You report concern among some US planetary scientists that the funding for the James Webb Space Telescope (JWST) is preventing NASA from providing a rocket for the 2016 ExoMars launch (see go.nature.com/zimumq and go.nature.com/emqasy). But this is not the root of the problem.

The real issue is the dismal five-year budget projection for NASA's Solar System exploration in US President Barack Obama's budget proposal from February 2011 — a projection driven by debt-reduction pressures across all federal agencies.

A look at more recent versions of the 2012 budget — one from the House of Representatives, which provides no money for the JWST, and the other from the Senate, which gives enough this year to support a launch in 2018 — reveals that identical amounts are provided for Solar System exploration. It is therefore clear that cancellation of the JWST will lead to debt reduction and not to a transfer of funds to planetary missions, a point made explicitly in the House budget text.

The JWST will peer back in time to the beginnings of the cosmos while measuring the composition of atmospheres of super-earths around nearby stars. The challenge for space scientists is to make the case that such exploration is worthwhile, even in difficult economic times. If we attack one another's programmes, we shall see funds for many of our hard-fought missions swept away in the name of deficit reduction — and we shall all lose out.