15 percent of the cost of a new mission built from scratch. Thus do two seasoned spacecraft acquire new targets that had previously been unplanned, including an extrasolar study that could prove productive indeed, as EPOCh’s sensitivity should exceed that of existing ground and space-based observatories.
Addendum: Dr. Deming just responded to my note asking about the target list for EPOCh, saying that his team was still evaluating the best candidates. He also said the observations would involve three stars, all relatively nearby, and all, of course, orbited by transiting giant planets.
You know, perhaps this “budget crisis” has produced one helpful side affect for NASA: they are beginning to be frugal with their resources.
I hope once they receive more funds in 2008 that this pattern continues, as the cost of space isn’t getting any cheaper.
i think that it’s good ideia use Deep Impact carema to search and confirm the planets around the eclipse binary star system CM Draconis AB http://www.solstation.com/stars2/cm-dra3.htm
Astronomers cannot find Comet Boethin, so Deep Impact is
going to be sent to another comet, Hartley 2, which will take
two years longer to reach:
http://planetary.org/blog/article/00001243/
The other problem is that NASA has not yet promised to pay
for this extended space mission – which I assume would still
be cheaper than sending a whole new spacecraft there.
Deep Impact Extended Mission Heads for Comet Hartley 2
COLLEGE PARK, Md. — NASA has given a University of Maryland-led team of
scientists the green light to fly the Deep Impact spacecraft to Comet Hartley 2
on a two-part extended mission know as EPOXI. The spacecraft will fly by Earth
on New Year’s Eve at the beginning of a more than two-and-a-half-year journey to
Hartley 2.
The EPOXI mission is actually two new missions in one. During the first six
months of the journey to Hartley 2, the Extrasolar Planet Observations and
Characterization (EPOCh) mission will use the larger of the two telescopes on
the Deep Impact spacecraft to search for Earth-sized planets around five stars
selected as likely candidates for such planets. Upon arriving at the comet the
Deep Impact eXtended Investigation (DIXI) will conduct an extended flyby of
Hartley 2 using all three of the spacecraft’s instruments (two telescopes with
digital color cameras and an infrared spectrometer).
“It’s exciting that we can send the Deep Impact spacecraft on a new mission that
combines two totally independent science investigations, both of which can help
us better understand how solar systems form and evolve,” said Deep Impact leader
and University of Maryland astronomer Michael A’Hearn, who is principal
investigator (PI) for both the overall EPOXI mission and its DIXI component.
The EPOXI mission brings back the Deep Impact partnership between the University
of Maryland, NASA’s Jet Propulsion Laboratory (JPL) and Ball Aerospace &
Technologies Corporation, and adds NASA’s Goddard Space Flight Center.
DAUGHTERS OF DEEP IMPACT
On July 4th 2005, the University of Maryland-led NASA mission Deep Impact made
history and world-wide headlines when it smashed a probe into Comet Tempel. The
mission yielded a wealth of new cometary information, but the data on Tempel 1
was in many cases startlingly different from that from comet missions Deep Space
1 and Stardust. As a result, rather than revealing the true nature of comets,
the sometimes conflicting data from these three missions has left scientists
questioning most of what they thought they knew about these fascinating, and
potentially dangerous, objects; and longing for new data from other comets.
“One of the great surprises of comet explorations has been the wide diversity
among the different cometary surfaces imaged to date,” said A’Hearn. “We want a
close look at Hartley 2 to see if the surprises of Tempel 1 are more common than
we thought, or if Tempel 1 really is unusual.”
After the completion of Deep Impact, the mission team knew they had a still
healthy and flight-proven spacecraft that was capable of traveling to a
never-before-visited comet at a fraction of the cost of a newly built and
launched mission. In 2006 the A’Hearn-led team began the proposal process that
eventually became EPOXI.
TRAJECTORY OF A DUAL MISSION
When the Deep Impact/EPOXI spacecraft passes by Earth on December 31, 2007, it
will use the pull of our planet’s gravity to direct and speed itself toward
comet Hartley 2. In doing this the spacecraft is aimed toward an encounter with
comet Hartley 2 at a time when tracking stations in two different locations on
Earth can “see” the spacecraft to receive data from it and send commands to it.
In late December 2007, the spacecraft’s instruments will be recalibrated using
the Moon as a target.
Hartley 2 was not the original destination of the new mission. It was selected
in October following the surprising realization that despite tremendous efforts
by many observatories and observers, the scientists could not reliably identify
their first choice, comet Boethin, and its orbit in time to plan the mission
flyby of Earth. The team then recommended to NASA that it be allowed to fly to
the backup target, comet Hartley 2.
“Hartley 2 is scientifically just as interesting as comet Boethin since both
have relatively small, active nuclei,” said A’Hearn. “As we have worked the
details of the comet Hartley 2 encounter, we are confident that the observations
will turn out to be even better than Boethin.”
THE JOURNEY’S EPOCh LEG
The first part of the Deep Impact extended mission — the search for alien
worlds — will begin in late January as the spacecraft cruises toward Hartley 2.
More than 200 alien (extrasolar) planets have been discovered to date. Most of
these are detected indirectly, by the gravitational pull they exert on their
parent star. Directly observing extrasolar planets by detecting the light
reflected from them is very difficult, because a star’s brilliance obscures
light coming from any planets orbiting it.
However, sometimes the orbit of an extrasolar world is aligned so that it
eclipses its star as seen from Earth. In these rare cases, light from that
planet can be seen directly.
“When the planet appears next to its star, your telescope captures their
combined light. When the planet passes behind its star, your telescope only sees
light from the star. By subtracting light from just the star from the combined
light, you are left with light from the planet,†said Goddard scientist Drake
Deming, who heads EPOCh and is deputy principal investigator for EPOXI. “We can
analyze this light to discover what the atmospheres of these planets are like.”
Planets as small as three Earth masses can be detected in this way. EPOCh will
also observe the Earth in visible and infrared wavelengths to allow comparisons
with future discoveries of Earth-like planets around other stars.
The mission will observe five nearby stars with “transiting extrasolar planets,”
so named because the planet transits, or passes in front of, its star. The
planets were discovered earlier and are giant planets with massive atmospheres,
like Jupiter in our solar system. They orbit their stars much closer than Earth
does the sun, so they are hot and belong to the class of extrasolar planets
nicknamed “Hot Jupiters.”
However, these giant planets may not be alone. If there are other worlds around
these stars, they might also transit the star and be discovered by the
spacecraft. Even if they don’t transit, Deep Impact could find them indirectly.
Their gravity will pull on the transit planets, altering their orbits and the
timing of their transits.
“Since Deep Impact will be able to stare at these stars for long periods, we can
observe multiple transits and compare the timing to see if there are any hidden
worlds,” said Deming.
ARE WE THERE YET?
In June of 2008, the extended mission will end its EPOCh portion and transition
to a long, quiet journey to comet Hartley 2. The total trip — measured from its
December 31, 2007 flyby of Earth to its closest encounter with the comet on
October 11, 2010 — will be roughly 1.6 billion miles or some 18 times the
distance from the Earth to the sun. It will take the spacecraft three trips
around the sun before it can intercept the comet, which at that time will be at
a distance of some 12.4 million miles from Earth.
At the nearest point of its flyby of Hartley 2, the spacecraft will be some 550
miles from the comet. Deep Impact does not have another probe, so Hartley 2 will
not get hit, but the close-up view will allow the spacecraft’s two telescopes to
closely observe surface features of the comet while its infrared spectrometer
maps the composition of any outbursts of gas from the surface.
Comet science goals for this phase of the mission are to:
• Search for and, if found, produce maps of outbursts of gas from
the surface of comet Hartley 2. Track the outburst as the comet rotates.
Correlate outbursts with surface features. Such outbursts were observed during
the spacecraft’s flyby of comet Tempel 1.
• Obtain infrared spectral maps of gasses in the innermost portion
of the coma. The coma is the cloud of gas and dust that surrounds the comet.
Investigate the distribution of dust and gas in the coma.
• Search for frozen volatiles (SUCH AS?) on the surface of the
comet. Water ice, for example, was discovered when the flyby explored Tempel 1.
• Produce broad band images of the comet that will establish limits
on the size of the nucleus. Produce a model of its shape.
• Map the brightness and color variations of the surface. Locate
landscape features that indicate the processes by which the comet was formed.
Compare the distribution of crater sizes with the distribution of the size of
craters on other comets, asteroids and planetary satellites.
• Map the temperature of the surface to assess how readily heat is
transmitted to the interior and the flow of subsurface volatiles, such as water
vapor, to the surface.
For A’Hearn and his DIXI team the most rewarding time will come after the flyby,
as they turn the raw data into new insights on the structure and formation of
comets and their place in the history of our solar system.
NASA’s DEEP IMPACT BEGINS HUNT FOR ALIEN WORLDS
GREENBELT, Md. – NASA’s Deep Impact spacecraft is aiming its largest telescope at five stars in a search for alien (exosolar) planets as it enters its extended mission, called EPOXI.
Deep Impact made history when the mission team directed an impactor from the spacecraft into comet Tempel 1 on July 4, 2005. NASA recently extended the mission, redirecting the spacecraft for a flyby of comet Hartley 2 on Oct. 11, 2010.
As it cruises toward the comet, Deep Impact will observe five nearby stars with “transiting exosolar planets,” so named because the planet transits, or passes in front of, its star. The EPOXI team, led by University of Maryland astronomer Dr. Michael A’Hearn, directed the spacecraft to begin these observations January 22. The planets were discovered earlier and are giant planets with massive atmospheres, like Jupiter in our solar system. They orbit their stars much closer than Earth does the sun, so they are hot and belong to the class of exosolar planets nicknamed “Hot Jupiters.”
However, these giant planets may not be alone. If there are other worlds around these stars, they might also transit the star and be discovered by the spacecraft. Deep Impact can even find planets that don’t transit, using a timing technique. Gravity from the unseen planets will pull on the transiting planets, altering their orbits and the timing of their transits.
“We’re on the hunt for planets down to the size of Earth, orbiting some of our closest neighboring stars,” said EPOXI Deputy Principal Investigator Dr. Drake Deming of NASA’s Goddard Space Flight Center in Greenbelt, Md. EPOXI is a combination of the names for the two extended mission components: the exosolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact eXtended Investigation (DIXI). Goddard leads the EPOCh component.
More than 200 exosolar planets have been discovered to date. Most of these are detected indirectly, by the gravitational pull they exert on their parent star. Directly observing exosolar planets by detecting the light reflected from them is very difficult, because a star’s brilliance obscures light coming from any planets orbiting it.
However, sometimes the orbit of an exosolar world is aligned so that it eclipses its star as seen from Earth. In these rare cases, called transits, light from that planet can be seen directly.
“When the planet appears next to its star, your telescope captures their combined light. When the planet passes behind its star, your telescope only sees light from the star. By subtracting light from just the star from the combined light, you are left with light from the planet,” said Deming, who is leading the search for exosolar worlds with Deep Impact. “We can analyze this light to discover what the atmospheres of these planets are like.”
Deep Impact will also look back to observe the Earth in visible and infrared wavelengths, allowing comparisons with future discoveries of Earth-like planets around other stars.
The University of Maryland is the Principal Investigator institution, leading the overall EPOXI mission, including the flyby of comet Hartley 2, called the Deep Impact eXtended Investigation (DIXI). NASA Goddard leads the exosolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh). EPOXI is a combination of the names for these two extended mission components. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages EPOXI for NASA’s Science Mission Directorate, Washington. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.
For information about EPOXI, visit:
http://www.nasa.gov/epoxi