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Scientists have observed a collision of two of the universe's most extreme objects. And a mission to Jupiter's moon Europa makes an important step forward.

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Seeing giant things smash into each other is one of the great pleasures of astronomy. So it's no surprise that astronomers were excited to announce that they've seen a collision between two of the universe's most extreme objects for the first time: a black hole and a neutron star.

Even cooler, this observation was made not with light, but with ripples in the fabric of space-time called gravitational waves. Black holes and neutron stars are the remnants of giant stars that have undergone a supernova explosion. If the dying star has a core with a mass roughly two to three times larger than our Sun it ends up as a neutron star -- an incredibly dense body made mostly of neutrons.

Stars bigger than that collapse all the way down to a black hole. Large stars often form in binary pairs. And since those big stars will eventually die, it's not too surprising to see black holes and neutron stars orbiting each other.

In the past, astronomers have seen black holes collide with other black holes and even two neutron stars hit one another. But these latest observations mark the first time an object of each type has been involved in the collision. The event was detected by LIGO and VIRGO, gravitational wave observatories located in the United States and Italy.

Gravitational waves work kind of like someone sitting down on the couch next to you. You don't have to see it happen because you can feel how their weight distorts the cushions. Einstein's theory of general relativity says that gravity distorts the fabric of the universe in much the same way.

Because neutron stars and black holes both have a ton of gravity, their collision sends out a massive disturbance that can travel an incredible distance -- in this case, around 900 million light-years. LIGO and VIRGO detect these gravitational waves using a technique called interferometry, which combines a pair of lasers pointed at right angles to one another. When a gravitational wave washes over the detector, it makes space literally shorter in one direction and longer in the other.

The two lasers thus travel different distances, causing a change in travel time that records the presence of the wave. This all sounds kind of straightforward, but getting it to actually work was so difficult that it almost immediately scored a Nobel Prize back in 2017. By combining multiple detectors, scientists can filter out any false positives and triangulate where the event must have taken place.

Seeing the merger between these two kinds of objects isn't just a nifty addition to our collection of cool space collisions. Astronomers hope by analyzing how the black hole ripped the neutron star apart, they can improve their understanding of the structure of neutron stars, and how resilient they are. Closer to home, this month NASA has announced that its upcoming Europa Clipper mission has taken an important step forward.

The project, which will probably cost about four billion dollars, has moved from a preliminary draft to its final design stage. Its goal is to study Jupiter's moon Europa, which planetary scientists believe conceals a vast ocean of liquid water under its icy surface. All that water makes Europa perhaps the very best place in the solar system to search for extraterrestrial life.

Although the mission is moving into the next phase of planning, it hasn't been entirely smooth sailing for the Clipper thus far. NASA announced back in March 2019 that it was cancelling development of a key instrument designed to measure the depth of Europa's hidden ocean. The device, called a magnetometer, would have precisely measured the magnetic fields created by currents of electricity within the ocean.

But tests showed that the sensor, which was already three times over-budget, would have struggled to handle the intense radiation environment around Jupiter. Instead NASA will replace the specially designed magnetometer with a more generic type used on other missions. This off-the-shelf part will be more reliable, but less precise: instead of measuring the ocean's depth to within 20 kilometers, it could be off by as much as a hundred.

And, if the ocean is particularly conductive to electricity, the sensor may not return much information at all. That's not ideal, but eliminating mission components that threaten the progress of the overall project is a key element in the design review process. There's also uncertainty about how the spacecraft will actually get to Jupiter.

While Congress has mandated that the Europa Clipper fly aboard NASA's upcoming Space. Launch System, that rocket won't even be available until years after the satellite itself is ready in 2023. The mission could instead launch on schedule aboard a commercial rocket like the Falcon.

Heavy for a fraction of the price, but the trip would be slower and require a tricky flyby of Venus along the way. Still, these kinds of uncertainties are normal for a mission as complex and ambitious as the Europa Clipper. The fact that NASA has given mission planners the green light to move ahead is a big deal and takes us one step closer to exploring yet another new place in the solar system.

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