Herschel’s population of trans-Neptunian objects

Herschel’s population of trans-Neptunian objects.

Credit: ESA

ESA’s Herschel space observatory has observed 132 of the known 1400 cold worlds that inhabit a region of the Solar System beyond the orbit of Neptune, some 4.5–7.5 billion km from the Sun.

These ‘trans-Neptunian objects’ (TNOs), include worlds such as Pluto, Eris, Haumea and Makemake, and make up a vast population of such objects thought to occupy these far-flung reaches of the Solar System.

TNOs are particularly cold, at around –230ºC, but these low temperatures lend themselves to observations by Herschel, which observes at far-infrared to sub-millimetre wavelengths.

Indeed, the space observatory observed the thermal emission from 132 such objects during its nearly four-year lifetime.

These measurements provided their sizes and albedos (the fraction of visible light reflected from the surface), properties that are not otherwise easily accessible.

The graphic presented here shows a sample of the population of TNOs observed with Herschel, arranged to showcase these properties.

What is most striking is their diversity. They range from just below 50 km to almost 2400 km in diameter; Pluto and Eris are the largest.

Two worlds have distinctly elongated shapes: Haumea (seen in white) and Varuna (brown). Some even host their own moons (not shown).

The albedo measurement implies a variety of surface compositions: low albedo (brown) is an indication of dark surface materials, such as organic material, while higher albedo (white) suggests pure ices.

TNOs are thought to be some of the most primitive remnants of the planet-forming era. Thus the results of the Herschel “TNOs are cool: A survey of the trans-Neptunian region” open key time programme are being used to test different models of Solar System formation and evolution.

ESA Proba-2: Ffuel tank refilled from ‘solid gas’

Proba-2 is flight-testing a total of 17 technology demonstrators for future ESA missions. 

It also serves as a scientific platform for solar and space weather observations.

Credits: ESA/Pierre Carril

Sometimes all it takes is fresh air to get a new lease of life. ESA’s Proba-2 microsatellite is a good example: an influx of nitrogen has replenished its fuel tank, in the process demonstrating a whole new space technology.

On 16 August a telecommand was sent from ESA’s Redu ground station in Belgium to boost the gases in Proba-2’s unusual ‘resistojet’ engine.

Used to maintain the microsatellite’s orbit at 600 km altitude, this experimental engine runs on xenon gas heated before ejection to provide added thrust.

The command added nitrogen gas to the fuel tank, bringing its pressure close to its launch level.

	cool-gas generators

“What makes this repressurisation unique is that the added gas was not stored in a pressurised state but produced from a solid material at room temperature, the first of four ‘cool-gas generators’ on Proba-2,” explained Laurens van Vliet of Dutch research organisation TNO, which developed the technology.

“Nitrogen, like xenon, is an inert, non-reactive gas, so the resistojet can work just as well with a xenon–nitrogen mixture.”

The bottle-shaped cool-gas generators are filled with a rigid solid material that, once triggered, produces more than 250 times its own volume in pure nitrogen gas.

Integrating TNO's four cool-gas generators aboard Proba-2

Credits: TNO

To read more on this subject visit the ESA Proba-2 portal

Pluto and it's 14 new neighbours

Beyond Neptune's orbit, roughly five billion miles from the sun, the solar system can seem like a dark, desolate place.

But like the murky depths of the ocean, the darkness hides millions of mysterious bodies—or at least, so we think.

Known collectively as trans-Neptunian objects, or TNOs, the first of this population to be discovered was Pluto in 1930. Since then we've found a thousand or so objects in Pluto's domain. Some have even been given exotic names, such as Chaos, Ixion, Quaoar, and Rhadamanthus.

A chart of the largest known trans-Neptunian objects as of 2007.
—Image courtesy NASA, ESA, and A. Feild (STScI)

So far, two probes have ventured that deep into the solar system (that'd be Voyager 1 and 2) but neither one paid much heed to TNOs on their way farther afield.

That means astronomers using Earthly telescopes can only guess at how many bodies are out there, what they look like, and what they're made of.

Now, using archived pictures from the Hubble Space Telescope, a team of scientists has found a way to spot TNOs, and they've added 14 more to the catalog.

"Trans-Neptunian objects interest us because they are building blocks left over from the formation of the solar system," lead author Cesar Fuentes, formerly with the Harvard-Smithsonian Center for Astrophysics and now at Northern Arizona University, said in a press release.

The trick to finding them is to look for the equivalent of meteor streaks in Hubble shots of other objects.